Crate fast_thumbhash 

fast-thumbhash

A fast ThumbHash encoder/decoder — 10x+ faster drop-in replacement for the thumbhash crate.

Optimizations

Encoder (rgba_to_thumb_hash):

• Separable 2D DCT splits O(W·H·N) into two 1D passes
• Chebyshev cosine recurrence: 2 cos() calls per frequency instead of W or H
• Stack-allocated cos/partial buffers (zero heap allocation in hot path)
• Integer average computation avoids N × f32 divisions
• Opaque fast path: RGBA→LPQA with zero per-pixel alpha dependency
• Single allocation for all 4 LPQA channels
• Branchless nibble packing
• unsafe get_unchecked eliminates bounds checks in inner loops

Decoder (thumb_hash_to_rgba):

• Separable 2D IDCT with SAXPY row accumulation (auto-vectorizes to NEON/SSE)
• Chebyshev cosine recurrence for cosine tables
• Stack-allocated cos tables, AC buffers, and scratch rows
• Direct nibble indexing (no std::io::Read overhead)
• unsafe get_unchecked in all hot loops

Functions

base91_decode

Decode a base91 string back to bytes. Returns Err(()) on invalid input.

base91_encode

Encode bytes to a base91 string.

rgba_to_thumb_hash

Encodes an RGBA image to a ThumbHash byte vector.

rgba_to_thumb_hash_b91

Encode an RGBA image directly to a base91-encoded ThumbHash string.

thumb_hash_from_b91

Decode a base91-encoded ThumbHash string to an RGBA image.

thumb_hash_to_approximate_aspect_ratio

Returns the approximate aspect ratio of a ThumbHash image as width / height.

thumb_hash_to_average_rgba

Extracts the average RGBA color from a ThumbHash.

thumb_hash_to_rgba

Decodes a ThumbHash to an RGBA8 image.