# PNG
Canonical URL: https://encode.wiki/codecs/images/png/
Content version: ca42e61ede0a24f789d7287a0001244ad22f4abf
Status: current
License: CC BY-SA 4.0
Checksum (SHA-256): 3a18275fa33a9592d585929b639f045fcf7a668c379f713240e355d5cdfe4ee5
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](/codecs/images/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](/codecs/images/webp/) or lossless [JPEG XL](/codecs/images/jxl/) too. Use [JPEG](/codecs/images/jpeg/), [AVIF](/codecs/images/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](https://github.com/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`.
```sh
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](https://github.com/fhanau/Efficient-Compression-Tool) (`ect`) optimises PNG and JPEG (and gzip/zip) in place. Level `-9` is very slow; `-3` is the default.
```sh
ect -9 -strip image.png
```
`ect` also recompresses JPEGs losslessly (`-progressive` makes them progressive). [ZopfliPNG](https://github.com/google/zopfli) and [OptiPNG](https://optipng.sourceforge.net/) 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](https://pngquant.org/) 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.
```sh
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](/reference/encoder-parameters/) 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](https://optipng.sourceforge.net/optipng-7.9.1.man1.html).
## 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.