tyler
4da9a48d7c
libwebp 1.6.0's decode + demux path (lossy VP8, lossless VP8L, VP8X container, ALPH alpha, and the WebPDemux/WebPAnimDecoder animation API) amalgamated into one stb-style header, built as portable C99 with all SIMD, threads and host file I/O disabled. amalgamate.py regenerates webpdec.h from a pinned, auto-cloned libwebp checkout; tests/run.sh checks decode output bit-exactly against reference libwebp. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
115 lines
3.8 KiB
Markdown
115 lines
3.8 KiB
Markdown
# webpdec.h — single-file WebP decoder
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A standalone, dependency-free **WebP decoder** in one C header, in the spirit of
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the [stb](https://github.com/nothings/stb) libraries. It is libwebp's decode +
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demux path — *not* a reimplementation — amalgamated into a single file and built
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as portable C99 with **all SIMD, threads and host file I/O disabled**. So it is
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as correct as upstream libwebp (it *is* libwebp) while staying trivial to drop
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into any project.
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Handles everything real WebP files use:
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- **Lossy** (VP8) and **lossless** (VP8L)
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- The **VP8X** extended container
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- The **ALPH** alpha chunk (transparency on lossy images)
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- **Animation** + metadata via the demux API (`WebPDemux`, `WebPAnimDecoder`)
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## Usage
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In exactly **one** translation unit:
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```c
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#define WEBPDEC_IMPLEMENTATION
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#include "webpdec.h"
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```
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Everywhere else, just `#include "webpdec.h"`. The public API is the standard
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libwebp decode + demux API.
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Still image:
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```c
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int w, h;
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uint8_t *rgba = WebPDecodeRGBA(data, len, &w, &h); /* R,G,B,A, top-to-bottom */
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if (rgba) {
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/* ... w*h*4 bytes ... */
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WebPFree(rgba);
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}
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```
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Animation (all frames, with blending/disposal handled for you):
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```c
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WebPData wd = { data, len };
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WebPAnimDecoderOptions opt; WebPAnimDecoderOptionsInit(&opt);
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opt.color_mode = MODE_RGBA;
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WebPAnimDecoder *dec = WebPAnimDecoderNew(&wd, &opt);
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WebPAnimInfo info; WebPAnimDecoderGetInfo(dec, &info); /* canvas, frame_count */
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while (WebPAnimDecoderHasMoreFrames(dec)) {
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uint8_t *rgba; int timestamp_ms;
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WebPAnimDecoderGetNext(dec, &rgba, ×tamp_ms); /* rgba owned by dec */
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}
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WebPAnimDecoderDelete(dec);
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```
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`WebPGetInfo`, `WebPDecodeRGB/BGRA/ARGB`, the advanced `WebPDecode` +
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`WebPDecoderConfig` path (including decode-time downscaling via
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`config.options.use_scaling`), and the full `WebPDemux*` chunk/metadata
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iterator are all available — see the declarations near the top of `webpdec.h`.
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Link with `-lm` (libwebp uses `pow()` for gamma tables). That's the only
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dependency.
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## What's not included
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Encoding and the mux (assembly/write) API — this is decode-only, by design.
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## Portability
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`webpdec.h` defines `HAVE_CONFIG_H` and bakes in an empty `config.h`, which is
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libwebp's own switch for "no detected SIMD": every architecture dispatcher falls
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back to the reference C path. This works even on SIMD-capable targets — e.g. on
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ARMv8 where the toolchain defines `__ARM_NEON`, NEON still stays off. No
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pthreads, no `<stdio.h>` file calls. Tested decoding bit-exactly against
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upstream libwebp on x86-64 and aarch64.
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## Repo layout
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```
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webpdec.h the single-file library (committed; this is all you need)
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amalgamate.py regenerates webpdec.h from libwebp
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UPSTREAM_COMMIT the exact pinned libwebp revision
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LICENSE, PATENTS upstream BSD-style license + patent grant
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tests/ test_decode.c, test_demux.c, run.sh
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libwebp/ upstream checkout (git-ignored; fetched on demand)
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```
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## Regenerating
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```sh
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python3 amalgamate.py # writes webpdec.h (+ refreshes UPSTREAM_COMMIT)
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```
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With no `libwebp/` present it shallow-clones libwebp at the pinned commit, so a
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fresh checkout just works — no submodules, no manual steps. The amalgamation is
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`LIBWEBPDECODER_OBJS` + `DEMUX_OBJS` minus every `*_sse2/_neon/_msa/...` SIMD
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translation unit; the few single-TU symbol clashes are namespaced by the script
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(the upstream tree is left pristine, so regeneration is byte-for-byte
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reproducible).
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## Tests
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```sh
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sh tests/run.sh
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```
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With [Pillow](https://python-pillow.org/) installed it generates lossy /
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lossless / alpha / animated samples and checks decode output **bit-exactly**
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against reference libwebp; otherwise pass it `.webp` paths to decode.
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## License
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BSD-style, identical to libwebp. See `LICENSE` and `PATENTS`. Copyright Google
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LLC and the WebM project authors; this file only mechanically concatenates their
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sources.
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