PNG decoder (RFC 2083) #67

Implement a pure Rust PNG decoder in the image crate per RFC 2083 (PNG specification).

Requirements#

• Validate PNG signature (8-byte magic)
• Parse critical chunks: IHDR, IDAT, IEND, PLTE
• Parse ancillary chunks: tRNS (transparency)
• Support color types:
  • 0: Grayscale
  • 2: RGB
  • 3: Indexed-color (palette)
  • 4: Grayscale + Alpha
  • 6: RGBA
• Bit depths: 1, 2, 4, 8, 16 (downconvert 16-bit to 8-bit)
• Scanline filtering: None, Sub, Up, Average, Paeth (reconstruct filtered rows)
• Interlacing: Adam7 (reorder passes into final image)
• Decompress IDAT chunks via zlib
• CRC-32 validation on chunks
• Output as RGBA8 Image
• API: fn decode_png(data: &[u8]) -> Result<Image, ImageError>

Acceptance Criteria#

• Decodes standard PNG test images (various color types, bit depths, interlacing)
• Handles tRNS transparency for palette and grayscale images
• Rejects corrupt PNGs (bad signature, CRC mismatch, invalid IHDR)
• Unit tests for each color type, filter type, and interlacing
• Integration tests with real PNG files
• No external dependencies, no unsafe

Dependencies#

• Requires DEFLATE decompressor
• Requires zlib wrapper
• Requires pixel format types

Phase 7 — Image Decoders