---
cycle: 7
phase: 1
status: open
date_opened: 2026-05-18
codec: H.264
kernel: IDCT 8x8 + add (High-profile residual)
parent: project_h264_scope_added.md (memory)
predicted_R: 0.4-0.8 (YELLOW/ORANGE) — comparable to VP9 IDCT 8x8 (cycle 1, R=0.92)
---

# Cycle 7, Phase 1 — H.264 IDCT 8×8 + add

Second H.264 kernel. 8×8 inverse integer transform used in
High-profile H.264 (most modern H.264 encodes High; broadcast
TV, web streams, file media). Smaller scope than IDCT 4×4 but
much more compute-heavy per block.

## Why IDCT 8x8 next

- Closely analogous to **cycle 1 (VP9 IDCT 8×8) which was R=0.92
  GREEN**. Best candidate for a near-immediate H.264 GREEN result.
- 64 coefficients per block (8×8) = same data shape as cycle 1.
- Integer butterfly (no trig multiplies) but more sub-stages than
  4×4. Per-block compute weight ~3-5× the 4×4.
- H.264 High-profile uses IDCT 8×8 for ~40-60 % of residual blocks
  (encoder choice). Decoder must support it for spec compliance.

## Kernel contract

Per H.264 spec §8.5.13 (8x8 inverse integer transform). 1D
butterfly (g[0..7] from input d[0..7]):

```
e[0] = d[0] + d[4]
e[1] = -d[3] + d[5] - d[7] - (d[7] >> 1)
e[2] = d[0] - d[4]
e[3] = d[1] + d[7] - d[3] - (d[3] >> 1)
e[4] = (d[2] >> 1) - d[6]
e[5] = -d[1] + d[7] + d[5] + (d[5] >> 1)
e[6] = d[2] + (d[6] >> 1)
e[7] = d[3] + d[5] + d[1] + (d[1] >> 1)

f[0] = e[0] + e[6]
f[1] = e[1] + (e[7] >> 2)
f[2] = e[2] + e[4]
f[3] = e[3] + (e[5] >> 2)
f[4] = e[2] - e[4]
f[5] = (e[3] >> 2) - e[5]
f[6] = e[0] - e[6]
f[7] = e[7] - (e[1] >> 2)

g[0..7] = butterfly of f[0..7]
```

Applied row-pass then column-pass (per H.264/FFmpeg convention,
with column-major block).

Final: dst[r,c] = clip(dst[r,c] + (g_2d[r,c] + 32) >> 6).

## NEON reference (M3 target)

FFmpeg's `ff_h264_idct8_add_neon`
(external/ffmpeg-snapshot/libavcodec/aarch64/h264idct_neon.S
line 267, ~60 instructions / pass × 2 + transpose + dst-add).
Signature mirrors cycle 6 IDCT 4×4:

```
void ff_h264_idct8_add_neon(uint8_t *dst, int16_t *block, ptrdiff_t stride);
```

Block: 64 int16, column-major (per cycle 6 Phase 9 lesson).

## 30fps@1080p H.264 8×8 floor

1920×1080 luma using all 8×8 transforms: 240 × 135 = 32 400
blocks/frame × 30 fps = 0.972 Mblock/s. Same as VP9 IDCT 8×8
(cycle 1) since the block density is the same.

**30fps@1080p floor: 0.972 Mblock/s.**

## Predicted R₇

Per the cycle 1 / cycle 6 patterns:
- VP9 IDCT 8×8 NEON M3 = 8.171 Mblock/s (cycle 1), per-block 122 ns
- H.264 IDCT 8×8 likely **less compute per block** than VP9 (no
  trig multiplies, just integer ops + shifts) → maybe 80-120 ns
  per block → 8-12 Mblock/s NEON
- QPU 8×8 IDCT R=0.92 GREEN in cycle 1 came from the matching
  16-lane / 8-row layout and shared-mem transpose
- H.264 IDCT 8×8 same shape → predicted **R₇ ≈ 0.5-0.9 YELLOW/GREEN**

## Acceptance for Phase 7

- M1: 100.0000% bit-exact (10000+ random blocks)
- M3: captured
- M2: captured
- R₇: classified
- M4: same-kernel mixed bench measured

## Cycle 7 deliverables

1. `tests/h264_idct8_ref.c` — column-major C reference
2. `tests/bench_neon_h264idct8.c` — Phase 3 bench
3. `src/v3d_h264idct8.comp` — Phase 6 shader (likely close to
   v3d_idct8.comp shape, but with different butterfly + integer
   math instead of Q14 trig)
4. `tests/bench_v3d_h264idct8.c` — Phase 6+7 bench
5. M4 via `bench_concurrent_mixed.c` extension

## Phase 4 effort estimate

Higher than cycle 1's iterations because the 8×8 IT butterfly is
more involved (3 sub-stages vs cycle 1's IDCT8 single butterfly).
~3-4 hours through Phase 7. Phase 5 Sonnet review again
non-skippable per CLAUDE.md.

## Next step (within this phase)

Move to Phase 3 (NEON baseline M3) after writing the C reference.

## Future H.264 cycles (preview, post cycle 7)

- Cycle 8 — H.264 chroma MC (4-tap; very lightweight; predicted
  RED per cycle 6 pattern but smaller still)
- Cycle 9 — H.264 luma quarter-pel MC (6-tap; analogous to cycle 3
  VP9 MC which was RED; predicted RED)
- Cycle 10 — H.264 in-loop deblock (analogous to cycle 2/4 VP9
  LPF which were GREEN; predicted GREEN)
- After cycle 10: scope re-evaluated based on cycle 7/10 results