pngcrush - Man Page

• Compress a PNG file: pngcrush in.png out.png
• Compress all PNGs and output them to the specified directory: pngcrush -d path/to/output *.png
• Compress PNG file with all 114 available algorithms and pick the best result: pngcrush -rem allb -brute -reduce in.png out.png

tldr.sh

pngcrush [ options ]  [ infile.png ]  [ outfile.png ]

pngcrush -e ext [ options ]  [ file1.png ]  [ file2.png ] ...

pngcrush -d dir [ options ]  [ file1.png ]  [ file2.png ] ...

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 pngcrush(1)) 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.

-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 pngcrush(1)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 (2bitdepth)-1. For output color type 0 and 4 (gray, see section pngcrush(1)) 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 pngcrush(1)). 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 pngcrush(1)) 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 pngcrush(1)).

-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 pngcrush(1)).

-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 pngcrush(1)).

-ztxt [b|a] keyword text

zTXt chunk to insert (see -text).

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.

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.

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.

png(5), libpng(3), zlib(3).

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"

optipng(1).