Provided by: libimage-metadata-jpeg-perl_0.159-1_all 
NAME
Image::MetaData::JPEG - Perl extension for showing/modifying JPEG (meta)data.
SYNOPSIS
use Image::MetaData::JPEG;
# Create a new JPEG file structure object
my $image = new Image::MetaData::JPEG('somepicture.jpg');
die 'Error: ' . Image::MetaData::JPEG::Error() unless $image;
# Get a list of references to comment segments
my @segments = $image->get_segments('COM', 'INDEXES');
# Get the JPEG picture dimensions
my ($dim_x, $dim_y) = $image->get_dimensions();
# Show all JPEG segments and their content
print $image->get_description();
# Retrieve a specific value from Exif meta-data
my $image_data = $image->get_Exif_data('IMAGE_DATA', 'TEXTUAL');
print $image_data->{DateTimeOriginal}->[0], "\n";
# Modify the DateTime tag for the main image
$image->set_Exif_data({'DateTime' => '1994:07:23 12:14:51'},
'IMAGE_DATA', 'ADD');
# Delete all meta-data segments (please, don't)
$image->drop_segments('METADATA');
# Rewrite file to disk after your modifications
$image->save('new_file_name.jpg');
# ... and a lot more methods for viewing/modifying meta-data, which
# are accessed through the $file or $segments[$index] references.
DESCRIPTION
The purpose of this module is to read/modify/rewrite meta-data segments in JPEG (Joint
Photographic Experts Group format) files, which can contain comments, thumbnails, Exif
information (photographic parameters), IPTC information (editorial parameters) and similar
data.
Each JPEG file is made of consecutive segments (tagged data blocks), and the actual row
picture data. Most of these segments specify parameters for decoding the picture data into
a bitmap; some of them, namely the COMment and APPlication segments, contain instead meta-
data, i.e., information about how the photo was shot (usually added by a digital camera)
and additional notes from the photograph. These additional pieces of information are
especially valuable for picture databases, since the meta-data can be saved together with
the picture without resorting to additional database structures. See the appendix about
the structure of JPEG files for technical details.
This module works by breaking a JPEG file into individual segments. Each file is
associated to an Image::MetaData::JPEG structure object, which contains one
Image::MetaData::JPEG::Segment object for each segment. Segments with a known format are
then parsed, and their content can be accessed in a structured way for display. Some of
them can even be modified and then rewritten to disk.
$JPEG::show_warnings
This package variable must be used to inhibit the printing of warnings: if it is
false, warnings are silently ignored. Otherwise, warning messages come with a detailed
back-trace and description of the warning location.
$Image::MetaData::JPEG::show_warnings = undef;
Managing a JPEG structure object
JPEG::new
[arguments: "($input, $regex, $options)"] The first thing you need in order to
interact with a JPEG picture is to create an Image::MetaData::JPEG structure object.
This is done with a call to the new method, whose first argument is an input source,
either a scalar, interpreted as a file name to be opened and read, or a scalar
reference, interpreted as a pointer to an in-memory buffer containing a JPEG stream.
This interface is similar to that of Image::Info, but no open file handle is
(currently) accepted. The constructor then parses the picture content and stores its
segments internally. The memory footprint is close to the size of the disk file plus a
few tens of kilobytes.
my $file = new Image::MetaData::JPEG('a_file_name.jpg');
my $file = new Image::MetaData::JPEG(\ $a_JPEG_stream);
The constructor method accepts two optional arguments, a regular expression and an
option string. If the regular expression is present, it is matched against segment
names, and only those segments with a positive match are parsed (they are nonetheless
stored); this allows for some speed-up if you just need partial information, but be
sure not to miss something necessary; e.g., SOF segments are needed for reading the
picture dimensions. For instance, if you just want to manipulate the comments, you
could set the string to 'COM'.
my $file = new Image::MetaData::JPEG('a_file_name.jpg', 'COM');
The third optional argument is an option string. If it matches the string
'FASTREADONLY', only the segments matching the regular expression are actually stored;
also, everything which is found after a Start Of Scan is completely neglected. This
allows for very large speed-ups, but, obviously, you cannot rebuild the file
afterwards, so this is only for getting information fast, e.g., when doing a directory
scan.
my $file = new Image::MetaData::JPEG('a_file.jpg', 'COM', 'FASTREADONLY');
Nota bene: an old version of "Arles Image Web Page Creator" had a bug which caused the
application to generate JPEG's with illegal comment segments, reportedly due to a bug
in the Intel JPEG library the developers used at that time (these segments had to 0x00
bytes appended). It is true that a JPEG file with garbage between segments is to be
considered invalid, but some libraries like IJG's try to forgive, so this module tries
to forgive too, if the amount of garbage isn't too large (only a warning is printed).
JPEG::Error
[arguments: none] If the file reference remains undefined after a call to new, the
file is to be considered not parseable by this module, and one should issue some error
message and go to another file. An error message explaining the reason of the failure
can be retrieved with the Error method:
die 'Error: ' . Image::MetaData::JPEG::Error() unless $file;
JPEG::get_segments
[arguments: "($regex, $do_indexes)"] If the new call is successful, the returned
reference points to an Image::MetaData::JPEG structure object containing a list of
references to Image::MetaData::JPEG::Segment objects, which can be retrieved with the
get_segments method. This method returns a list containing the references (or their
indexes in the Segment references' list, if the second argument is the string INDEXES)
to those Segments whose name matches the $regex regular expression. For instance, if
$regex is 'APP', all application Segments will be returned. If you want only APP1
Segments you need to specify '^APP1$'. The output can become invalid after
adding/removing any Segment. If $regex is undefined, all references are returned.
my @segments = $file->get_segments($regex, $do_indexes);
JPEG::drop_segments
[arguments: "($regex)"] Similarly, if you are only interested in eliminating some
segments, you can use the drop_segments method, which erases from the internal segment
list all segments matching a given regular expression. If the regular expression is
undefined or evaluates to the empty string, this method throws an exception, because I
don't want the user to erase the whole file just because he/she did not understand
what he was doing. One should also remember that it is not wise to drop non-meta-data
segments, because this in general invalidates the file. As a special case, if $regex
== 'METADATA', all APP* and COM segments are erased.
$file->drop_segments('^APP1$');
JPEG::insert_segments
[arguments: "($segref, $pos, $overwrite)"] Inserting a Segment into the picture's
segment list is done with the insert_segments method. This method inserts the segments
referenced by $segref into the current list of segments at position $pos. If $segref
is undefined, the method fails silently. If $pos is undefined, the position is chosen
automatically (using find_new_app_segment_position ); if $pos is out of bound, an
exception is thrown; this happens also if $pos points to the first segment, and it is
an SOI. $segref may be a reference to a single segment or a reference to a list of
segment references; everything else throws an exception. If $overwrite is defined, it
must be the number of segments to overwrite during the splice.
$file->insert_segments([$my_comment_1, $my_comment_2], 3, 1);
JPEG::get_description
JPEG::get_dimensions
[arguments: none] Getting a string describing the findings of the parsing stage is as
easy as calling the get_description method. Those Segments whose parsing failed have
the first line of their description stating the stopping error condition. Non-
printable characters are replaced, in the string returned by get_description, by a
slash followed by the two digit hexadecimal code of the character. The (x,y)
dimensions of the JPEG picture are returned by get_dimensions from the Start of Frame
(SOF*) Segment:
print $file->get_description();
my ($dim_x, $dim_y) = $file->get_dimensions();
JPEG::find_new_app_segment_position
[arguments: "($name)"] If a new comment or application Segment is to be added to the
file, the module provides a standard algorithm for deciding the location of the new
Segment, in the find_new_app_segment_position method. The argument is the name of the
Segment to be inserted (it defaults to 'COM', producing a warning). The position is
chosen immediately before the first (or after the last) element of some list, provided
that the list is not empty, otherwise the next list is taken into account: 1) [for COM
segments only] after 'COM' segments; otherwise after APP segments; 2) [for APPx
segments only] after APPy's (trying y = x..0, in sequence); otherwise before APPy's
(trying y = x+1..15, in sequence); 3) before DHP segments; 4) before SOF segments. If
all these tentatives fail, the position immediately after the SOI segment is returned
(i.e., 1).
my $new_position = $file->find_new_app_segment_position('APP2');
JPEG::save
[arguments: "($filename)"] The data areas of each Segment in the in-memory JPEG
structure object can be rewritten to a disk file or to an in-memory scalar, thus
recreating the (possibly modified) JPEG picture. This is accomplished by the save
method, accepting a filename or a scalar reference as argument; if the file name is
undefined, it defaults to the file originally used to create the JPEG structure
object. This method returns "true" (1) if it works, "false" (undefined) otherwise.
Remember that if the file had initially been opened with the 'FASTREADONLY' option, it
is not possible to save it, and this call fails immediately.
print "Creation of $newJPEG failed!" unless $file->save($newJPEG);
An example of how to proficiently use the in-memory feature to read the content of a
JPEG thumbnail is the following (see later for get_Exif_data, and also do some error
checking!):
my $thumbnail = $file->get_Exif_data('THUMBNAIL');
print Image::MetaData::JPEG->new($thumbnail)->get_description();
Managing a JPEG Segment object
JPEG::Segment::name
JPEG::Segment::error
An Image::MetaData::JPEG::Segment object is created for each Segment found in the JPEG
image during the creation of a JPEG object (see JPEG::new), and a parser routine is
executed at the same time. The name member of a Segment object identifies the "nature"
of the Segment (e.g. 'APP0', ..., 'APP15' or 'COM'). If any error occurs (in the
Segment or in an underlying class), the parsing of that Segment is interrupted at some
point and remains therefore incomplete: the error member of the relevant Segment
object is then set to a meaningful error message. If no error occurs, the same
variable is left undefined.
printf 'Invalid %s!\n', $segment->{name} if $segment->{error};
JPEG::Segment::records
The reference to the Segment object is returned in any case. In this way, a faulty
Segment cannot inhibit the creation of a JPEG structure object; faulty segments cannot
be edited or modified, basically because their structure could not be fully
understood. They are always rewritten to disk unmodified, so that a file with
corrupted or non-standard Segments can be partially edited without fearing of damaging
it. Once a Segment has successfully been built, its parsed information can be accessed
directly through the records member: this is a reference to an array of JPEG::Record
objects, an internal class modelled on Exif records (see the subsection about record
management for further details).
my $records = $segment->{records};
printf '%s has %d records\n', $segment->{name}, scalar @$records;
JPEG::Segment::search_record
JPEG::Segment::search_record_value
[arguments: "([$dirref], $keys ...)"] If a specific record is needed, it can be
selected with the help of the search_record method, which searches for a record with a
given key (see "JPEG::Record::key") in a given record directory, returning a reference
to the record if the search was fruitful, the undefined value otherwise. The
algorithm for the search is as follows: 1) a start directory is chosen by looking at
the last argument: if it is an ARRAY ref it is popped out and used, otherwise the top-
level directory is selected; 2) a string is created by joining all remaining arguments
on '@', then it is exploded into a list of keys on the same character (all undefined
or "false" arguments are simply discarded); 3) these keys are used for an iterative
search starting from the initially chosen directory: all but the last key must
correspond to $REFERENCE records. If $key is exactly "FIRST_RECORD" / "LAST_RECORD",
the first/last record in the current dir is used.
my $segments = $file->get_segments('APP0');
my $segment = $$segments[0];
print "I found it!\n" if $segment->search_record('Identifier');
If you are interested only in the Record's value, you can use the search_record_value
method, a simple wrapper around search_record(): it returns the record value (with
"JPEG::Record::get_value") if the search is successful, undef otherwise.
print "Its value is: ", $segment->search_record_value('Identifier');
Nota bene: the returned record is initialised with a "fake" $REFERENCE record pointing
to the records member of the current segment; this record is therefore returned if
search_record is invoked without arguments. For the same reason, search_record_value
invoked without arguments returns the records member:
$segment->search_record_value() eq $this->{records} || print "error!";
JPEG::Segment::update
[arguments: none] If a Segment's content (i.e. its Records' values) is modified, it is
necessary to dump it into the private binary data area of the Segment in order to have
the modification written to disk at "JPEG::save" time. This is accomplished by
invoking the update method (necessary only if you changed record values "by hand"; all
"high-level" methods for changing a Segment's content in fact call "update" on their
own). However, only Segments without errors can be updated (don't try to undef the
Segment's error flag, unless you know what you are doing!); trying to update a segment
with errors throws an exception. The same happens when trying to update a segment
without update support or without records (this catches segments created with the
'NOPARSE' flag). In practise, never use this method unless you are writing an
extension for this module.
Note that this method preliminarly saves a reference to the old segment data area and
restores it if the update process fails (if this happens, a warning is generated). One
wonders wheather there are there cleverer ways to handle this case (any suggestion is
welcome). It is however better to have a corrupt object in memory, than a corrupt
object written over the original. Currently, this is restricted to the possibility
that an updated segment becomes too large.
$segment->update();
JPEG::Segment::reparse_as
[arguments: "($new_name)"] The reparse_as method re-executes the parsing of a Segment
after changing the Segment name. This is very handy if you have a JPEG file with a
"correct" application Segment exception made for its name. I used it the first time
for a file having an ICC_profile Segment (normally in APP2) stored as APP13. Note that
the name of the Segment is permanently changed, so, if the Segment is updated and the
file is rewritten to disk, it will be "correct".
for my $segment ($file->get_segments('APP13')) {
$segment->reparse_as('APP2') if $segment->{error} &&
$segment->search_record('Identifier') =~ 'ICC_PROFILE';
$segment->update(); }
JPEG::Segment::output_segment_data
[arguments: none] The current in-memory data area of a Segment can be output to a file
through the output_segment_data method (exception made for entropy coded Segments,
this includes the initial two bytes with the Segment identifier and the two bytes with
the length if present); the argument is a file handle (this is likely to become more
general in the future). If there are problems at output time (e.g., the segment
content is too large), an exception is thrown
eval { $segment->output_segment_data($output_handle) } ||
print "A terrible output error occurred! Help me.\n";
JPEG::Segment::get_description
JPEG::Segment::size
[arguments: none] A string describing the parsed content of the Segment is obtained
through the get_description method (this is the same string used by the
get_description method of a JPEG structure object). If the Segment parsing stage was
interrupted, this string includes the relevant error. The size method returns the size
of the internal data area of a Segment object. This can be different from the length
of the scalar returned by get_segment_data, because the identifier and the length is
not included.
print $segment->get_description();
print 'Size is 4 + ' . $segment->size();
Managing a JPEG Record object
JPEG::Record::key
JPEG::Record::type
JPEG::Record::values
JPEG::Record::extra
The JPEG::Record class is an internal class for storing parsed information about a
JPEG Segment, inspired by Exif records. A Record is made up by four fields: key, type,
values and extra. The key is the record's identifier; it is either numeric or textual
(numeric keys can be translated with the help of the %JPEG_lookup function in
Tables.pm, included in this package). The type is obviously the type of stored info
(like unsigned integers, ASCII strings and so on ...). extra is a helper field for
storing additional information. Last, values is an array reference to the record
content (almost always there is just one value). For instance, for a non-IPTC
Photoshop record in APP13:
printf 'The numeric key 0x%04x means %s',
$record->{key}, JPEG_lookup('APP13@Photoshop_RECORDS', $record->{key});
printf 'This record contains %d values\n', scalar @{$record->{values}};
A Record's type can be one among the following predefined constants:
0 $NIBBLES two 4-bit unsigned integers (private)
1 $BYTE An 8-bit unsigned integer
2 $ASCII A variable length ASCII string
3 $SHORT A 16-bit unsigned integer
4 $LONG A 32-bit unsigned integer
5 $RATIONAL Two LONGs (numerator and denominator)
6 $SBYTE An 8-bit signed integer
7 $UNDEF A generic variable length string
8 $SSHORT A 16-bit signed integer
9 $SLONG A 32-bit signed integer (2's complement)
10 $SRATIONAL Two SLONGs (numerator and denominator)
11 $FLOAT A 32-bit float (a single float)
12 $DOUBLE A 64-bit float (a double float)
13 $REFERENCE A Perl list reference (internal)
$UNDEF is used for not-better-specified binary data. A record of a numeric type can
have multiple elements in its @{values} list ($NIBBLES implies an even number); an
$UNDEF or $ASCII type record instead has only one element, but its length can vary.
Last, a $REFERENCE record holds a single Perl reference to another record list: this
allows for the construction of a sort of directory tree in a Segment.
JPEG::Record::get_category
[arguments: none] The category of a record can be obtained with the get_category
method, which returns 'p' for Perl references, 'I' for integer types, 'S' for $ASCII
and $UNDEF, 'R' for rational types and 'F' for floating point types.
for my $record (@{$segment->{records}}) {
print "Subdir found\n" if $record->get_category() eq 'p'; }
JPEG::Record::get_description
[arguments: "($names)"] A human-readable description of a Record's content is the
output of the get_description method. Its argument is a reference to an array of
names, which are to be used as successive keys in a general hash keeping translations
of numeric tags. No argument is needed if the key is already non-numeric (see the
example of get_value for more details). In the output of get_description unreasonably
long strings are trimmed and non-printing characters are replaced with their
hexadecimal representation. Strings are then enclosed between delimiters, and null-
terminated $ASCII strings have their last character chopped off (but a dot is added
after the closing delimiter). $ASCII strings use a " as delimiter, while $UNDEF
strings use '.
print $record->get_description($names);
JPEG::Record::get_value
[arguments: "($index)"] In absence of "high-level" routines for collecting
information, a Record's content can be read directly, either by accessing the values
member or by calling the get_value method: it returns the $index-th value in the value
list; if the index is undefined (not supplied), the sum/concatenation of all values is
returned. The index is checked for out-of-bound errors. The following code, an
abridged version of get_description, shows how to proficiently use these methods and
members.
sub show_directory {
my ($segment, $records, $names) = @_;
my @subdirs = ();
for my $record (@$records) {
print $record->get_description($names);
push @subdirs, $record if $record->get_category() eq 'p'; }
foreach my $subdir (@subdirs) {
my $directory = $subdir->get_value();
push @$names, $subdir->{key};
printf 'Subdir %s (%d records)', $names, scalar @$directory;
show_directory($segment, $directory, $names);
pop @$names; } }
show_directory($segment, $segment->{records}, [ $segment->{name} ]);
JPEG::Record::get
[arguments: "($endianness)"] If the Record structure is needed in detail, one can
resort to the get method; in list context this method returns (key, type, count,
dataref). The data reference points to a packed scalar, ready to be written to disk.
In scalar context, it returns the dereferenced dataref. This is tricky (but handy for
other routines). The argument specifies an endianness (this defaults to big endian).
my ($key, $type, $count, $dataref) = $record->get();
Comments ("COM" segments)
JPEG::get_number_of_comments
JPEG::get_comments
[arguments: none] Each "COM" Segment in a JPEG file contains a user comment, whose
content is free format. There is however a limitation, because a JPEG Segment cannot
be longer than 64KB; this limits the length of a comment to $max_length = (2^16 - 3)
bytes. The number of comment Segments in a file is returned by get_number_of_comments,
while get_comments returns a list of strings (each string is the content of a COM
Segment); if no comments are present, they return zero and the empty list
respectively.
my $number = $file->get_number_of_comments();
my @comments = $file->get_comments();
JPEG::add_comment
[arguments: "($string)"] A comment can be added with the add_comment method, whose
only argument is a string. Indeed, if the string is too long, it is broken into
multiple strings with length smaller or equal to $max_length, and multiple comment
Segments are added to the file. If there is already at least one comment Segment, the
new Segments are created right after the last one. Otherwise, the standard position
search of find_new_app_segment_position
is applied.
$file->add_comment('a' x 100000);
JPEG::set_comment
[arguments: "($index, $string)"] An already existing comment can be replaced with the
set_comment method. Its two arguments are an $index and a $string: the $index-th
comment Segment is replaced with one or more new Segments based on $string (the index
of the first comment Segment is 0). If $string is too big, it is broken down as in
add_comment. If $string is undefined, the selected comment Segment is erased. If
$index is out-of-bound a warning is printed out.
$file->set_comment(0, 'This is the new comment');
JPEG::remove_comment
JPEG::remove_all_comments
[arguments: "($index)" for remove_comment] However, if you only need to erase the
comment, you can just call remove_comment with just the Segment $index. If you want to
remove all comments, just call remove_all_comments.
$file->remove_comment(0);
$file->remove_all_comments();
JPEG::join_comments
[arguments: "($separation, @selection)"] It is known that some JPEG comment readers
out there do not read past the first comment. So, the join_comments method, whose goal
is obvious, can be useful. This method creates a string from joining all comments
selected by the @selection index list (the $separation scalar is a string inserted at
each junction point), and overwrites the first selected comment while deleting the
others. A exception is thrown for each illegal comment index. Similar considerations
as before on the string length apply. If no separation string is provided, it defaults
to \n. If no index is provided in @selection, it is assumed that the method must join
all the comments into the first one, and delete the others.
$file->join_comments('---', 2, 5, 8);
JFIF data ("APP0" segments)
APP0 Segments are written by older cameras adopting the JFIF (JPEG File Interchange
Format), or one of its extensions, for storing images. JFIF files use the APP0
application Segment for inserting configuration data and a JPEG or RGB packed thumbnail
image. The format is described in the appendix about the APP0 structure, including the
names of all possible tags. It is of course possible to access each APP0 Segment
individually by means of the get_segments and search_record_value methods. A snippet of
code for doing this is the following:
for my $segment ($file->get_segments('APP0')) {
my $iden = $segment->search_record_value('Identifier');
my $xdim = $segment->search_record_value('Xthumbnail');
my $ydim = $segment->search_record_value('Ythumbnail');
printf 'Segment type: %s; dimensions: %dx%d\n',
substr($iden, 0, -1), $xdim, $ydim;
printf '%15s => %s\n', $_->{key}, $_->get_value()
for $segment->{records}; }
JPEG::get_app0_data
[arguments: none] However, if you want to avoid to deal directly with Segments, you
can use the get_app0_data method, which returns a reference to a hash with a plain
translation of the content of the first interesting APP0 segment (this is the first
'JFXX' APP0 segment, if present, the first 'JFIF' APP0 segment otherwise). Segments
with errors are excluded. An empty hash means that no valid APP0 segment is present.
my $data = $file->get_app0_data();
printf '%15s => %s\n', $_, (($_=~/..Thumbnail/)?'...':$$data{$_});
Exif data ("APP1" segments)
The DCT Exif (Exchangeable Image File format) standard provides photographic meta-data in
the APP1 section. Various tag-values pairs are stored in groups called IFDs (Image File
Directories), where each group refers to a different kind of information; one can find
data about how the photo was shot, GPS data, thumbnail data and so on ... (see the
appendix about the APP1 segment structure for more details). This module provides a number
of methods for managing Exif data without dealing with the details of the low level
representation. Note that, given the complicated structure of an Exif APP1 segment (where
extensive use of "pointers" is made), some digital cameras and graphic programs decide to
leave some unused space in the JPEG file. The dump routines of this module, on the other
hand, leave no unused space, so just calling update() on an Exif APP1 segment even without
modifying its content can give you a smaller file (some tens of kilobytes can be saved).
JPEG::retrieve_app1_Exif_segment
[arguments: "($index)"] In order to work on Exif data, an Exif APP1 Segment must be
selected. The retrieve_app1_Exif_Segment method returns a reference to the $index-th
such Segment (the first Segment if the index is undefined). If no such Segment exists,
the method returns the undefined reference. If $index is (-1), the routine returns the
number of available APP1 Exif Segments (which is non negative).
my $num = $file->retrieve_app1_Exif_segment(-1);
my $ref = $file->retrieve_app1_Exif_segment($num - 1);
JPEG::provide_app1_Exif_segment
[arguments: none] If you want to be sure to have an Exif APP1 Segment, use the
provide_app1_Exif_segment method instead, which forces the Segment to be present in
the file, and returns its reference. The algorithm is the following: 1) if at least
one Segment with this properties is already present, we are done; 2) if [1] fails, an
APP1 segment is added and initialised with a big-endian Exif structure (its position
is chosen by find_new_app_segment_position, as usual). Note that there is no $index
argument here.
my $ref = $file->provide_app1_Exif_segment();
JPEG::remove_app1_Exif_info
[arguments: "($index)"] If you want to eliminate the $index-th Exif APP1 Segment from
the JPEG file segment list use the remove_app1_Exif_info method. As usual, if $index
is (-1), all Exif APP1 Segments are affected at once; if $index is undefined, it
defaults to -1, so both (-1) and undef cause all Exif APP1 segments to be removed. Be
aware that the file won't be a valid Exif file after this.
$file->remove_app1_Exif_info(-1);
How to inspect your Exif data
JPEG::Segment::get_Exif_data
JPEG::get_Exif_data
[arguments: "($what, $type)"] Once you have a Segment reference pointing to your
favourite Exif Segment, you may want to have a look at the records it contains, by
using the get_Exif_data method: it accepts two arguments ($what and $type) and returns
the content of the APP1 segment packed in various forms. Error conditions (invalid
$what's and $type's) manifest themselves through an undefined return value.
All Exif records are natively identified by numeric tags (keys), which can be
"translated" into a human-readable form by using the Exif standard docs; only a few
fields in the Exif APP1 preamble (they are not Exif records) are always identified by
this module by means of textual tags. The $type argument selects the output format for
the record keys (tags):
* NUMERIC: record tags are native numeric keys
* TEXTUAL: record tags are human-readable (default)
Of course, record values are never translated. If a numeric Exif tag is not known, a
custom textual key is created with "Unknown_tag_" followed by its numerical value
(this solves problems with non-standard tags). The subset of Exif tags returned by
this method is determined by the value of $what, which can be one of:
$what returned info returned type
---------------------------------------------------------------------
ALL (default) everything but THUMBNAIL ref. to hash of hashes
IMAGE_DATA a merge of IFD0_DATA and SUBIFD_DATA ref. to flat hash
THUMB_DATA this is an alias for IFD1_DATA ref. to flat hash
THUMBNAIL the actual (un)compressed thumbnail ref. to scalar
ROOT_DATA header records (TIFF and similar) ref. to flat hash
IFD0_DATA primary image TIFF tags ref. to flat hash
SUBIFD_DATA Exif private tags ref. to flat hash
MAKERNOTE_DATA MakerNote tags (if struct. is known) ref. to flat hash
GPS_DATA GPS data of the primary image ref. to flat hash
INTEROP_DATA interoperability data ref. to flat hash
IFD1_DATA thumbnail-related TIFF tags ref. to flat hash
Setting $what equal to 'ALL' returns a reference to a hash of hashes, whose top-level
hash contains the following keys: ROOT_DATA, IFD0_DATA, SUBIFD_DATA, GPS_DATA,
INTEROP_DATA, MAKERNOTE_DATA and IFD1_DATA; each key corresponds to a second-level
hash containing a copy of all Exif records present in the IFD (sub)directory
corresponding to the key (if this directory is not present or contains no records, the
second-level hash exists and is empty). Note that the Exif record values' format is
not checked to be valid according to the Exif standard. This is, in some sense,
consistent with the fact that also "unknown" tags are included in the output. This
complicated structure is more easily explained by showing an example (see also the
section about valid Exif tags for details on possible records):
my $hash_ref = $segment->get_Exif_data('ALL', 'TEXTUAL');
can give
$hash_ref = {
'ROOT_DATA' =>
{ 'Signature' => [ 42 ],
'Endianness' => [ 'MM' ],
'Identifier' => [ "Exif\000\000" ],
'ThumbnailData' => [ ... image ... ], },
'IFD1_DATA' =>
{ 'ResolutionUnit' => [ 2 ],
'JPEGInterchangeFormatLength' => [ 3922 ],
'JPEGInterchangeFormat' => [ 2204 ],
'Orientation' => [ 1 ],
'XResolution' => [ 72, 1 ],
'Compression' => [ 6 ],
'YResolution' => [ 72, 1 ], },
'SubIFD_DATA' =>
{ 'ApertureValue' => [ 35, 10 ],
'PixelXDimension' => [ 2160 ],
etc., etc. ....
'ExifVersion' => [ '0210' ], },
'MAKERNOTE_DATA' => {},
'IFD0_DATA' =>
{ 'Model' => [ "KODAK DX3900 ZOOM DIGITAL CAMERA\000" ],
'ResolutionUnit' => [ 2 ],
etc., etc. ...
'YResolution' => [ 230, 1 ], },
'GPS_DATA' => {},
'INTEROP_DATA' =>
{ 'InteroperabilityVersion' => [ '0100' ],
'InteroperabilityIndex' => [ "R98\000" ], }, };
Setting $what equal to '*_DATA' returns a reference to a flat hash, corresponding to
one or more IFD (sub)dirs. For instance, 'IMAGE_DATA' is a merge of 'IFD0_DATA' and
'SUBIFD_DATA': this interface is simpler for the end-user, because there is only one
dereference level; also, he/she does not need to be aware of the partition of records
related to the main image into two IFDs. If the (sub)directory is not present or
contains no records, the returned hash exists and is empty. With reference to the
previous example:
my $hash_ref = $segment->get_Exif_data('IMAGE_DATA', 'TEXTUAL');
gives
$hash_ref = {
'ResolutionUnit' => [ 2 ],
'JPEGInterchangeFormatLength' => [ 3922 ],
'JPEGInterchangeFormat' => [ 2204 ],
'Orientation' => [ 1 ],
'XResolution' => [ 72, 1 ],
'Compression' => [ 6 ],
'YResolution' => [ 72, 1 ],
'ApertureValue' => [ 35, 10 ],
'PixelXDimension' => [ 2160 ],
etc., etc. ....
'ExifVersion' => [ '0210' ], };
Last, setting $what to 'THUMBNAIL' returns a reference to a copy of the actual Exif
thumbnail image (this is not included in the set returned by 'THUMB_DATA'); if there
is no thumbnail, a reference to the empty string is returned (the undefined value
cannot be used, because it is assumed that it corresponds to an error condition here).
Note that the pointed scalar may be quite large (~ 10^1 KB). If the thumbnail is in
JPEG format (this corresponds to the 'Compression' property, in IFD1, set to 6), you
can create another JPEG picture object from it, like in the following example:
my $data_ref = $segment->get_Exif_data('THUMBNAIL');
my $thumb = new Image::MetaData::JPEG($data_ref);
print $thumb->get_description();
If you are only interested in reading Exif data in a standard configuration, you can
skip the segment-search calls and use directly JPEG::get_Exif_data (a method of the
JPEG class, so you only need a JPEG structure object). This is an interface to the
method with the same name in the Segment class, acting on the first Exif APP1 Segment
(if no such segment is present, the undefined value is returned) and passing the
arguments through. Note that most JPEG files with Exif data contain at most one Exif
APP1 segment, so you are not going to loose anything here. A snippet of code for
visualising Exif data looks like this:
while (my ($d, $h) = each %{$image->get_Exif_data('ALL')}) {
while (my ($t, $a) = each %$h) {
printf '%-25s\t%-25s\t-> ', $d, $t;
s/([\000-\037\177-\377])/sprintf '\\%02x',ord($1)/ge,
$_ = (length $_ > 30) ? (substr($_,0,30) . ' ... ') : $_,
printf '%-5s', $_ for @$a; print "\n"; } }
How to modify your Exif data
JPEG::Segment::set_Exif_data
JPEG::set_Exif_data
[arguments: "($data, $what, $action)"]
Similarly to the getter case, there is a set_Exif_data method callable from a picture
object, which does nothing more than looking for the first Exif APP1 segment (creating
it, if there is none) and invoke the method with the same name in the Segment class,
passing its arguments through. So, the remaining of this section will concentrate on
the Segment method. The problem of setting a new thumbnail or erasing it is dealt with
in the last paragraphs of this section. (The APP1 Exif structure is quite complicated,
and the number of different possible cases when trying to modify it is very large;
therefore, designing a clean and intuitive interface for this task is not trivial.
Fell free to suggest improvements and cleaner interfaces).
Exif records are usually characterised by a numeric key (a tag); this was already
discussed in the "getter" section. Since these keys, for valid records, can be
translated from numeric to textual form and back, the end user has the freedom to use
whichever form better fits his needs. The two forms can even be mixed in the same
"setter" call: the method will take care to translate textual tags to numeric tags
when possible, and reject the others; then, it will proceed as if all tags were
numeric from the very beginning. Records with unknown textual or numeric tags are
always rejected.
The arguments to set_Exif_data are $data, $what and $action. The $data argument must
be a hash reference to a flat hash, containing the key - record values pairs supplied
by the user. The "value" part of each hash element can be an array reference
(containing a list of values for the record, remember that some records are multi-
valued) or a single scalar (this is internally converted to a reference to an array
containing only the supplied scalar). If a record value is supposed to be a null
terminated string, the user can supply a Perl scalar without the final null character
(it will be inserted automatically).
The $what argument must be a scalar, and it selects the portion of the Exif APP1
segment concerned by the set_Exif_data call. So, obviously, the end user can modify
only one section at a time; this is a simplification (for the developer of course) but
also for the end user, because trying to set all Exif-like values in one go would
require an offensively complicated data structure to specify the destination of each
record (note that some records in different sections can have the same numerical tag,
so a plain hash would not trivially work). Valid values for $what are (MakerNote data
are not currently modifiable):
$what modifies ... $data type
--------------------------------------------------------------------
IMAGE_DATA as IFD0_DATA and SUBIFD_DATA ref. to flat hash
THUMB_DATA this is an alias for IFD1_DATA ref. to flat hash
THUMBNAIL the actual (un)compressed thumbnail ref. to scalar/object
ROOT_DATA header records (endianness) ref. to flat hash
IFD0_DATA primary image TIFF tags ref. to flat hash
SUBIFD_DATA Exif private tags ref. to flat hash
GPS_DATA GPS data of the primary image ref. to flat hash
INTEROP_DATA interoperability data in SubIFD ref. to flat hash
IFD1_DATA thumbnail-related TIFF tags ref. to flat hash
The $action argument controls whether the setter adds ($action = 'ADD') records to a
given data directory or replaces ($action = 'REPLACE') them. In the first case, each
user-supplied record replaces the existing version of that record if present, and
simply inserts the record if it was not already present; however, existing records
with no counterpart in the user supplied $data hash remain untouched. In the second
case, the record directory is cleared before inserting user data. Note that, since
Exif and Exif-like records are non-repeatable in nature, there is no need of an
'UPDATE' action, like for IPTC (see the IPTC section).
The set_Exif_data routine first checks that the concerned segment is of the
appropriate type (Exif APP1), that $data is a hash reference (a scalar reference for
the thumbnail), and that $action and $what are valid. If $action is undefined, it
defaults to 'REPLACE'. Then, an appropriate (sub)IFD is created, if absent, and all
user-supplied records are checked for consistency (have a look at the appendixes for
this). Last, records are set in increasing (numerical) tag order, and mandatory data
are added, if not present. The return value of the setter routine is always a hash
reference; in general it contains records rejected by the specialised routines. If an
error occurs in a very early stage of the setter, this reference contains a single
entry with key='ERROR' and value set to some meaningful error message. So, returning a
reference to an empty hash means that everything was OK. An example, concerning the
much popular task of changing the DateTime record, follows:
$dt = '1994:07:23 12:14:51';
$hash = $image->set_Exif_data({'DateTime' => $dt}, 'IMAGE_DATA', 'ADD');
print "DateTime record rejected\n" if %$hash;
Depending on $what, some of the following notes apply:
ROOT_DATA
The only modifiable item is the 'Endianness' (and it can only be set to big-
endian, 'MM', or little-endian, 'II'); everything else is rejected (see the APP1
structure for further details). This only influences how the image is written back
to disk (the in-memory representation is always native).
IMAGE_DATA
By specifying this target one can address the IFD0_DATA and SUBIFD_DATA targets at
once. First, all records are tried in the IFD0, then, rejected records are tried
into SubIFD (then, they are definitively rejected).
IFD0_DATA
See the canonical, additional and company-assigned tags' sections in the
appendixes (this target refers to the primary image). The 'XResolution',
'YResolution', 'ResolutionUnit', and 'YCbCrPositioning' records are forced if not
present (to [1,72], [1,72], 2 and 1 respectively). Note that the situation would
be more complicated if we were dealing with uncompressed (TIFF) primary images.
SUBIFD_DATA
See the private Exif section in the appendixes. The 'ExifVersion',
'ComponentsConfiguration', 'FlashpixVersion', 'ColorSpace', and
'Pixel[XY]Dimension' records are forced if not present (to '0220', '1230', '0100',
1 and 0x0 respectively). Image dimensions can be retrieved from the SOF segment
with the JPEG structure object's method get_dimensions() and set explicitly by the
user if necessary (this cannot be done from within the APP1 segment, because it
does not link back to its parent); however, the horizontal field in the SubIFD
should not include data padding, while that in the SOF segment does, so the
meaning is slightly different and these fields cannot be automatically calculated.
THUMB_DATA (or its alias IFD1_DATA)
See the canonical, additional and company-related tag lists' sections in the
appendixes (this target refers to thumbnail properties). The 'XResolution',
'YResolution', 'ResolutionUnit', 'YCbCrSubSampling', 'PhotometricInterpretation'
and 'PlanarConfiguration' records are forced if not present (to [1,72], [1,72], 2,
[2,1], 2 and 1 respectively). Note that some of these records are not necessary
for all types of thumbnails, but JPEG readers will probably skip unnecessary
information without problems.
GPS_DATA
See the GPS tags section in the appendixes. The 'GPSVersionID' record is forced,
if it is not present at the end of the process, because it is mandatory (ver 2.2
is chosen). There are some record inter-correlations which are still neglected
here (for instance, the 'GPSAltitude' record can be inserted without providing the
corresponding 'GPSAltitudeRef' record).
INTEROP_DATA
JPEG::forge_interoperability_IFD
[arguments: none] See the Interoperability directory section in the appendixes.
The 'InteroperabilityIndex' and 'InteroperabilityVersion' records are forced, if
they are not present at the end of the process, because they are mandatory ('R98'
and ver 1.0 are chosen). Note that an Interoperability subIFD should be made as
standard as possible: if you just want to add it to the file, it is better to use
the forge_interoperability_IFD method, which takes care of all values
('RelatedImageFileFormat' is set to 'Exif JPEG Ver. 2.2', and the dimensions are
taken from get_dimensions()).
MAKERNOTE_DATA
See the appendix on MakerNotes for a detailed discussion on how the content of a
MakerNote is managed. If there is an error during the parsing of the MakerNote,
only those tags which could be fully decoded before the error are returned. Note
that MakerNote structures are often partially known, so many tags will likely be
translated as 'Unknown_tag_...'. MakerNotes cannot be currently modified.
THUMBNAIL
$data must be a reference to a scalar containing the new thumbnail or to a valid
Image::MetaData::JPEG object; if it points to an empty string, the thumbnail is
erased (the undefined value DOES NOT erase the thumbnail, it generates instead an
error). All thumbnail specific records (see the canonical tags section) are
removed, and only those corresponding to the newly inserted thumbnail are
calculated and written back. Currently, it is not possible to insert an
uncompressed thumbnail (this will probably happen in the form of a TIFF image),
only JPEG ones are accepted (automatic records contain the type, length and
offset). The following code shows how to set and delete a thumbnail.
my $image = new Image::MetaData::JPEG('original_image.jpg');
my $thumb = new Image::MetaData::JPEG('some_thumbnail.jpg');
$image->set_Exif_data($thumb, 'THUMBNAIL');
$image->save('modified_image.jpg');
$image->set_Exif_data(\ '', 'THUMBNAIL');
$image->save('thumbless_image.jpg');
XMP data ("APP1" segments)
XMP (eXtensible Metadata Platform) is a technology, conceived by Adobe Systems, to tag
graphic files with metadata, and to manage them during a lifetime made of multiple
processing steps. Its serialisation (the actual way metadata are saved in the file) is
based on RDF (Resource Description Framework) implemented as an application of XML. Its
flexibility allows to accomodate existing, future and private metadata schemas. In a JPEG
file, XMP information is included alongside Exif and IPTC data, and is stored in an APP1
segment on its own starting with the XMP namespace URI and followed by the actual XMP
packet (see XMP APP1 segment structure for more details).
XMP was introduced in 2001 as part of Adobe Acrobat version 5.01. Adobe has a trademark on
XMP, and retains control over its specification. Source code for the XMP software-
development kit was released by Adobe, but with a custom license, whose compatibility with
the GNU public license and open-source nature altogether is questioned.
Photoshop and IPTC data ("APP13" segments)
Adobe's Photoshop program, a de-facto standard for image manipulation, has, since long,
used the APP13 segment for storing non-graphical information, such as layers, paths,
ecc..., including editorial information modelled on IPTC/NAA recommendations. This module
provides a number of methods for managing Photoshop/IPTC data without dealing with the
details of the low level representation (although sometimes this means taking some
decisions for the end user ....). The structure of the IPTC data block(s) is managed in
detail and separately from the rest, although this block is a sort of "sub-case" of
Photoshop information. The interface is intentionally similar to that for Exif data.
All public methods have a $what argument selecting which part of the APP13 segment you are
working with. The default is 'IPTC'. If $what is invalid, an exception is always raised.
The kind of information you can access with different values of $what is explained in the
following (have a look at the appendices about valid Photoshop-style and IPTC tags for
further details):
$what: Concerned pieces of information:
----------- --------------------------------
'IPTC' or Editorial information like caption, abstract, author,
'IPTC_2' copyright notice, byline, shot site, user defined keywords,
and many more; in practise, all what is covered by the IPTC
Application Record 2. This is the most common option; the
default value of $what, 'IPTC', is a synonym for 'IPTC_2'
for backward compatibility (NOT a merge of 'IPTC_1/2').
'IPTC_1' This refers to more obscure pieces of information, contained
in the IPTC Envelope Record 1. One is rarely interested by
this, exception made for the "Coded Character Set" tag,
which is necessary to define a character set different
from ASCII (i.e., when you don't write or read in English).
'PHOTOSHOP' Alpha channels, colour information, transfer functions,
or 'PS_8BIM' and many other details concerning the visual rendering of
or 'PS_8BPS' the picture. These fields are most often only modified by
or 'PS_PHUT' an image manipulation program, and not directly by the user.
Recent versions of Photoshop (>= 4.0) use a resource data
block type equal to '8BIM', and this is the default in
this module (so, 'PHOTOSHOP' and 'PS_8BIM' are synonyms).
However, some other older or undocumented resource data
block types are also allowed.
JPEG::retrieve_app13_segment
[arguments: "($index, $what)"] In order to work on Photoshop/IPTC data, a suitable
Photoshop-style APP13 Segment must first be selected. The retrieve_app13_segment
method returns a reference to the $index-th Segment (the first Segment if the $index
is undefined) which contains information matching the $what argument. If such Segment
does not exist, the method returns the undefined reference. If $index is (-1), the
routine returns the number of available suitable APP13 Segments (which is non
negative). Beware, the meaning of $index is influenced by the value of $what.
my $num_IPTC = $file->retrieve_app13_segment(-1, 'IPTC');
my $ref_IPTC = $file->retrieve_app13_segment($num - 1, 'IPTC');
JPEG::provide_app13_segment
[arguments: "($what)"] If you want to be sure to have an APP13 Segment suitable for
the kind of information you want to write, use the provide_app13_segment method
instead, which forces the Segment to be present in the file, and returns its
reference. If at least one segment matching $what is already present, the first one is
returned. Otherwise, the first Photoshop-like APP13 is adapted by inserting an
appropriate subdirectory record (update is called automatically). If no such segment
exists, it is first created and inserted (the "Photoshop 3.0\000" identifier is used).
Note that there is no $index argument here.
my $ref_Photoshop = $file->provide_app13_segment('PHOTOSHOP');
JPEG::remove_app13_info
[arguments: "($index, $what)"] If you want to remove all traces of some flavour of
APP13 information from the $index-th APP13 Photoshop-style Segment, use the
remove_app13_info method with $what set to the appropriate value. If, after this, the
segment is empty, it is eliminated from the list of segments in the file. If $index is
(-1), all APP13 Segments are affected at once. Beware, the meaning of $index is
influenced by the value of $what.
$file->remove_app13_info(3, 'PHOTOSHOP');
$file->remove_app13_info(-1, 'IPTC');
$file->remove_app13_info(0, 'IPTC_1');
How to inspect and modify your IPTC data
JPEG::Segment::get_app13_data
[arguments: "($type, $what)"]
Once you have a Segment reference pointing to your favourite IPTC-enabled APP13
Segment, you may want to have a look at the records it contains. Use the
get_app13_data method for this: its behaviour is controlled by the $type and $what
argument (here, $what is 'IPTC_1' or 'IPTC_2' alias 'IPTC', of course). It returns a
reference to a hash containing a copy of the list of the appropriate IPTC records, if
present, undef otherwise: each element of the hash is a pair (key, arrayref), where
arrayref points to an array with the real values (some IPTC records are repeatable so
multiple values are possible). The record keys can be the native numeric keys ($type
eq 'NUMERIC') or translated textual keys ($type eq 'TEXTUAL', default); in any case,
the record values are untranslated. If a numeric key stored in the JPEG file is
unknown, and a textual translation is requested, the name of the key becomes
"Unknown_tag_$tag". Note that there is no check on the validity of IPTC records'
values: their format is not checked and one or multiple values can be attached to a
single tag independently of its repeatability. This is, in some sense, consistent with
the fact that also "unknown" tags are included in the output. If $type or $what is
invalid, an exception is thrown out. An example of how to extract and display IPTC
data is given here:
my $hash_ref = $segment->get_app13_data('TEXTUAL', 'IPTC');
while (my ($key, $vals) = each %$hash_ref) {
printf "# %20s =", $key; print " '$_'" for @$vals; print "\n"; }
### This could print:
# DateCreated = '19890207'
# ByLine = 'Interesting picture' 'really'
# Category = 'POL'
# Keywords = 'key-1' 'key-2' 'key-99'
# OriginatingProgram = 'Mapivi'
JPEG::Segment::set_app13_data
[arguments: "($data, $action, $what)"] The hash returned by get_app13_data can be
edited and reinserted with the set_app13_data method, whose arguments are $data,
$action and, as usual, $what. If $action or $what is invalid, an exception is
generated. This method accepts IPTC data in various formats and updates the
corresponding subdirectory in the segment. The key type of each entry in the input
hash can be numeric or textual, independently of the others (the same key can appear
in both forms, the corresponding values will be put together). The value of each entry
can be an array reference or a scalar (you can use this as a shortcut for value arrays
with only one value). The $action argument can be:
- ADD : new records are added and nothing is deleted; however, if you
try to add a non-repeatable record which is already present,
the newly supplied value ejects (replaces) the pre-existing value.
- UPDATE : new records replace those characterised by the same tags,
but the others are preserved. This makes it possible to modify
some repeatable IPTC records without deleting the other tags.
- REPLACE : all records present in the IPTC subdirectory are deleted
before inserting the new ones (this is the default action).
If, after implementing the changes required by $action, any mandatory dataset
(according to the IPTC standard), is still undefined, it is added automatically. This
often concerns version datasets, with numeric index 0.
The return value is a reference to a hash containing the rejected key-values entries.
The entries of %$data are not modified. An entry in the %$data hash can be rejected
for various reasons (you might want to have a look at appendix about valid IPTC tags
for further information): a) the tag is undefined or not known; b) the entry value is
undefined or points to an empty array; c) the non-repeatability constraint is
violated; d) the tag is marked as invalid; e) a value is undefined f) the length of a
value is invalid; g) a value does not match its mandatory regular expression.
$segment->set_app13_data($additional_data, 'ADD', 'IPTC');
A snippet of code for changing IPTC data looks like this:
my $segment = $file->provide_app13_segment('IPTC');
my $hashref_1 = { CodedCharacterSet => "\033\045G" }; # UTF-8
my $hashref_2 = { ObjectName => 'prova',
ByLine => 'ciao',
Keywords => [ 'donald', 'duck' ],
SupplementalCategory => ['arte', 'scienza', 'diporto'] };
$segment->set_app13_data($hashref_2, 'REPLACE', 'IPTC');
$segment->provide_app13_subdir('IPTC_1');
$segment->set_app13_data($hashref_1, 'ADD', 'IPTC_1');
JPEG::get_app13_data
[arguments: "($type, $what)"] If you are only interested in reading IPTC data in a
standard configuration, you can skip most of the previous calls and use directly
JPEG::get_app13_data (a method in the JPEG class, so you only need a JPEG structure
object). This is an interface to the method with the same name in the Segment class,
acting on the first relevant APP13 Segment (if no such segment is present, the
undefined value is returned) and passing the arguments through. Note that most JPEG
files with Photoshop/IPTC data contain at most one APP13 segment, so you are not going
to "loose" anything here. A snippet of code for visualising IPTC data looks like this:
my $hashref = $file->get_app13_data('TEXTUAL', 'IPTC');
while (my ($tag, $val_arrayref) = each %$hashref) {
printf '%25s --> ', $tag;
print "$_ " for @$val_arrayref; print "\n"; }
JPEG::set_app13_data
[arguments: "($data, $action, $what)"] There is, of course, a symmetric
JPEG::set_app13_data method, which writes data to the JPEG object without asking the
user to bother about Segments: it uses the first available suitable Segment; if this
is not possible, a new Segment is created and initialised (because the method uses
"JPEG::provide_app13_segment" internally, and not "JPEG::retrieve_app13_segment" as
"JPEG::get_app13_data" does).
$file->set_app13_data($hashref, 'UPDATE', 'IPTC');
How to inspect and modify your Photoshop data
The procedure of inspecting and modifying Photoshop data (i.e., non-IPTC data in a
Photoshop-style APP13 segment) is analogous to that for IPTC data, but with $what set to
'PHOTOSHOP' (alias 'PS_8BIM'), or to the seldom used 'PS_8BPS' and 'PS_PHUT'. The whole
description will not be repeated here, have a look at the IPTC section for it: this
section takes only care to point out differences. If you are not acquainted with the
structure of an APP13 segment and its terminology (e.g., "resource data block"), have a
look at the Photoshop-style tags' section.
About get_app13_data, it should only be pointed out that resource block names are appended
to the list of values for each tag (even if they are undefined), so the list length is
alway even. Things are more complicated for set_app13_data: non-IPTC Photoshop
specifications are less uniform than IPTC ones, and checking the correctness of user
supplied data would be an enumerative task. Currently, this module does not perform any
syntax check on non-IPTC data, but this could change in the future (any contribution is
welcome); only tags (or, how they are called in this case, "resource block identifiers")
are checked for being in the allowed tags list (see the Photoshop-style tags' table for
details). The IPTC/NAA tag is of course rejected: IPTC data must be inserted with $what
set to 'IPTC' or its siblings.
Although not explicitly stated, it seems that non-IPTC Photoshop tags are non-repeatable
(let me know if not so), so two resource blocks with the same tag shouldn't exist. For
this reason, the 'UPDATE' action is changed internally to 'ADD'. Moreover, since the
resource block structure is not explored, all resource blocks are treated as single-valued
and the value type is $UNDEF. So, in the user-supplied data hash, if a tag key returns a
data array reference, only the first element (which cannot be undefined) of the array is
used as resource block value: if a second element is present, it is used as resource block
name (which is otherwise set to the null string). Suppling more than two elements is an
error and causes the record to be rejected.
my $segment = $file->provide_app13_segment('PHOTOSHOP');
my $hashref = {
GlobalAngle => pack('N', 0x1e),
GlobalAltitude => pack('N', 0x1e),
CopyrightFlag => "\001",
IDsBaseValue => [ pack('N', 1), 'Layer ID Generator Base' ] };
$segment->set_app13_data($hashref, 'ADD', 'PHOTOSHOP');
NOTES
On the subject of year specification in a date
There are currently eight fields whose purpose is to store a date in a JPEG picture,
namely 'DateTime', 'DateTimeOriginal' and 'DateTimeDigitized' (in IFD0/1 or SubIFD),
'GPSDateStamp' (in the GPS section), and 'ReleaseDate', 'ExpirationDate', 'DateCreated'
and 'DigitalCreationDate' (in the IPTC section). Most of these dates refer to some
electronic treatment of images, a kind of process which was not available before the late
twentieth century. Two of them refer to release and expiration dates in the IPTC standard,
and should therefore not be set to a date before the introduction of the standard itself.
However, there exist users who want to use some of these fields in a non-conventional way
to refer to dates when analog photography but not digital photography was available. For
this reason, all tags (but one) can be written with a year starting from 1800 (and not
from 1900 as in earlier releases). Users are however advised to check the "specifications"
for these tags before setting the date and take responsibility for their non-
conventionality.
There is one notable exception to the previous considerations, that is the IPTC
'DateCreated' dataset, which should explicitly refer to the creation date of the object
represented in the picture, which can be many centuries in the past. For this dataset a
special regular expression is provided which allows a date in the full ISO-8601 YYYY-MM-DD
format (however, it should be noted that even ISO-8601 does not allow a date before 0AD,
so not all masterworks from ancient Greece can be tagged in this way ... let me know if I
am wrong). I am, of course, still open to suggestions and reconsiderations on this
subject.
On the problem of MakerNote corruption and ways to overcome it
A widespread problem with Exif maker notes is that there is no common standard for how to
parse and rewrite the information in the MakerNote data area. This is the reason why most
programs dealing with Exif JPEG files corrupt the MakerNote on saving, or decide to drop
it altogether (be aware that there existed programs known to hang when they try to read a
corrupt maker note).
In fact, many maker notes contain a non-standard IFD structure, with some tags storing
file offsets (see the documentation page describing the IFD structure). Therefore, saving
a maker note without regard for internal offsets' adjustment reduces the note mostly to
garbage. Re-dumping a maker note after changing the Exif APP1 segment endianness incurs
the same problem, because no internal byte-swap is performed.
A few countermeasures have been introduced in this package to try to cure some maker note
problems. The first one concerns the correct byte order (the endianness, which is not
always the same used in the Exif segment), which needs not to be known in advance; it is
in fact determined by using the fact that, if the note is IFD-like (even non-standard),
the number of tags is always in the range [1,255], so the two-bytes tag count has always
the most significant byte set to zero, and the least significant byte set to non-zero.
There is also a prediction and correction mechanism for the offsets in the
interoperability arrays, based on the simple assumption that the absolute value of offsets
can be wrong, but their differences are always right, so, if one can get the first one
right ... a good bet is the address of the byte immediately following the next_IFD link
(or the tag list, if this link is absent). If the parsing process does not end
successfully, this mechanism is enabled and its "corrected" findings are stored instead of
the original ones if it is able to cure the problems (i.e., if the second try at parsing
the note is successful).
CURRENT STATUS
A lot of other routines for modifying other meta-data could be added in the future. The
following is a list of the current status of various meta-data Segments (only APP and COM
Segments).
Segment Possible content Status
* COM User comments parse/read/write
* APP0 JFIF data (+ thumbnail) parse/read
* APP1 Exif or XMP data parse/read[Exif]/write[Exif]
* APP1 Maker notes parse/read
* APP2 FPXR data or ICC profiles parse
* APP3 additional Exif-like data parse
* APP4 HPSC nothing
* APP12 PreExif ASCII meta parse
* APP13 IPTC and PhotoShop data parse/read/write
* APP14 Adobe tags parse
KNOWN BUGS
USE WITH CAUTION! THIS IS EXPERIMENTAL SOFTWARE!
This module is still experimental, and not yet finished. In particular, it is far from
being well tested, and some interfaces could change depending on user feedback. The
ability to modify maker notes is not yet implemented (moreover, have a look at the
MakerNote appendix for a general note on the problem of MakerNote corruption). APP13 data
spanning multiple Segments are not correctly read/written. Most of APP12 Segments do not
fit the structure parsed by parse_app12(), probably there is some standard I don't know.
OTHER PACKAGES
Other packages are available in the free software arena, with a feature set showing a
large overlap with that found in this package; a probably incomplete list follows.
However, none of them is (or was) completely satisfactory with respect to the package's
objectives, which are: being a single package dealing with all types of meta-information
in read/write mode in a JPEG (and possibly TIFF) file; depending on the least possible
number of non standard packages and/or external programs or libraries; being open-source
and written in Perl. Of course, most of these objectives are far from being reached ....
"Image::ExifTool" by Phil Harvey
ExifTool is a Perl module with an included command-line application for reading and
writing meta information in image files. It recognizes EXIF, GPS, IPTC, XMP, JFIF,
GeoTIFF, ICC Profile, Photoshop IRB and ID3 meta information as well as the maker
notes of many digital cameras including Canon, Casio, FujiFilm, Kodak, Leaf,
Minolta/Konica-Minolta, Nikon, Olympus/Epson, Panasonic/Leica, Pentax/Asahi, Ricoh,
Sanyo and Sigma/Foveon. It was started as a highly customisable, read-only report
tool, capable of organising the results in various ways. Since version 4.10 (beginning
of 2005) it added the ability to modify and rewrite JPEG tags. So sad there are now
two projects with such a large overlap.
"Image::IPTCInfo" by Josh Carter
This is a CPAN module for for extracting IPTC image meta-data. It allows reading IPTC
data (there is an XML and also an HTML output feature) and manipulating them through
native Perl structures. This library does not implement a full parsing of the JPEG
file, so I did not consider it as a good base for the development of a full-featured
module. Moreover, I don't like the separate treatment of keywords and supplemental
categories.
"JPEG::JFIF" by Marcin Krzyzanowski, "Image::EXIF" by Sergey Prozhogin and "exiftags" by
Eric M. Johnston
JPEG::JFIF is a very small CPAN module for reading meta-data in JFIF/JPEG format
files. In practice, it only recognises a subset of the IPTC tags in APP13, and the
parsing code is not suitable for being reused for a generic JPEG segment. Image::Exif
is just a Perl wrapper around exiftags, which is a program parsing the APP1 section in
JPEG files for Exif meta-data (it supports a variety of MakerNotes). exiftags can also
rewrite comments and date and time tags.
"Image::Info" by Gisle Aas
This CPAN module extracts meta information from a variety of graphic formats
(including JPEG and TIFF). So, it is not specifically about JPEG segments: reported
information includes media type, extension, width, height, colour type, comments,
Interlace, Compression, Gamma, and LastModificationTime. For JPEG files, it
additionally reports from JFIF (APP0) and Exif (APP1) segments (including MakerNotes).
This module does not allow for editing.
"exif" by Martin Krzywinski and "exifdump.py" by Thierry Bousch
These are two basic scripts to extract Exif information from JPEGs. The first script
is written in Perl and targets Canon pictures. The second one is written in Python,
and it only works on JPEG files beginning with an APP1 section after the SOI. So, they
are much simpler than all other programs/libraries described here. Of course, they
cannot modify Exif data.
"jhead" by Matthias Wandel
The jhead program (written in C) is used to display JPEG comments and Exif data, and
to perform limited manipulation of Exif headers (such as changing the internal time-
stamps, removing the thumbnail, or transferring headers back into edited images) and
comments. Exif header data modification is very limited, as jhead's internal
implementation of the file system contained in the Exif header is read-only; there,
for instance, no way to replace the thumbnail in the Exif header with another.
"exifprobe" by Duane H. Hesser
This is a C program which examines and reports the contents and structure of JPEG and
TIFF image files. It recognises all standard JPEG markers and reports the contents of
any properly structured TIFF IFD encountered, even when entry tags are not recognised.
Camera MakerNotes are included. GPS and GeoTIFF tags are recognised and entries
printed in "raw" form, but are not expanded. The output is nicely formatted, with
indentation and colouration; this program is a great tool for inspecting a JPEG/TIFF
structure while debugging.
"libexif" by Lutz Müller
This is a library, written in C, for parsing, editing, and saving Exif data. All Exif
tags described in Exif standard 2.1 are supported. Libexif can only handle some maker
notes, and even those not very well. It is used by a number of front-ends, including:
Exif (read-only command-line utility), gexif (a GTK+ front-end for editing Exif data),
gphoto2 (command-line front-end to libgphoto2, a library to access digital cameras),
gtkam (a GTK+ front-end to libgphoto2), thirdeye (a digital photos organiser and
driver for eComStation).
"jpegrdf" by Norman Walsh
This is a Java application for manipulating (read/write) RDF meta-data in the comment
sections of JPEG images (is this the same thing which can be found in APP1 segments in
XMP format?). It can also access and convert into RDF the Exif tags and a few other
general properties. However, I don't want to rely on a Java environment being
installed in order to be able to access these properties.
"OpenExif" by Eastman Kodak Company
This is an object-oriented interface written in C++ to Exif formatted JPEG image
files. It is very complete and sponsored by a large company, so it is to be considered
a sort of reference. The toolkit allows creating, reading, and modifying the meta-data
in the Exif file. It also provides means of getting and setting the main image and the
thumbnail image. OpenExif is also extensible, and Application segments can be added.
AUTHOR
Stefano Bettelli, bettelli@cpan.org
COPYRIGHT AND LICENSE
Copyright (C) 2004,2005,2006 by Stefano Bettelli
This library is free software; you can redistribute it and/or modify it under the terms of
the GNU General Public License. See the COPYING and LICENSE file for the license terms.
SEE ALSO
Have a look at the technical appendixes of the Image::MetaData::JPEG module [M in the
following], packaged as separate documents: they contain a description of segment
structures [M::Structures], and lists of valid tags [M::TagLists], including a tentative
description of some MakerNote formats [M::MakerNotes]. See also your current perl(1)
documentation, an explanation for the General Public License and the manual pages of the
following optional Perl modules: Image::ExifTool(3pm), Image::IPTCInfo(3pm),
JPEG::JFIF(3pm), Image::EXIF(3pm) and Image::Info(3pm).