PNG

PNG (1996) is the lossless format for screenshots, UI, diagrams, pixel art and anything that needs real transparency. It was created as a patent-free replacement for GIF, and the third edition of the spec (a W3C Recommendation since June 2025) adds animation (APNG), HDR colour signalling (cICP, mDCV, cLLI) and Exif.

Property	Value
Compression	Lossless (per-row filters plus DEFLATE)
Bit depth	1, 2, 4, 8 or 16 bits per sample (palette up to 8 bits)
Colour types	Grayscale, RGB, indexed palette, with or without alpha
Alpha	Full alpha channel, or per-entry alpha in palette images
Animation	APNG, supported by all current browsers
Progressive	Adam7 interlacing (usually makes files larger)
Royalty-free	Yes

When to use it

• Use PNG for screenshots, graphics with flat colour or sharp edges, masters you will edit again, and anything that must come out pixel-exact.
• Not for photos. A photo as PNG is several times larger than a good JPEG, and usually larger than lossless WebP or lossless JPEG XL too. Use JPEG, AVIF or WebP for delivery.
• For smaller PNGs, you have two routes: lossless recompression (oxipng, ECT) which never changes pixels, or palette quantisation (pngquant) which does and often saves 60–80%.

Shrink PNGs without changing pixels

oxipng

oxipng is the usual choice: multithreaded, fast, and able to do limited optimisation of APNG. It rewrites files in place unless you give --out or --dir.

oxipng -o 4 --strip safe image.png --out image.opt.png
oxipng -o max --strip safe *.png

Option	What it does
-o 0…-o 6, -o max	Effort. Default is 2, which is fast and already good; higher levels have diminishing returns. max is level 6.
--strip safe (-s)	Removes non-critical chunks but keeps cICP, iCCP, sRGB, pHYs and the APNG chunks. Use --strip all only if you are sure no colour or animation data matters.
--alpha (-a)	Rewrites the colour of fully transparent pixels to compress better. Looks identical, but the hidden RGB values change.
-z	Uses Zopfli instead of libdeflate. Slower, small extra gain.
--scale16	Converts 16-bit to 8-bit. Lossy; oxipng otherwise keeps 16-bit data intact.

Without --strip oxipng keeps every chunk that is still valid, including metadata.

ECT

Efficient Compression Tool (ect) optimises PNG and JPEG (and gzip/zip) in place. Level -9 is very slow; -3 is the default.

ect -9 -strip image.png

ect also recompresses JPEGs losslessly (-progressive makes them progressive). ZopfliPNG and OptiPNG do the same job; OptiPNG is the older tool, ZopfliPNG strips all ancillary chunks (including colour profiles) unless told to keep them.

Shrink PNGs by reducing colours (lossy)

pngquant converts RGB/RGBA to an 8-bit palette with per-colour alpha, using dithering to hide the loss. It suits icons, illustrations and screenshots; smooth gradients and photos can band or posterise.

pngquant --quality 65-90 --skip-if-larger --strip --output small.png input.png

• --quality min-max picks the fewest colours that reach max; if it cannot reach min it writes nothing and exits with status 99 (98 for --skip-if-larger).
• --speed runs 1 (slowest, best) to 11. The default is 4 and the author recommends leaving it unless you need real-time speed.
• --nofs turns off dithering (cleaner flat areas, more banding in gradients). --floyd=0.5 reduces it.
• --ext .png --force overwrites input files in place. There is no backup.

Run oxipng on the result for a little more.

Pitfalls

• Metadata and colour: PNG colour can come from sRGB, iCCP, cICP, or legacy gAMA/cHRM. oxipng’s --strip safe drops gAMA and cHRM, and --strip all drops everything non-critical, so check images that depended on them. Missing profiles change how colour-managed viewers show the image.
• Interlacing: Adam7 costs compression and is rarely worth it. oxipng removes it by default; use -i keep or -i on if you need it.
• Hidden RGB in transparent pixels: --alpha and ZopfliPNG’s --lossy_transparent change it. Harmless for display, wrong if the same file feeds a later edit or a texture pipeline.
• 16-bit images: many tools silently convert to 8-bit. Keep an eye on --scale16, ImageMagick’s -depth 8 and any “web export” preset.
• APNG: only some tools keep every frame. oxipng --strip all and optipng -snip flatten animations. For animation that is not a looping sticker, an actual video or animated WebP/AVIF is far smaller.
• Large flat-colour screenshots may compress better as lossless WebP or JPEG XL if the destination supports them.

Full option reference

Every option and default for oxipng 10.2.1, OptiPNG 7.9.1, pngquant 3.0.3 and ZopfliPNG. Reading option tables explains the Default column.

oxipng parameter reference

Use -- before an input filename that begins with a hyphen. The hidden -Z is still accepted as an alias of --zopfli.

Complete oxipng 10.2.1 options and defaults

Parameter and aliases	Type / accepted values	Default	Meaning and limits
--opt, -o	enum 0–6/max	2	Preset from 0 (fastest) to 6 or max (smallest); explicit coding options override it.
--recursive, -r	switch	off	Optimizes every .png and .apng file found in input directories.
--dir	directory	not specified	Writes outputs into this directory, creating it; recursion flattens the folder layout.
--out	path	not specified (input replaced if optimized)	Writes the result to this file instead of replacing the input.
--stdout	switch	off	Writes the PNG to stdout; cannot combine with —dir, —out or —json.
--preserve, -p	switch	off	Keeps the original permissions and timestamps where possible.
--dry-run, -d	switch	off	Reports the savings without writing any file.
-s	switch	off	Same as —strip safe: removes non-essential chunks and keeps those that affect display.
--strip	mode/list	keep valid chunks, except caBX/iDOT	safe, all or a comma-separated chunk list; all also turns APNG into a plain PNG.
--keep	comma-separated chunk names	not specified	Strips everything except the listed chunks; display keeps the appearance-affecting set.
--alpha, -a	switch	off	Rewrites colour values of fully transparent pixels to compress better; visually lossless.
--interlace, -i	enum	off; keep with —nx	off, on (Adam7) or keep; interlacing adds 25–50% to the size.
--scale16	switch	off	Reduces 16-bit channels to 8-bit by scaling; lossy.
--verbose, -v	repeatable switch	0 occurrences	Shows per-file detail; repeat for more. Cannot combine with —quiet.
--quiet, -q	switch	off	Suppresses all output.
--json, -j	switch	off	Prints results as JSON; cannot combine with —stdout.
--filters, -f	integer list/range 0–9	0,1,6,7 at level 2	Row filters to try: 0–4 fixed, 5 MinSum, 6 Entropy, 7 Bigrams, 8 BigEnt, 9 Brute.
--fast	switch	on at levels 0–4; off at 5–6	Scores filters with a quick pass, then runs one full trial on the best.
--zc	integer 0–12	11 at level 2	libdeflate level 0–12 for main trials; cannot combine with —zopfli.
--nb	switch	off	Keeps the original bit depth.
--nc	switch	off	Keeps the original colour type.
--np	switch	off	Does not convert to indexed colour or change an existing palette.
--ng	switch	off	Does not convert to or from greyscale.
--nx	switch	off	Disables every reduction and makes the default interlace mode keep.
--nz	switch	off	Skips IDAT recompression unless a transformation needs it; with —nx, disables all optimization.
--fix	switch	off	Skips chunk checksum validation so damaged files can be processed; output checksums are valid.
--force	switch	off	Writes the output even when it is larger than the input.
--zopfli, -z, -Z	switch	off	Uses Zopfli instead of libdeflate for main trials; much slower, a little smaller.
--zi	positive u64	15	Zopfli iteration count; fewer is faster. Requires —zopfli.
--ziwi	positive u64	not specified	Stops Zopfli after this many iterations without improvement; pair with a high —zi.
--brute-level	integer 1–12	1; 5 at level 6	libdeflate level for the Brute filter; 1–5 is sensible.
--brute-lines	nonnegative integer	3; 4 at level 4; 8 at levels 5–6	Rows compressed together by the Brute filter; 2–16 is sensible.
--timeout	u64 seconds	no limit	Time budget in seconds, checked between trials; a running trial can overrun it.
--max-raw-size	size with unit suffix	no limit	Skips images whose decompressed data exceeds this size; accepts suffixes like k or MB.
--threads, -t	nonnegative integer	logical CPU count	Maximum worker threads; 0 lets Rayon choose.
--sequential	switch	off (parallel file processing)	Processes files one at a time, in order.
--help, -h	action	not requested	Shows help.
--version, -V	action	not requested	Prints the version.
--	action	not requested	Ends option parsing so later arguments are input paths.

Presets: -o 0 uses filters 0,5 with libdeflate level 5; -o 1 uses 0,7 and level 10; -o 2 (default) uses 0,1,6,7 and level 11 with fast evaluation; -o 3 adds filters 8,9; -o 4 uses level 12; -o 5 and -o 6 disable fast evaluation, -o 6 uses every filter. Explicit --filters, --zc, --brute-* and --fast options override the preset regardless of order.

--strip safe / --keep display keep cICP, iCCP, sRGB, pHYs, acTL, fcTL and fdAT, and drop other ancillary chunks such as gAMA, cHRM, sBIT, mDCV and cLLI. Without --strip, oxipng keeps every chunk that is still valid, but drops caBX and Apple iDOT.

OptiPNG parameter reference

OptiPNG takes the same kind of optimisation switches but uses single-hyphen options, and documented prefixes are accepted as abbreviations. Prefer the full lowercase spelling. Attached forms such as -o2, -f0,5 and -zc9 are the same options.

Complete OptiPNG 7.9.1 options, aliases, and defaults

Parameter and aliases	Type / accepted values	Default	Meaning and limits
-o	nonnegative integer	2	Preset 0–7, higher is slower and smaller; larger values act as 7.
-out, -ou	path	not specified (input can be replaced)	Output file for a single input; cannot combine with -dir.
-dir, -d, -di	directory	not specified	Output directory; cannot combine with -out.
-backup, -b, -ba, -bac, -back, -backu, -keep, -k, -ke, -kee	switch	off	Keeps a .bak copy when overwriting the input.
-clobber, -c, -cl, -clo, -clob, -clobb, -clobbe	switch	off	Allows replacing an existing output file.
-fix, -fi	switch	off	Tries to recover malformed input; check the result.
-force, -fo, -for, -forc	switch	off	Writes an output file even when no improvement is found.
-preserve, -p, -pr, -pre, -pres, -prese, -preser, -preserv	switch	off	Keeps file attributes such as permissions and timestamps where possible.
-quiet, -q, -qu, -qui, -quie, -silent, -sil, -sile, -silen	switch	off	Suppresses normal output.
-simulate, -sim, -simu, -simul, -simula, -simulat	switch	off	Runs the optimization without writing files.
-f	set/ranges 0–5	0,5 at level 2	Row filters to try: 0–4 fixed, 5 adaptive. Accepts lists and ranges such as 0,5.
-i	integer 0–1	retain input interlace	Output interlacing: 0 none, 1 Adam7.
-zc	set/ranges 1–9	9 at level 2	zlib compression levels to try, 1–9.
-zm	set/ranges 1–9	8 at level 2	zlib memory levels to try, 1–9.
-zs	set/ranges 0–3	0–3 at level 2	zlib strategies to try: 0 default, 1 filtered, 2 Huffman-only, 3 RLE.
-zw	power-of-two size	libpng default 32 KiB; smaller for Huffman/RLE	zlib window size, 256 to 32768 bytes; accepts suffixes like 4k.
-full, -fu, -ful	switch	off	Produces a full IDAT report, which can slow the search.
-nb	switch	off	Disables bit-depth reduction.
-nc	switch	off	Disables colour-type reduction.
-np	switch	off	Disables palette reduction.
-nx	switch	off	Disables all three reductions.
-nz	switch	off	Skips IDAT recompression unless a reduction needs it.
-snip, -sn, -sni	switch	off	Extracts one image from an animated or multi-image file and drops the rest.
-strip, -st, -str, -stri	object	not enabled	Only all is accepted; individual chunks cannot be named.
-v	switch	off	Verbose output; with no input files, prints the version and copyright.
-verbose, -verb, -verbo, -verbos	switch	off	Verbose output only.
-version, -vers, -versi, -versio	action	not requested	Prints the version when no input files are given.
-debug	switch	off	Enables internal debug features; cannot be abbreviated.
-log, -lo	removed option	not supported	Removed; the parser rejects it. Redirect output with the shell instead.
-help, -?, -h, -he, -hel	action	not requested	Shows extended help.
--	action	not requested	Ends option parsing so later arguments are input paths.

The default -o2 searches -zc9 -zm8 -zs0-3 -f0,5. Higher levels search more combinations but never add visual loss. Repeated -f and -z* sets are unioned. -strip accepts only all. -snip extracts a single image from an animated or multi-image input and discards the animation. See the versioned manual.

pngquant parameter reference

This is the Rust frontend (3.0.3). The optional positional ncolors is the same as the hidden --colors.

Complete pngquant 3.0.3 Rust options and defaults

Parameter and aliases	Type / accepted values	Default	Meaning and limits
--colors	integer 2–256	256 maximum	Maximum palette size, 2–256; the positional ncolors argument sets the same limit.
--quality, -Q	min-max in 0–100	no minimum; target 100	Quality range min-max, 0–100. Below min the image is refused (exit 99). A single N means 90% of N to N.
--speed, -s	integer 1–11	4	1 (slowest, best) to 11 (fastest); 10 compresses the PNG faster, 11 also disables dithering.
--floyd	optional float	1	Floyd–Steinberg dithering strength 0–1; write —floyd=0.5.
--nofs, --ordered	switch	off	Disables dithering; —ordered is an alias.
--posterize	integer 0–4	0	Drops this many low bits of colour precision, 0–4; lossy but smaller.
--strip	switch	off	Removes optional metadata chunks.
--map	PNG path	not specified	Uses the colours of this PNG as a fixed palette.
--ext	suffix	-fs8.png; -or8.png without dithering	Output filename suffix for batch runs; cannot combine with —output.
--output, -o	path	derived suffix; stdout for stdin	Single output file; - writes to stdout.
--force, -f	switch	off	Overwrites existing output files without a backup.
--no-force	switch	off	Never overwrites existing files, even when —force is also given.
--skip-if-larger	switch	off	Writes nothing when the result is not smaller than the input (exit 98).
--transbug	switch	off	Puts a transparent entry last in the palette for old, buggy viewers.
--iebug	ignored switch	off	Accepted but ignored in 3.0.3.
--verbose, -v	switch	off	Prints status and quantization details.
--quiet, -q	switch	off	Accepted but has no effect; output is already quiet unless —verbose is given.
--help, -h	action	not requested	Shows help.
--version, -V	action	not requested	Prints the version.
--	action	not requested	Ends option parsing so later arguments are the color count and input paths.

--quality 65-85 tries a palette that meets 85 and refuses images that cannot reach 65, with exit code 99; 98 is the --skip-if-larger refusal. When reading from stdin the original may be passed through unchanged, so check what the pipeline actually produced. --iebug is registered but ignored in this version. The old C-only spellings (-nofs, -fs) are not accepted. --nofs and --ordered both mean no error diffusion.

ZopfliPNG parameter reference

Pinned to Google’s source commit df1517dd8518 (30 September 2026); there is no versioned release. Long-option values are attached with =.

Complete pinned Google ZopfliPNG options and defaults

Parameter and aliases	Type / accepted values	Default	Meaning and limits
-m	repeatable switch	off (15/5 iterations)	Multiplies the iteration counts by 4; repeat for more.
-y	switch	off	Overwrites existing output without asking; a smaller earlier output is still kept.
-d	switch	off	Dry run: prints sizes without saving.
-q	switch	off	Fast compression without Zopfli; for quickly trying filters and colour types.
--prefix	optional text attached with =	not enabled; zopfli_ when bare	Batch mode: writes prefixed copies of each input, optionally into a directory. Use —prefix=name.
--always_zopflify	switch	off	Writes the Zopfli output even when larger than the input; for benchmarking.
--verbose	switch	off	Prints a detailed coding report.
--lossy_transparent	switch	off	Discards RGB values behind fully transparent pixels.
--lossy_8bit	switch	off	Converts 16-bit channels to 8-bit; lossy.
--iterations	integer attached with =	15 below 200000 raw bytes; 5 otherwise	Zopfli iterations for any file size; more is slower and slightly smaller. Minimum 1.
--splitting	ignored compatibility argument	ignored	Ignored; kept for backward compatibility.
--filters	characters attached with =	automatic fast selection	Strategies to try: 0–4 fixed, m MinSum, e entropy, p input filters, b brute force. 0me is a good set.
--keepchunks	comma-separated four-letter chunk names	none (IHDR,PLTE,tRNS,IDAT,IEND only)	Extra chunks to keep, as exact four-letter names separated by commas (iCCP,gAMA). bKGD or sBIT also pins the colour type.
--keepcolortype	switch	off	Keeps the input colour type and bit depth, at some cost in compression.
--help, -h	action	not requested	Shows help.

The default iteration split depends on the raw DEFLATE input size (15 below 200,000 bytes, otherwise 5). An existing output file is treated as a previous candidate and kept if smaller. ZopfliPNG returns success on some failure paths, so confirm the output exists and decodes. By default it keeps only IHDR, PLTE, tRNS, IDAT and IEND, which removes colour profiles; list the ones the image needs, such as --keepchunks=iCCP,sRGB,gAMA,cHRM,cICP; names are exact four-letter, case-sensitive and comma-separated with no spaces. It is not an APNG optimiser.