TDD: pure C# PNG/JPEG/BMP and full-frame animated GIF/APNG codecs with coverage gates

[mikasoukhov]

Pure-managed PNG/JPEG/BMP/GIF/APNG with end-to-end animated frame preservation

No GDI+, SkiaSharp, ImageSharp, P/Invoke or OS-native image codecs. Cross-platform .NET 6 / .NET 10 (Windows, Linux, macOS).

Public functionality

• GetImageSize: PNG/APNG, JPEG, BMP and GIF dimensions.
• ResizeImage: preserve source container and aspect ratio. PNG → PNG, JPEG → JPEG, BMP → BMP (portable top-down BGRA32 V4), GIF → animated GIF and APNG → animated APNG. Every frame is resized, retaining delays and looping; no upscaling.
• ConvertToPng: all still formats → RGBA8 PNG; GIF → animated PNG (APNG) with every displayed frame and its delay/loop count. APNG → validated unchanged APNG.
• AddTextWatermark: render on every GIF/APNG frame, returning GIF/APNG respectively; BMP → BMP; PNG/JPEG → PNG. Original image data never mutated; managed TrueType/Verdana fallback.
• GetGifFrames, GetGifLoopCount, GetAnimationFrames, GetAnimationLoopCount: access full-canvas RGBA8 frames and timing, including APNG.

Image formats implemented

• PNG: all 15 legal color type/depth pairs, 5 filters, alpha, palette, Adam7; APNG decoder supports frame rectangles, source/over blending and three disposal modes.
• JPEG: baseline + progressive 8-bit, grayscale, YCbCr, Adobe CMYK/YCCK, restart markers; managed baseline JPEG output.
• BMP: palette 1/4/8-bit; 16/24/32-bit; RGB555/RGB565/custom bitfields; top-down/bottom-up; Windows/OS2 headers; RLE4/RLE8; encoder retains alpha using BGRA32.
• GIF87a/GIF89a: every frame, global/local palettes, LZW, interlace, GCE transparency, disposal methods 0–3, per-frame timings, Netscape loop extension; managed GIF encoder outputs full-canvas independently composited frames.

Test-first evidence

• Previous original image utility RED→GREEN: RED → GREEN.
• BMP/GIF regression tests were committed before implementation in BmpGifTddTests.cs; BMP/GIF code then added in BmpCodec.cs, GifCodec.cs, ApngEncoder.cs and ApngDecoder.cs.
• New tests build on independent Pillow-created BMP and four-frame GIF fixtures (including disposal 2/3, differing delays and NETSCAPE looping). Additional programmatically generated BMP RLE4/RLE8/indexed/masks/OS2 and GIF interlace/local palette/LZW fixtures. APNG tests independently decompress each frame's IDAT/fdAT and verify pixel colors, ordered frame controls, alpha blend/disposal, and corrupt chunk rejection.
• 101 Drawing tests including 91 dedicated image tests; PNG/JPEG original fixtures and TrueType fuzz tests remain. Tests check corrupted/truncated files and exact RGBA pixel buffers.
• Drawing coverage collects Cobertura with Coverlet and requires >=90% lines and >=85% branches; most recent expanded measurements: 91.8% line / 86.1% branch (green run 38050821446). Both BMP and GIF codecs are included.
• Full test suite across Windows/Linux/macOS on .NET 6 and .NET 10, plus DB/Azurite integrations, must pass before merge.

Known format limitations

GIF has palette/binary-transparency limitations by specification: output preserves exact RGB for up to 255 opaque distinct colors/frame; otherwise uses palette quantization. Alpha under 128 becomes transparent. GIF encoder favors deterministic validity over maximum compression.

Unsupported specialized BMP embedded JPEG/PNG, GIF plain-text overlay rendering, arithmetic/lossless/12-bit JPEG and unrelated WebP/TIFF/HEIC/AVIF/SVG formats. No bundled licensed fonts. Native code packages are not introduced. Input/frame limits guard memory usage.

PR is ready for review; do not merge to master until review.