Second piece of the intra-prediction primitive set after PR #12
(Intra_4x4 luma 9 modes). Covers the Intra_16x16 luma MB type
per H.264 §8.3.2: 4 modes (Vertical, Horizontal, DC, Plane).
Scope:
- tests/h264_intra_pred_16x16_ref.c — 4 spec-derived modes.
Same FFmpeg-style interface as the 4x4 sibling:
void daedalus_h264_pred_16x16_<name>_ref(uint8_t *dst, ptrdiff_t stride);
Assumes all neighbours valid (interior-MB case).
The Plane mode is the algorithmically heaviest of the four —
spec §8.3.2.4 has two slope sums (H, V) over the asymmetric
top/left contexts, a clipped quadratic evaluation per cell,
and a top-left-corner participant at i=7 / j=7. Implementation
follows the spec straightforwardly with `clip_u8` on the final
saturating cast.
- tests/test_intra_pred_16x16.c — 5 test cases:
* V, H, DC: standard contexts (gradient top / gradient left /
small uniform pair).
* Plane (uniform): all neighbours = 100 → H = V = 0 →
output = (16*200 + 16) >> 5 = 100. Verifies the
orientation-free portion of the formula.
* Plane (gradient): top + left both 0..15, spec-derived
corner expectations pred[0][0] = 1 and pred[15][15] = 31.
The arithmetic chain (H = V = 400 → b = c = 31, a = 480)
is fully hand-traced in the test comment so the expected
values are auditable.
- CMakeLists.txt — new test_intra_pred_16x16 binary; pure-CPU
library, no daedalus_core dependency (same separation as the
4x4 ref).
Verified on hertz:
$ ./build/test_intra_pred_16x16
Vertical (mode 0) PASS
Horizontal (mode 1) PASS
DC (mode 2) PASS
Plane (mode 3, uniform) PASS
Plane (mode 3, gradient) PASS (corners 1, 31)
ALL Intra_16x16 mode references PASS
Plane mode being right first try is meaningful — H/V sums, b/c
slope shifts, and the a-baseline arithmetic have many sign / index
error opportunities. The asymmetric gradient test would have caught
any of them; it didn't.
What this does NOT cover (still in the intra-pred backlog):
- Intra_8x8 chroma (4 modes per H.264 §8.3.3).
- Intra_8x8 luma (High profile, 9 modes per §8.3.2.1 + the 1-2-1
smoothing pre-filter — distinct algorithm from Intra_4x4).
- Neighbour-availability fallback for boundary MBs.
- Dispatch wrappers (same architectural question as before — wait
for decoder Stage 2a strategy decision).
claude-noether