Rgb24 JPEG decode/encode in LibJpegWrap and JpegCodec (for v1.5.0)

README.md

@@ -221,10 +221,32 @@ codec.DctMethod = DctMethod.Float;  // Faster encoding
 var header = new FrameHeader(1920, 1080, 1920 * 3, PixelFormat.Rgb24, 1920 * 1080 * 3);
 var frame = new FrameImage(header, rawPixelData);
 
-byte[] jpegBuffer = new byte[header.Length];
-int jpegLength = codec.Encode(frame, jpegBuffer);
+using var jpeg = codec.EncodeRgb24(frame, quality: 90);  // Encode() takes I420 or Gray8 only
 ```
 
+### Rgb24 Decode/Encode
+
+`Decode`/`Encode` work on Gray8 and I420. For packed Rgb24 (R, G, B) use the four `IJpegCodec` members below
+(added in 1.5.0). **Breaking change for custom `IJpegCodec` implementations:** they must add these four members.
+
+| Method | Result |
+|--------|--------|
+| `FrameHeader DecodeRgb24(ReadOnlyMemory<byte> jpeg, Memory<byte> output)` | Full-resolution Rgb24, stride = width * 3; output must hold width * height * 3 bytes |
+| `FrameImage DecodeRgb24(ReadOnlyMemory<byte> jpeg)` | Same, buffer rented from `MemoryPool<byte>.Shared` (dispose it) |
+| `int EncodeRgb24(in FrameImage frame, int quality, Memory<byte> output, JpegSubsampling subsampling = Ratio420)` | Baseline JPEG; returns bytes written |
+| `FrameImage EncodeRgb24(in FrameImage frame, int quality, JpegSubsampling subsampling = Ratio420)` | Same, buffer rented from the shared pool (worst-case `tjBufSize` bound) |
+
+- Decoding uses libjpeg-turbo's default (fancy) chroma upsampling; output is bit-identical to `djpeg -ppm`.
+- Encoding uses `jpeg_set_defaults` + `jpeg_set_quality(quality, force_baseline)` and the integer DCT. The default
+  subsampling is libjpeg's default, 4:2:0 (`JpegSubsampling.Ratio420`); pass `JpegSubsampling.Ratio444` to keep
+  chroma at full resolution (sharp coloured edges, larger files). `FrameHeader.Stride` may include row padding.
+- The Rgb24 calls are stateless native calls: they ignore the codec's `Quality`/`DctMethod` and are safe to call
+  concurrently on one `JpegCodec`.
+- Errors: `InvalidOperationException` for undecodable data or a too-small output buffer; `ArgumentException` when the
+  frame data is shorter than `Stride * (Height - 1) + Width * 3`; `ArgumentOutOfRangeException` for quality outside 1-100.
+- Platforms: the native Rgb24 entry points ship in the **linux-x64** `LibJpegWrap.so`. On linux-arm64 and win-x64 the
+  bundled binaries predate them, and the Rgb24 methods throw `NotSupportedException`.
+
 ---
 
 ## HDR Blending API
@@ -716,6 +738,15 @@ cmake --build build --config Release
 ### Dependencies
 
 No external NuGet dependencies for JPEG codec - native binaries are bundled.
+On Linux the bundled `LibJpegWrap.so` links `libjpeg.so.8` dynamically: install `libjpeg-turbo8` (Ubuntu/Debian).
+
+The linux-x64 binary is built in an `ubuntu:22.04` container (glibc 2.35 baseline) against `libjpeg-turbo8-dev`:
+
+```bash
+docker run --rm -v $PWD/src/LibJpegWrap:/src:ro -v $PWD/out:/out ubuntu:22.04 bash -c \
+  'apt-get update && apt-get install -y g++ cmake libjpeg-turbo8-dev && cmake -S /src -B /b -DCMAKE_BUILD_TYPE=Release \
+   && cmake --build /b && strip --strip-unneeded /b/libLibJpegWrap.so && cp /b/libLibJpegWrap.so /out/LibJpegWrap.so'
+```
 
 | Component | Purpose | Platforms |
 |-----------|---------|-----------|