Provided by: libnetpbm-dev_11.05.02-1.1build1_amd64 
NAME
libnetpbm_ug - netpbm sample code
DESCRIPTION
The Libnetpbm programming library is part of Netpbm(1).
Contents
•
Example
•
libnetpbm Classes
•
Library Initialization classes
•
The pam Structure
•
Plain versus Raw Format
•
Reference
Example
Here is an example of a C program that uses libnetpbm to read a Netpbm image input and produce a Netpbm
image output.
/* Example program fragment to read a PAM or PNM image
from stdin, add up the values of every sample in it
(I don't know why), and write the image unchanged to
stdout. */
#include <netpbm/pam.h>
struct pam inpam, outpam;
tuple * tuplerow;
unsigned int row;
pm_init(argv[0], 0);
pnm_readpaminit(stdin, &inpam, PAM_STRUCT_SIZE(tuple_type));
outpam = inpam; outpam.file = stdout;
pnm_writepaminit(&outpam);
tuplerow = pnm_allocpamrow(&inpam);
for (row = 0; row < inpam.height; ++row) {
unsigned int column;
pnm_readpamrow(&inpam, tuplerow);
for (column = 0; column < inpam.width; ++column) {
unsigned int plane;
for (plane = 0; plane < inpam.depth; ++plane) {
grand_total += tuplerow[column][plane];
}
}
pnm_writepamrow(&outpam, tuplerow);
}
pnm_freepamrow(tuplerow);
libnetpbm Classes
In this section, Guide To Using Libnetpbm, we cover only the PAM functions in libnetpbm. As described in
the introduction to libnetpbm" (1), there are four other classes of image processing functions (PBM, PGM,
PPM, PNM). They are less important, since you can do everything more easily with the PAM functions, but
if you're working on old programs or need the extra efficiency those older functions can sometimes
provide, you can find them documented as here: PBM Function Manual(1), PGM Function Manual(1), PPM
Function Manual(1), and PNM Function Manual(1).
In case you're wondering, what makes the PAM functions easier to use is:
• Each function handles all the formats. It does so without converting to a common format, so your
program can treat the different formats differently if it wants. However, the interface makes it
easy for your program to ignore the differences between the formats if that's what you want.
• The PAM function parameter lists convey most information about the image with which you're working
with a single pam structure, which you can build once and use over and over, whereas the older
functions require you to pass up to 5 pieces of image information (height, width, etc.) as
separate arguments to every function.
Library Initialization
Every program that uses the library must initialize the library, i.e. set up the process to use the
library, as described in Initialization . That is the purpose of the call to pm_init() in the example
above.
The pam Structure
The PAM functions take most of their arguments in the form of a single pam structure. This is not an
opaque object, but just a convenient way to organize the information upon which most the functions
depend. So you are free to access or set the elements of the structure however you want. But you will
find in most cases it is most convenient to call pnm_readpaminit() or pnm_writepaminit() to set the
members in the pam structure before calling any other pam functions, and then just to pass the structure
unchanged in all future calls to pam functions.
It depends upon the function to which you pass the structure what members are inputs, what members are
outputs, and what members are irrelevant.
It is possible for a pam structure not to specify some members, by operation of its len member. When you
supply a pam structure as an argument to a function, the function has default behavior defined for
unspecified members. All the functions require that you specify at least up through maxval, and some
require more.
Likewise, a function the returns a pam structure can return only a subset of the members defined here,
according to its setting of the len member. But this normally happens only because the library is old
and predates the existence of the omitted members.
The members are:
size The storage size in bytes of this entire structure.
len The length, in bytes, of the information in this structure. The information starts in the first
byte and is contiguous. This cannot be greater than size. size and len can be used to make
programs compatible with newer and older versions of the Netpbm libraries.
file The file.
format The format code of the image, which tells which of the various Netpbm image formats is being
processed. The following macros stand for those format codes:
PAM_FORMAT
PAM
RPBM_FORMAT
raw PBM format
RPGM_FORMAT
raw PGM format
RPPM_FORMAT
raw PPM format
PBM_FORMAT
plain PBM format
PGM_FORMAT
plain PGM format
PPM_FORMAT
plain PPM format
There is an important quirk in the meaning of this member when you use the pam structure to write
an image: Only the type portion of it is meaningful. A Netpbm format code conveys two pieces of
information: The format type (PBM, PGM, PPM, or PAM) and the plainness (plain PBM vs raw PBM,
etc.). But when writing, libnetpbm ignores the plainness part and instead takes the plainness
from the plainformat member. So PBM_FORMAT and RPBM_FORMAT are identical when writing.
This quirk exists for historical purposes; it's necessary for consistency with the older functions
such as pnm_writepnmrow() whose format and forceplain arguments are analogous.
Before Netpbm 10.32 (February 2006), libnetpbm did not ignore the plainness. This caused many
programs to behave poorly, producing plain format output when they should, for backward
compatibility at the very least, produce raw format output.
A common way to use this member is to copy it and the plainformat member from a pam for an input
image to a pam for an output image. When you do that, your output image will be raw format
regardless of whether your input image was plain or raw, and this is the conventional behavior of
Netpbm programs.
plainformat
This is a boolean value (0 = false, 1 = true), meaningful only when writing an image file. It
means to write in the plain (text) version of the format indicated by format as opposed to the raw
(binary) version. Note that the format code in format would appear to completely specify the
format, making plainformat redundant. But see the description of format for why that isn't true.
Until Netpbm 10.32 (February 2006), this was defined a little differently. The format member did
in fact completely identify the format and plainformat was redundant and existed as a separate
member only for computational speed. But this was inconsistent with the older libnetpbm interface
(e.g. pnm_writepnm(), and it made it difficult to write backward compatible programs. Before
Netpbm 10.32, it affected reading as well as writing.
libnetpbm image reading functions set this member to false, for your convenience in building an
output image pam from an input image pam.
height The height of the image in rows.
width The width of the image in number of columns (tuples per row).
depth The depth of the image (degree of or number of samples in each tuple).
maxval The maxval of the image. See definitions in pam(1).
bytes_per_sample
The number of bytes used to represent each sample in the image file. See the format definition in
pam(1). This is entirely redundant with maxval. It exists as a separate member for computational
speed.
tuple_type
The tuple type of the image. See definitions in pam(1). Netpbm defines values for the most
common types of visual images, but any value is legal. There are macros for these values:
PAM_PBM_TUPLETYPE
black and white image, such as would alternatively be represented by a PBM image.
PAM_PGM_TUPLETYPE
grayscale image, such as would alternatively be represented by a PGM image.
PAM_PPM_TUPLETYPE
color image, such as would alternatively be represented by a PPM image.
PAM_PBM_ALPHA_TUPLETYPE
black and white with a transparency (alpha) information.
PAM_PGM_ALPHA_TUPLETYPE
grayscale with a transparency (alpha) information.
PAM_PPM_ALPHA_TUPLETYPE
color with a transparency (alpha) information.
allocation_depth
The number of samples for which memory is allocated for any tuple associated with this PAM
structure. This must be at least as great as 'depth'. Only the first 'depth' of the samples of a
tuple are meaningful.
The purpose of this is to make it possible for a program to change the type of a tuple to one with
more or fewer planes.
0 means the allocation depth is the same as the image depth.
comment_p
Pointer to a pointer to a NUL-terminated ASCII string of comments. When reading an image, this
contains the comments from the image's PAM header; when writing, the image gets these as comments,
right after the magic number line. The individual comments are delimited by newlines and are in
the same order as in the PAM header. The "#" at the beginning of a PAM header line that indicates
the line is a comment is not part of the comment.
On output, NULL means no comments.
On input, libnetpbm mallocs storage for the comments and placed the pointer at *comment_p. Caller
must free it. NULL means libnetpbm does not return comments and does not allocate any storage.
Examples:
const char * comments;
...
pam.comment_p = &comments;
pnm_readpaminit(fileP, &pam, PAM_STRUCT_SIZE(comment_p));
printf("The comments are:\n");
printf("%s", comments)
free(comments);
const char * comments;
...
comments = strdup("This is a comment 1\nThis is comment 2\n");
pam.comment_p = &comments;
pnm_writepaminit(&pam);
free(comments);
This works only for PAM images. If you read a PNM image, you always get back a null string. If
you write a PNM image, you always get an image that contains no comments.
This member does not exist before Netpbm 10.35 (August 2006). Before that, there is no way with
libnetpbm to get or set comments. The macro PAM_HAVE_COMMENT_P is defined in pam.h where the
member exists.
Plain Versus Raw Format
The PNM formats each come in two varieties: the older plain (text) format and the newer raw (binary)
format. There are different format codes for the plain and raw formats, but which of the two formats the
pnm and pam functions write is independent of the format code you pass to them.
The pam functions always write raw formats. If you specify the format code for a plain format, a pam
function assumes instead the raw version of that format.
The pnm functions choose between plain and raw based on the forceplain parameter that every write-type
pnm function has. If this boolean value is true, the function writes the plain version of the format
specified by the format code. If it is false, the function writes the raw version of the format
specified by the format code.
We are trying to stamp out the older plain formats, so it would be a wise choice not to write a program
that sets forceplain true under any circumstance. A user who needs a plain format can use the
pnmtoplainpnm program to convert the output of your program to plain format.
Reference
The Libnetpbm Netpbm Image Processing Manual" (1) describes the the libnetpbm functions for processing
image data.
The Libnetpbm Utility Manual(1) describes the functions that are not specifically related to the Netpbm
image formats.
DOCUMENT SOURCE
This manual page was generated by the Netpbm tool 'makeman' from HTML source. The master documentation
is at
http://netpbm.sourceforge.net/doc/libnetpbm_ug.html
netpbm documentation Libnetpbm Image Processing Manual(3)