bc_crunch

bc_crunch is a tiny, dependency-free C99 library for lossless compression of GPU-compressed texture blocks BC1, BC4, BC3, and BC5.

• ~700 LOC total (single-file encoder/decoder, no build system tricks)
• No malloc, no external libs, no threads, no SIMD requirement
• Deterministic, bit-exact reconstruction
• Fully tested with bytes-per-bytes comparison
• Focused on production textures: albedo, masks, normals, heightmaps, etc.
• GPU-ready output: decompressed blocks are written in standard BC1/BC4/BC3/BC5 format, ready to be uploaded directly to GPU memory (or directly written in shared memory)
• No extra memory for decompression: only the encoder needs a temporary buffer; decoding writes straight to the output buffer.

This is not another general-purpose compressor. bc_crunch is specialized for already-compressed GPU formats — it exploits the internal structure of BC1/BC4 blocks, spatial patterns, endpoint deltas, popcount distances, and Morton-ordered indices to achieve significant size reductions with extremely low CPU cost.

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Benchmarks

Compression ratio naturally depends on the input content. Repetitive patterns, smooth gradients, and large uniform regions compress very well, while randomness or high-frequency noise significantly reduces efficiency (e.g., dirt textures are typically very noisy).

BC1 benchmarks

Average compression ratio : 1.493586

Category	# Samples	Original BC1 Size (bytes)	Avg Crunched Size (bytes)	Avg Compression Ratio
Kodak photos	24	196,608	136,254	1.44×
Dirt	20	131,072	105,386	1.24×
Wood	20	131,072	79,306	1.65×
Elements	20	131,072	90,412	1.45×
Brick	20	131,072	88,212	1.49×
Metal	20	131,072	87,132	1.51×
Plaster	20	131,072	99,118	1.33×
Stone	20	131,072	97,014	1.35×
Tile	20	131,072	101,087	1.30×
Blaz Tree	7	32,768–32,768	16,900	1.84×
SW Tree	7	20,480–32,768	14,227	2.08×
pkf Concrete	4	32,768	24,583	1.33×
pk02 Floor	20	32,768–524,288	100,604	1.83×
pk02 Trim	5	4,096–131,072	11,777	1.89×
Rock	4	524,288	412,365	1.27×
Brick Large	2	524,288	408,746	1.28×

BC1 textures samples

BC1 decompression has been measured at 35 MB/s on an M4. Some range encoder optimizations were removed to keep the library small, but they could be reinstated.

BC4 benchmarks

Average compression ratio : 1.232521

Category	Samples	Uncompressed (bytes)	Avg Compressed (bytes)	Avg Compression Ratio
AO	4	524,288	472,347	1.110685
Displacement	4	524,288	390,158	1.354357

BC4 textures samples

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Technical details

BC1

• Zigzag block traversal
• Endpoint deltas using left/up predictors
• Top-table of the 256 most frequent index bitfields (prepass histogram)
• Nearest-match selection via popcount distance
• Delta-encoded index patches (no raw fallback)

BC4

• Zigzag traversal
• Cyclic wrapped endpoint deltas (mod 256) using left/up predictors
• 256-entry sliding dictionary
• Move-to-front heuristic when hit
• Nearest-match selection via popcount distance
• Block zigzag traversal with xor-delta encoding for index fallback

Composite Formats

• BC3 is just (BC1 + BC4) — no extra logic
• BC5 is a dual BC4 channels

How to build tests

• open a terminal in the folder
• mkdir build
• cd build
• cmake -DCMAKE_BUILD_TYPE=Release ..
• cmake --build .
• ./test

More tests will be added soon.

FAQ

How can I compress a texture with all mipmaps

Right now you need to call bc_crunch separately for each mip level. I may add a helper later, but I want to keep the encoder simple and stateless, without any mip-level awareness or cross-mip data.