NAME

oxipng - Losslessly improve compression of PNG files

SYNOPSIS

oxipng [-o|--opt] [-r|--recursive] [--dir] [--out] [--stdout] [-p|--preserve] [-P|--pretend] [-s ] [--strip] [--keep] [-a|--alpha] [-i|--interlace] [--scale16] [-v|--verbose]... [-q|--quiet] [-f|--filters] [--fast] [--zc] [--nb] [--nc] [--np] [--ng] [--nx] [--nz] [--fix] [--force] [-Z|--zopfli] [--zi] [--timeout] [-t|--threads] [--sequential] [-h|--help] [-V|--version] <files>

DESCRIPTION

Losslessly improve compression of PNG files

OPTIONS

-o, --opt=level [default: 2]

Set the optimization level preset. The default level 2 is quite fast and provides good compression. Lower levels are faster, higher levels provide better compression, though with increasingly diminishing returns.

0 => --zc 5 --fast (1 trial, determined heuristically)
1 => --zc 10 --fast (1 trial, determined heuristically)
2 => --zc 11 -f 0,1,6,7 --fast (4 fast trials, 1 main trial)
3 => --zc 11 -f 0,7,8,9 (4 trials)
4 => --zc 12 -f 0,7,8,9 (4 trials)
5 => --zc 12 -f 0,1,2,5,6,7,8,9 (8 trials)
6 => --zc 12 -f 0-9 (10 trials)
max => (stable alias for the max level)

Manually specifying a compression option (zc, f, etc.) will override the optimization preset, regardless of the order you write the arguments.

-r, --recursive

When directories are given as input, traverse the directory trees and optimize all PNG files found (files with “.png” or “.apng” extension).

--dir=directory

Write output file(s) to <directory>. If the directory does not exist, it will be created. Note that this will not preserve the directory structure of the input files when used with '--recursive'.

--out=file

Write output file to <file>

--stdout

Write output to stdout

-p, --preserve

Preserve file permissions and timestamps if possible

-P, --pretend

Do not write any files, only show compression results

-s

Strip safely-removable chunks, same as '--strip safe'

--strip=mode

Strip metadata chunks, where <mode> is one of:

safe => Strip all non-critical chunks, except for the following:
cICP, iCCP, sRGB, pHYs, acTL, fcTL, fdAT
all => Strip all non-critical chunks
<list> => Strip chunks in the comma-separated list, e.g. 'bKGD,cHRM'

CAUTION: 'all' will convert APNGs to standard PNGs.

Note that 'bKGD', 'sBIT' and 'hIST' will be forcibly stripped if the color type or bit depth is changed, regardless of any options set.

The default when --strip is not passed is to keep all metadata.

--keep=list

Strip all metadata chunks except those in the comma-separated list. The special value 'display' includes chunks that affect the image appearance, equivalent to '--strip safe'.

E.g. '--keep eXIf,display' will strip chunks, keeping only eXIf and those that affect the image appearance.

-a, --alpha

Perform additional optimization on images with an alpha channel, by altering the color values of fully transparent pixels. This is generally recommended for better compression, but take care as while this is “visually lossless”, it is technically a lossy transformation and may be unsuitable for some applications.

-i, --interlace=type

Set the PNG interlacing type, where <type> is one of:

0 => Remove interlacing from all images that are processed
1 => Apply Adam7 interlacing on all images that are processed
keep => Keep the existing interlacing type of each image

Note that interlacing can add 25-50% to the size of an optimized image. Only use it if you believe the benefits outweigh the costs for your use case.

[default: 0]

--scale16

Forcibly reduce images with 16 bits per channel to 8 bits per channel. This is a lossy operation but can provide significant savings when you have no need for higher depth. Reduction is performed by scaling the values such that, e.g. 0x00FF is reduced to 0x01 rather than 0x00.

Without this flag, 16-bit images will only be reduced in depth if it can be done losslessly.

-v, --verbose

Run in verbose mode (use twice to increase verbosity)

-q, --quiet

Run in quiet mode

-f, --filters=list

Perform compression trials with each of the given filter types. You can specify a comma-separated list, or a range of values. E.g. '-f 0-3' is the same as '-f 0,1,2,3'.

PNG delta filters (apply the same filter to every line)
0 => None (recommended to always include this filter)
1 => Sub
2 => Up
3 => Average
4 => Paeth

Heuristic strategies (try to find the best delta filter for each line)
5 => MinSum Minimum sum of absolute differences
6 => Entropy Highest Shannon entropy
7 => Bigrams Lowest count of distinct bigrams
8 => BigEnt Highest Shannon entropy of bigrams
9 => Brute Smallest compressed size (slow)

The default value depends on the optimization level preset.

--fast

Perform a fast compression evaluation of each enabled filter, followed by a single main compression trial of the best result. Recommended if you have more filters enabled than CPU cores.

--zc=level

Deflate compression level (0-12) for main compression trials. The levels here are defined by the libdeflate compression library.

The default value depends on the optimization level preset.

--nb

Do not change bit depth

--nc

Do not change color type

--np

Do not change color palette

--ng

Do not change to or from grayscale

--nx

Do not perform any transformations and do not deinterlace by default.

--nz

Do not recompress IDAT unless required due to transformations. Recompression of other compressed chunks (such as iCCP) will also be disabled. Note that the combination of '--nx' and '--nz' will fully disable all optimization.

--fix

Do not perform checksum validation of PNG chunks. This may allow some files with errors to be processed successfully.

--force

Write the output even if it is larger than the input

-Z, --zopfli

Use the much slower but stronger Zopfli compressor for main compression trials. Recommended use is with '-o max' and '--fast'.

--zi=iterations [default: 15]

Set the number of iterations to use for Zopfli compression. Using fewer iterations may speed up compression for large files. This option requires '--zopfli' to be set.

--timeout=secs

Maximum amount of time, in seconds, to spend on optimizations. Oxipng will check the timeout before each transformation or compression trial, and will stop trying to optimize the file if the timeout is exceeded. Note that this does not cut short any operations that are already in progress, so it is currently of limited effectiveness for large files with high compression levels.

-t, --threads=num

Set the maximum number of threads to use. Oxipng uses multithreading to evaluate multiple optimizations on the same file in parallel as well as process multiple files in parallel. You can set this to a lower value if you need to limit memory or CPU usage.

[default: num logical CPUs]

--sequential

Process multiple files sequentially rather than in parallel. Use this if you need determinism in the processing order. Note this is not necessary if using '--threads 1'.

-h, --help

Print help (see a summary with '-h')

-V, --version

Print version

<files>

File(s) to compress (use '-' for stdin)

VERSION

v0.1.0

AUTHORS

Joshua Holmer <jholmer.in@gmail.com>