webpdec.h: single-file WebP decoder

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>

README.md

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