oxipng 9.1.0 Losslessly improve compression of PNG files

Usage: oxipng [OPTIONS] <files>...

Arguments:
<files>...
File(s) to compress (use '-' for stdin)

Options:
-o, --opt <level>
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.

[default: 2]

-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 (1-12) 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'.

--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 number of threads to use [default: num CPU cores]

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

-V, --version
Print version