Validates marfrit-packages patch 0016 (PR #106) end-to-end against
the daedalus_decode_h264 CLI. Callback fires once per macroblock
in coded order; this PR checks the count + uniqueness invariants
WITHOUT yet driving daedalus-decoder differently — that's PR-A3.
Infrastructure landed
---------------------
CMake gains DAEDALUS_FFMPEG_PREFIX option pointing at a private
FFmpeg install carrying patch 0016. When set, the CLI links
against it (static .a's from $prefix/lib) and the inspection
codepath is compiled in (DAEDALUS_HAVE_H264_MB_INSPECT_CB). When
unset, the CLI falls back to the pkg-config-discovered system
FFmpeg and behaves as PR-A1b did (identity-passthrough only, no
callback).
The H264Context struct stays opaque (forward-decl only — its
real definition lives in libavcodec's internal h264dec.h which
isn't installed). Real per-MB state extraction (sl->mb coeffs,
mb_type, intra modes, deblock params) will land in PR-A3
alongside an internal-header include path.
The callback's only job in this PR: assert (mb_x, mb_y) lies in
the coded grid, mark "seen" in a per-frame bitmap, count
invocations. At end-of-frame: assert seen-count == mb_w*mb_h,
0 duplicates, 0 out-of-bounds.
Per-frame mb-grid init goes BEFORE first avcodec_send_packet
(callbacks fire from inside send_packet, before the first
receive_frame ever returns — lazy init from AVFrame would miss
all of frame 0). Dims come from codecpar->width/height rounded
up to 16-mod (H.264 codes 1080 display as 1088 coded).
Raster-order check considered but dropped: libavcodec uses
MB-level threading in some configs so callbacks fire out of
raster order. The contract is "each MB exactly once", not "in
raster order"; the bitmap check captures that.
Result on hertz (Pi 5, patched FFmpeg at /tmp/ffmpeg-inspect-prefix)
-------------------------------------------------------------------
320x240 I-only, 3 frames:
mb-grid 20x15
callback invocations: 900 (= 3 * 300)
missing/duplicates/oob: 0/0/0
identity-passthrough Y diff 0/230400, UV diff 0/115200
PASS
1920x1088 I-only, 3 frames:
mb-grid 120x68
callback invocations: 24480 (= 3 * 8160)
missing/duplicates/oob: 0/0/0
identity-passthrough Y diff 0/6266880, UV diff 0/3133440
PASS
Followups
---------
- PR-A3: include libavcodec/h264dec.h via -I to access H264Context
internals; extract sl->mb coefficients in the callback, compute
P = pre-deblock pixels - IDCT(C) using a transcribed C reference;
feed daedalus_decoder with REAL (P, C, edges) instead of identity.
Use avctx->skip_loop_filter = AVDISCARD_ALL to make libavcodec
output pre-deblock so the subtraction is exact.
- PR-A4 onwards: extend to P/B frames + chroma DC + intra prediction
coverage.
claude-noether