h264: Intra_16x16 luma prediction — 4-mode C reference + spec gates #13

2026-05-24T22:35:41Z

marfrit commented

2026-05-24 22:35:41 +00:00

Second intra-prediction primitive after PR #12 (Intra_4x4). Covers Intra_16x16 luma per H.264 §8.3.2: 4 modes (V, H, DC, Plane).

Plane mode is the algorithmically heaviest of the four — spec §8.3.2.4 has two slope sums (H, V) over asymmetric top/left contexts, a clipped quadratic per cell, and a top-left-corner participant. Tested with a gradient context where pred[0][0]=1 and pred[15][15]=31 are hand-derived from the spec arithmetic; the full chain (H = V = 400 → b = c = 31, a = 480) is in the test comment so the expected values are auditable.

ALL 5 tests PASS first try — Plane mode being correct first-try is meaningful given the sign/index error opportunities.

Remaining intra-pred backlog: Intra_8x8 chroma (4 modes), Intra_8x8 luma High profile (9 modes + 1-2-1 smoothing pre-filter), availability fallbacks.

Second intra-prediction primitive after PR #12 (Intra_4x4). Covers Intra_16x16 luma per H.264 §8.3.2: 4 modes (V, H, DC, Plane). Plane mode is the algorithmically heaviest of the four — spec §8.3.2.4 has two slope sums (H, V) over asymmetric top/left contexts, a clipped quadratic per cell, and a top-left-corner participant. Tested with a gradient context where `pred[0][0]=1` and `pred[15][15]=31` are hand-derived from the spec arithmetic; the full chain (H = V = 400 → b = c = 31, a = 480) is in the test comment so the expected values are auditable. **ALL 5 tests PASS first try** — Plane mode being correct first-try is meaningful given the sign/index error opportunities. Remaining intra-pred backlog: Intra_8x8 chroma (4 modes), Intra_8x8 luma High profile (9 modes + 1-2-1 smoothing pre-filter), availability fallbacks.

marfrit added 1 commit 2026-05-24 22:35:42 +00:00

h264: Intra_16x16 luma prediction — 4-mode C reference + spec gates c43ee84d8e

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).

marfrit merged commit dff610e13d into main

2026-05-24 22:40:32 +00:00

marfrit deleted branch noether/h264-intra-pred-16x16

2026-05-24 22:40:34 +00:00

claude-noether referenced this issue from a commit

2026-05-24 22:42:51 +00:00

h264: Intra_8x8 chroma prediction — 4-mode C reference + spec gates

marfrit referenced this pull request

2026-05-24 22:43:03 +00:00

h264: Intra_8x8 chroma prediction — 4-mode C reference + spec gates #14

claude-noether referenced this issue from a commit

2026-05-25 07:35:52 +00:00