WebP VP8L Encoder Implementation Notes

Current Status

The WebP VP8L (lossless) encoder now implements:

• ✅ Simple Huffman coding (1-2 unique symbols per channel)
• ✅ Literal pixel encoding (uncompressed)
• ✅ Complex Huffman codes (>2 symbols) - FULLY WORKING
• ❌ LZ77 backward references - not yet implemented
• ❌ Color cache - not yet implemented

This produces valid, lossless WebP files that decode correctly. Complex Huffman coding provides better compression than simple codes, though not as good as with LZ77 and color cache.

Complex Huffman Codes - Implementation Details

What Works

• ✅ Standard Huffman tree construction from symbol frequencies
• ✅ Canonical Huffman code generation
• ✅ RLE encoding of code lengths (using codes 16, 17, 18 for runs)
• ✅ Code length Huffman table generation and encoding
• ✅ Single channel with many unique values (tested up to 50 colors)
• ✅ Multiple channels with many unique values (tested 50x50 gradients)

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WebP VP8L Implementation Notes

• Status: complex Huffman coding implemented; LZ77 backward references and color cache not implemented. Current encoder produces valid VP8L streams for Huffman-only encodings; full compression requires LZ77 & color cache.
• Decoder/Encoder files: src/formats/webp.ts, src/utils/webp_encoder.ts, src/utils/webp_decoder.ts.
• Tests: test/webp.test.ts, test/purejs_roundtrip.test.ts.
• Notes: implemented: canonical Huffman generation, code-length RLE, and writing of all required Huffman groups (green, red, blue, alpha, distance). Remaining work: implement LZ77 match finding & encoding, and color cache to reach full lossless compression parity with libwebp.