XBC7/XUBC7 (supercompressed weight grid residual DCT+endpoint DPCM+Zstd) prototype is done

The next step is integration and connecting the BC7->ASTC LDR 4x4, ETC1, etc. transcoders into place in the Basis Universal library. Bitrate and quality at low Q's (Q=1/100) are exceptional vs. XUASTC LDR 4x4 (~1.5-3.0 bpp), and at Q=100 it's lossless in BC7 space at up to ~5.6 bpp or less depending on the content. Q=1 is totally usable, unlike XUASTC LDR where it falls apart because it's stuck using absolute DCT, while XBC7 uses a much stronger residual DCT method with a small army of synthetic and dictionary predictors. At Q=1 it's like RDO in weight space: except it can modify what it's predicting from, or create entirely synthetic weight predictors from nearby blocks.

XBC7 is stuck with 4x4 blocks, so it cannot compete vs. XUASTC LDR for raw bitrate, but it's BC7-first which I think a lot of developers will like. At Q=90-100 it's visually lossless to actually lossless (relative to the BC7 encoder used, which is currently our real-time bc7f encoder which supports the entire BC7 format).

Claude Fable 5's description: "It's a genuinely interesting taxonomic hybrid — the closest honest description is: an intra-only video codec whose "pixels" are BC7's parameter space. Let me break down the lineage, because nearly every component has a recognizable ancestor:

The weight path is JPEG transplanted into index space. Prediction → 4×4 DCT → psychovisually-shaped quant table → quality-factor scaling (literally the JPEG 5000/q formula) → zigzag RLE → entropy coding. But the signal isn't luma — it's BC7 weight planes, with the AQ twist (endpoint-span-scaled quantization) playing the role of adaptive quantization.

The prediction machinery is like H.264/HEVC intra coding. The predictor bank contains H.264's directional modes (your DDL/DDR are intra modes 3/4 by name), JPEG-LS's MED, CALIC's gradient-adaptive predictor, and plane fits. The 32 XY-delta block references are intra block copy from HEVC Screen Content Coding. The command byte is a skip/merge/mode syntax. Solid-DPCM is a DC mode. The reserved P-frame bit makes the video ancestry explicit — and your encoder stripes are, structurally, H.264 slices: independently decodable regions traded against compression for parallelism, down to the same seam cost.

The lossless side is JPEG-LS/PNG lineage: wrapped-modular DPCM, the R−G/B−G transform is the classic reversible color transform, and the Q=100 mode makes it a true lossless coder over canonical BC7.

Within its own genus — GPU supercompression — it's a third architecture. Crunch/ETC1S quantize globally into vocabularies (low ceiling, great rate); UASTC+RDO+Zstd nudges bits to be LZ-friendly (shallow, format-bound). XBC7 instead reparameterizes the block format into semantically separated streams and codes each with real signal-processing tools — per-block adaptive, with a continuous lossy↔lossless dial neither predecessor has. It's what crunch might have become if it had gone transform-coding instead of VQ.

And the dual image/texture nature is where your ecosystem makes it strategic. One XBC7 file fans out to: BC7 directly (bit-exact at Q=100 — desktop GPUs, zero re-encode), ASTC 4×4 via your near-lossless transcoder (modern mobile), ETC1 via ETC1F from decoded pixels in real time (legacy mobile), and plain RGBA for CPU/image use. That's the Basis Universal proposition with a much higher quality ceiling than ETC1S and better rate than UASTC — and as a pure image codec it occupies an unusual niche: a high-quality-band coder (ceiling = BC7's ~46–50 dB) at 1.5–5.3 bpp whose decode output is already GPU-ready, which JPEG/WebP/AVIF can never claim. The honest comparison point for the image use-case is "JPEG at quality 85–95, but the decode step doubles as texture upload."

If you wanted a one-line description for the eventual README: "a predictive transform codec (DPCM + DCT, H.264-style intra prediction with block copy) operating on BC7's logical parameter space, with a continuous quality dial from ~1.5 bpp lossy to bit-exact lossless BC7, designed for multithreaded encode/decode via independently-coded stripes." Anyone who's worked on a video encoder will know exactly what they're looking at — which is also why the Lagrangian RDO step you're deferring is so natural here: the syntax is already a video codec's syntax, it's just missing the λ."