Lays the bit-exact gate for H.264 §8.3.1.4 Intra_4x4 luma prediction.
Spec-derived C reference covering all 9 modes; standalone test
exercises each against hand-computed expected 4x4 patterns.
Why fourier (not the decoder) gets this: it's a reusable spec-level
primitive — both daedalus-decoder (Phase 1 Stage 2a intra prediction)
and any future shader work will need the same bit-exact reference.
Putting it in fourier alongside the IDCT / deblock refs keeps the
"spec implementations" library cohesive.
Why CPU C reference, not NEON or QPU: the vendored FFmpeg snapshot
(external/ffmpeg-snapshot/libavcodec/aarch64/) has h264dsp/idct/qpel
but NOT h264pred. Vendoring h264pred_neon.S would expand the snapshot
surface; deferring that pending real perf data. Per the cycle 9
NEON benches that take ~5 ns per 8x8 qpel block, intra prediction
at ~5 ns per 4x4 block × 16 blocks/MB × 8160 MBs = ~650 us/frame at
1080p — well inside budget even at NEON, and much further inside at
plain C. Not the critical-path concern.
Scope:
- tests/h264_intra_pred_4x4_ref.c — 9 prediction modes per
H.264 spec §8.3.1.4 sub-clauses, FFmpeg-style interface:
void daedalus_h264_pred_4x4_<name>_ref(uint8_t *dst, ptrdiff_t stride);
Reads top/top-right/left/top-left neighbours from dst[-stride/-1]
offsets, writes 4×4 output at dst[0..3][0..3]. Assumes all 13
neighbour bytes are valid (interior-MB case; availability
fallbacks are caller-side per spec).
- tests/test_intra_pred_4x4.c — 10 cases:
* 9 uniform-context degenerate tests (one per mode), establishing
that nothing is structurally broken (all output cells must
equal the uniform input value).
* 1 asymmetric Vertical_Right sanity test with 16 distinct
expected cells hand-computed from spec §8.3.1.4.6 — the
"really exercise orientation + row/col arithmetic" gate.
- CMakeLists.txt — new test_intra_pred_4x4 binary (no daedalus_core
dependency; pure-CPU library doesn't need a context to construct).
Verified on hertz:
$ ./build/test_intra_pred_4x4
Vertical (mode 0) PASS
Horizontal (mode 1) PASS
DC (mode 2) PASS
DiagDownLeft (mode 3) PASS
DiagDownRight (mode 4) PASS
VerticalRight (mode 5) PASS
HorizontalDown (mode 6) PASS
VerticalLeft (mode 7) PASS
HorizontalUp (mode 8) PASS
VR asym (sanity) PASS
ALL 10 intra-4x4 mode references PASS
The VR asym test passed first try; the DC test fell on the first
attempt because my test expectation miscomputed the rounding shift
(I wrote 4, actual is 2 = (16+4)>>3). Fixed in the test. Reference
itself never had the bug.
What this does NOT cover (next-step backlog):
- Intra_16x16 luma prediction (4 modes per H.264 §8.3.2): vertical,
horizontal, DC, plane.
- Intra_8x8 chroma prediction (4 modes per H.264 §8.3.3): DC,
horizontal, vertical, plane.
- Intra_8x8 luma prediction (High profile, 9 modes per §8.3.2.1) —
these are the High-profile siblings of the modes in this PR with
the 1-2-1 smoothing pre-filter. Different but well-defined.
- Neighbour availability fallback (top-edge MB, left-edge MB,
slice-boundary, top-right unavailable in some positions).
- Dispatch wrappers — these refs aren't surfaced through
daedalus_dispatch_*(). Whether to do that depends on the
daedalus-decoder Stage 2a architecture (per-block CPU vs
per-diagonal GPU wavefront — TBD).
claude-noether