Provided by: pngcrush_1.7.85-1build1_amd64 
NAME
pngcrush — optimizes (or modifies) PNG (Portable Network Graphics) files.
SYNOPSIS
pngcrush [options] [infile.png] [outfile.png]
pngcrush -e ext [options] [file1.png] [file2.png] ...
pngcrush -d dir [options] [file1.png] [file2.png] ...
DESCRIPTION
pngcrush is an optimizer for PNG (Portable Network Graphics) files. Its main purpose is to reduce the
size of the file's IDAT chunk (or stream). pngcrush can also be used to modify a PNG's ancillary chunks
(example: transparency information or textual comments). Some familiarity with the PNG (pronounced
'ping') format may be helpful to users of pngcrush. pngcrush has reasonable defaults so running with no
options may produce smaller files. This document briefly describes the PNG format where necessary to
understand pngcrush. For complete documentation, see :
http://www.libpng.org/pub/png/.
The actual pixel data of a PNG is contained in one or many IDAT chunks. To make an IDAT chunk PNG
encoders take the raw pixel data, filter it with one of 5 different filters (See Section "Filter Types" )
then compress it. pngcrush reduces the size of PNG files by choosing a different filter or compression
methods from those used in the input file. pngcrush tries multiple filter/compression methods and
compares the size of the resulting IDAT chunk. The filter/compression method with the smallest IDAT
chunk is chosen for the output file. The filter and compression levels pngcrush tries are controlled by
command line options. All of the filter methods and compression levels are lossless; they will not
reduce image quality.
In addition to reducing the size of a PNG file, pngcrush can modify ancillary chunks. Ancillary chunks
are optional information including transparency (tRNS), gamma (gAMMA), standard RGB color space
conformance (sRGB) or textual information (iTXt, tEXt or zTXt). See the associated command line options
for details on manipulating these chunks.
OPTIONS
-already size
If file has an IDAT chunk greater than the integer size, it will be considered to be already
crushed and will not be processed further.
-bit_depth n
Force output bit depth to n. See Section "Color Types" for bit depth restrictions.
-bkgd r g b
Specify the default background color for the image. Some PNG viewers will use this background
when displaying the image. Background is specified by r, g and b integers between 0 and
(2^bitdepth)-1. For output color type 0 and 4 (gray, see section "Color Types" ) the green
index is used as the gray level of the background.
-brute Use brute force, try 114 different filter/compression methods [11-124]. This option is very
time-consuming and generally not worthwhile. You can restrict this option to certain filter
types, compression levels, or strategies by following it with -f filter, -l level, or -z
strategy. For example:
pngcrush -brute -f 0 infile.png
Will try only methods that use filter type 0.
-c type Set the output image color type to type, one of [0, 2, 4, or 6] (see section "Color Types" ).
Future versions of pngcrush will also allow color type 3, if there are 256 or fewer colors
present in the input file. Color types 4 and 6 are padded with an opaque alpha channel if the
input file does not have alpha information. You can use 0 or 4 to convert color to grayscale.
Use 0 or 2 to delete an unwanted alpha channel. Default is to use same color type as the input
file.
-d dir Specify a directory for all output files. All output files will have the same name as their
respective input files.
-dou Double the images gamma. This is used for fixing gamma in PhotoShop 5.0c5.02 files. It has
been claimed that the PS5 bug is actually more complex than that, in some unspecified way.
-e ext Specify a new extension ext for all output files.
-exit Forces pngcrush to call exit() rather than return() when it is finished. It is unclear to this
writer why this option exists.
-f filter Specify filter (see section "Filter Types" ) to use with the method specified in the preceding
-m or -brute option. Valid filter types are [0-4] : use specified filter, [5]: use adaptive
filtering.
-fix Fix otherwise fatal conditions such as bad CRCs. The CRC (Cyclic Redundancy Check) is present
in every chunk and can alert the decoder of corrupt data.
-force Write a new output file even if larger than input. Otherwise the input file will be copied to
output if it is smaller than any generated file and no chunk additions, removals, or changes
were requested.
-g gamma Value to insert in gAMA chunk, only if the input file has no gAMA chunk. To replace an
existing gAMA chunk, use the -replace_gamma option. Some decoders will use the gAMA chunk to
more accurately display the image. gamma relates the output intensity to the input samples
(input_sample = light_out GAMMA). gamma can be any integer between 0 and 2 32 and is
interpreted as GAMMA*100000. For example, if you wanted a GAMMA of 1/2.2, you would enter 45455
for gamma.
-h Display help and legal notices.
-itxt b|a keyword text
Insert an (uncompressed) iTXt chunk. b|a indicates whether chunk should be inserted before or
after the IDAT chunk, (see Section "Text Chunks" ).
-keep chunk_name
keep named chunk chunk_name even if the PNG datastream becomes invalid. Currently only dSIG is
recognized as an acceptable value.
-l level zlib compression level to use on the filtered IDAT chunk with the method specified by the
preceding -m or -brute option. zlib compression levels are integers between 0 and 9. 0 = no
compression, 1 = fastest compression, and 9 = best compression.
-loco Make the file more compressible by performing a lossless, reversible, color transformation. The
resulting file is a MNG format file, not a PNG, and should be given the .mng file extension.
The loco option has no effect on grayscale or indexed-color PNG files.
-m method pngcrush method [0-200] to try (0 means try all of 1-10). Can be repeated as in -m 1 -m 4 -m 7.
This can be useful if you run out of memory when pngcrush tries methods 2, 3, 5, 6, 8, 9, or 10
which use filtering and are memory intensive. Method 1, 4, and 7 use no filtering; methods 11
and up use specified filter, compression level, and strategy.
-max maximum_IDAT_size
Set the maximum idat size to be used when creating the compression buffer.[1 through 524288]
-mng write a new MNG, do not crush embedded PNGs.
-n Do not do compression or write output file. This is useful in conjunction with the -v option to
get info, or to test decode speed.
-p This option tells pngcrush to pause and wait for [enter] key whenever the screen fills.
-plte_len n
Truncates the PLTE. The PLTE chunk contains from 1 to 256 palette entries. Be sure not to
truncate it to less than the greatest index present in IDAT.
-q quiet, the opposite of verbose.
-rem chunkname
Name of an ancillary chunk or optional PLTE to be removed. Be careful with this. Please don't
use this feature to remove transparency, gamma, copyright, or other valuable information. To
remove several different chunks, repeat: -rem tEXt -rem pHYs. Known chunks (those in the PNG
spec or extensions document) can be named with all lower-case letters, so -rem bkgd is
equivalent to -rem bKGD. But note: -rem text removes all forms of text chunks; Exact case is
required to remove unknown chunks. To do surgery with a chain-saw, -rem alla removes all known
ancillary chunks except for tRNS, and -rem allb removes all but tRNS and gAMA.
-replace_gamma gamma
Force a specified gamma in the output file even if gAMA is present in the input. See the -g for
more information.
-res dpi Write a pHYs chunk with a resolution of dpi. The pHYs chunk indicates the desired pixel size.
-save Force writing of unknown chunks. If the input image has chunks that are not part of the PNG
specification, they should not be discarded.
-srgb n Set value of rendering intent for sRGB chunk to n where n is between 0 and 3. The appropriate
rendering intent depends on how the image will be used:
0 - Perceptual: when good adaptation to the output device gamut at the expense of
colorimetric accuracy is desired, example: photographs.
1 - Relative colorimetric: images requiring color appearance matching (relative to
the output device white point), example: logos.
2: Saturation: preservation of saturation at the expense of hue and lightness is
preferred, example: charts and graphs.
3: Absolute colorimetric: images requiring preservation of absolute colorimetry,
example: proofs (previews of images destined for a different output device).
-text [b|a] keyword text
Insert a tEXt chunk. [b|a] indicates whether chunk should be inserted before or after the IDAT
chunk, (see Section "Text Chunks" ).
-newtimestamp
Reset file modification time to newer time stamp. This is the default.
-oldtimestamp
Retain input file modification time stamp. The default is -newtimestamp which is the opposite
of this option.
-trns index red green blue gray
Insert a tRNS (transparency) chunk, if no tRNS chunk found in file. You must give all five
parameters regardless of the color type, scaled to the output bit depth. See the PNG
documentation for details.
-trns_array n trns[0] trns[1] ... trns[n-1]
Insert a tRNS (transparency) chunk, if no tRNS chunk found in file. See the PNG documentation
for details.
-v Display more detailed information. Repeat the option (use "-v -v") for even more.
-w size Specify size in kbytes (or bytes in the case of 512) of the sliding compression window where
size is one of [32, 16, 8, 4, 2, 1, or 512]. It's best to use the default (32) unless you run
out of memory. The program will use a smaller window anyway when the uncompressed file is
smaller than 16k.
-z strategy
Specify the zlib compression strategy [0, 1, or 2] to be used on the filtered IDAT chunk for
the method of the preceding -m. The zlib strategy parameter tunes the compression algorithm
and is one of:
• 0 : default, most compression is aimed towards string matching
• 1 : some string matching, some Huffman coding
• 2 : use only Huffman coding
-zitxt [b|a] keyword text
Insert a zTXt chunk. [b|a] indicates whether chunk should be inserted before or after the IDAT
chunk, (see Section "Text Chunks" ).
-ztxt [b|a] keyword text
zTXt chunk to insert (see -text).
Color Types
The PNG specification provides for five color types. The color type determines how the IDAT chunk will
be interpreted by the decoder. Choosing a color type appropriate for the color information in an image
can in principle reduce the size. However, certain combinations of -c and -bit_depth require color
counting which is currently disabled. In such cases no reduction will take place. Following are the PNG
color types followed by their supported bit depths (Note pngcrush does not support changing a file to
color type 3 from another color type.):
• 0 : grayscale without alpha channel (1,2,4,8,16)
• 2 : true color without alpha channel (8,16)
• 3 : indexed color (1,2,4,8)
• 4 : grayscale with alpha channel (8,16)
• 6 : true color with alpha channel (8,16)
An alpha channel represents transparency on a per pixel basis. An alpha value of zero is completely
transparent. An alpha channel of 2 bitdepth-1 is completely opaque.
Filter Types
The IDAT chunk can optionally be filtered before compression. These filters can make the IDAT chunk more
compressible without losing any data and result in a smaller PNG file. These filters are applied to the
bytes of the IDAT chunk, not the pixels. Following is a brief description of the filters, see the PNG
specification for details:
• 0 : no filter
• 1 : 'sub' transmits the difference between each byte and the value of the corresponding byte of the
prior pixel.
• 2 : 'up' transmits the difference between each byte and the value of the corresponding byte of the
pixel above this pixel
• 3 : 'average' transmits the difference between each byte and the average of the bytes described in
filters 1 and 2
• 4 : 'paeth' computes a simple linear function of the corresponding byte in three neighboring pixels
(paeth_predictor = left + above - upper left), then transmits the difference between the byte in
question and the neighboring byte closest to the value of paeth_predictor.
Text Chunks
Textual information pertaining to an image can be conveyed with the tEXt, iTXt and zTXt chunks. All text
chunks consist of a keyword followed by a string. The following keywords are defined in the PNG
specification: (you may invent keywords for other purposes):
Title: Short title or caption for image
Author: Name of image's creator
Description: Longer description of image
Copyright: Copyright notice
Creation Time: Time of original image creation
Software: Software used to create the image
Disclaimer: Legal disclaimer
Warning: Warning of nature of content
Source: Device used to create the image
Comment: Miscellaneous comment.
A tEXt chunk stores text in the ISO/IEC 8859-1 (Latin-1) character set. zTXt chunks also use the Latin-1
character set, but the text is compressed. This can be useful for large text chunks. iTXt chunks
consist of text in the UTF-8 of the Unicode character set.
keyword must be at least 1 character and less than 80 characters. text must be less than 2048
characters when using pngcrush For now, you can only add ten tEXt, iTXt, or zTXt chunks per pngcrush
run.
SEE ALSO
png(5), libpng(3), zlib(3).
AUTHOR
This manual page was written by David Whedon dwhedon@gordian.com for the Debian GNU/Linux system (but may
be used by others). Much of the information was gleaned from "PNG (Portable Network Graphics)
Specification, Version 1.2"
pngcrush(1)