Sdcb.SimdCodecs.Webp 1.2.0

Sdcb.SimdCodecs.Webp

Pure-managed WebP decoding for .NET 10, derived from RFC 6386 and RFC 9649. There is no encoder.

Supported input includes VP8 lossy, VP8L lossless, ALPH transparency, and animated WebP with full-canvas RGBA frames, random seeking, and frame timing. ICC, EXIF, and XMP metadata are preserved. The Sdcb.SimdCodecs facade decodes a still image or frame 0 of an animation with TryDecode, and every frame with TryOpenFrames; this package's animation API adds seeking by timestamp and the frame rectangles. Images larger than 67,108,864 pixels are rejected to bound decoder allocations.

using Sdcb.SimdCodecs.Webp;

byte[] data = File.ReadAllBytes("image.webp");
WebpImage image = WebpDecoder.Decode(data);
Console.WriteLine($"{image.Width} × {image.Height}");

Decode into your own RGBA buffer (stride 0 = tightly packed). A tightly packed destination is also the lossless decoder's working buffer, so a lossless decode needs almost no memory of its own:

WebpInfo info = WebpDecoder.ReadInfo(data);
byte[] rgba = new byte[info.Width * info.Height * 4];
WebpDecoder.DecodeTo(data, rgba);

Decoding is single-threaded by default. Opt in to decoding the transparency plane of a lossy image on a second thread, concurrently with the colour image:

WebpDecoder.DecodeTo(data, rgba, 0, new WebpDecodeOptions { Multithreaded = true });

Decode every frame into a full-canvas RGBA buffer:

WebpAnimation animation = WebpDecoder.DecodeAnimation(data);
foreach (WebpFrame frame in animation.Frames)
{
    Console.WriteLine($"{frame.TimestampMs} ms: {frame.DurationMs} ms");
    byte[] rgba = frame.Rgba;
}

For on-demand playback, seek to the next frame to produce, then decode it:

var decoder = new WebpAnimationDecoder(data);
decoder.Seek(12);
byte[] canvas = new byte[decoder.CanvasWidth * decoder.CanvasHeight * 4];
if (decoder.TryDecodeNextFrame(canvas, out WebpFrameInfo frameInfo))
{
    Console.WriteLine($"Displayed frame {frameInfo.Index}");
}

The decoder is spec-derived from RFC 6386 / RFC 9649; libwebp/libvpx source was not consulted — their binaries are used only as test oracles.

Known oracle divergence: anim-vp8l-alpha-hint.webp has a VP8L animation frame with transparent decoded pixels despite a cleared alpha_is_used hint. The RFC 9649 blend flag requires alpha blending, which this decoder follows; anim_dump 1.6.0 emits that frame uncomposited. It is excluded from the anim_dump comparison but remains covered by compositing and seek-versus-sequential tests.