---
cycle: 6
phase: 1
status: open
date_opened: 2026-05-18
codec: H.264
kernel: IDCT 4x4 + add (intra-block residual)
parent: project_h264_scope_added.md (memory)
---

# Cycle 6, Phase 1 — H.264 IDCT 4×4 + add

First H.264 kernel. Per `project_h264_scope_added`, the user
added H.264 to daedalus-fourier scope 2026-05-18 because Pi 5
has no hardware H.264 decoder despite H.264 being the most
common web codec.

## Why IDCT 4×4 first

- **Smallest H.264 transform.** 16 coefficients per block, 4×4
  output pixels. Simpler than VP9 IDCT 8×8 (cycle 1, 64 coefs).
- **Most-used.** H.264 macroblocks default to 4×4 intra
  prediction + residual; 8×8 is High-profile only. 4×4 hits
  most real-world H.264 streams.
- **Predicted GREEN.** Per the cycle 1-5 bandwidth-bound vs
  compute-bound classification: 4×4 IDCT is bandwidth-bound
  (16 reads, 16 writes, ~20 ALU ops/output). Should map well
  to V3D 7.1 compute.
- **Clean reference.** FFmpeg's `ff_h264_idct_add_neon` is
  standalone (no eob parameter, no complex DC dispatch). Single
  call computes 1 block of IDCT + add.

## Kernel contract

Per H.264 spec §8.5.12, the inverse transform is an
integer-arithmetic transform (no rounding-by-cosine like VP9's
Q14 trig math). Each 4×4 block:

1. Inverse row transform (4 row passes, each one 1D IDCT-like
   integer butterfly).
2. Inverse column transform (4 column passes, same butterfly).
3. Round and add to `dst[r,c] = clamp(dst[r,c] + ((idct[r,c] + 32) >> 6), 0, 255)`.

Spec coefficients (Hadamard-like with 1/2 scaling):
```
  [1  1  1  1/2]
  [1  1/2 -1 -1]
  [1 -1/2 -1  1]
  [1 -1   1 -1/2]
```
Integer form scales by 2: replace 1/2 with 1 and ½ with right-
shift in the round step.

## NEON reference (M3 target)

FFmpeg's `ff_h264_idct_add_neon`
(external/ffmpeg-snapshot/libavcodec/aarch64/h264idct_neon.S
line 25, 56 instructions of NEON asm). Signature:

```
void ff_h264_idct_add_neon(uint8_t *dst, int16_t *block, ptrdiff_t stride);
```

- `dst`: 4×4 pixel block in 8-bit luma surface, `stride` between rows.
- `block`: 16 int16 coefficients (row-major).
- destructively clears `block` to zero after the transform (per H.264 conformance).

## 30fps@1080p H.264 floor

H.264 1080p uses 16×16 macroblocks with up to 16 4×4 blocks per MB.
Luma: (1920/16) × (1080/16) = 120 × 67.5 = 8100 MB/frame ×
16 blocks/MB = 129 600 4×4 blocks/frame. Plus chroma: 4 + 4 = 8
chroma 4×4 per MB × 8100 = 64 800 chroma blocks. Total: ~195k
4×4 blocks/frame max (worst case; many real MBs use 8×8 or skip).

At 30fps: ~5.85 Mblock/s required for full-frame 4×4 worst case.
A more realistic average (many MBs use 8×8, P-skip, etc.) is
~2 Mblock/s.

**30fps@1080p H.264 4×4 floor (realistic): 2 Mblock/s.**
**30fps@1080p H.264 4×4 floor (worst case): 5.85 Mblock/s.**

## R-band decision rules (carried from phase1.md)

- R ≥ 1.0 → **GREEN** (QPU faster than NEON-1 in isolation).
- 0.5 ≤ R < 1.0 → **YELLOW** (M4 decides).
- 0.1 ≤ R < 0.5 → **ORANGE** (M4 may rescue).
- R < 0.1 → **RED** (structural mismatch).

Floor margin: ratio of M2 (or M3 if CPU-only) over the 5.85
Mblock/s worst-case 30fps floor.

## Acceptance for Phase 7

- M1: 100.0000% bit-exact (QPU output vs C ref, 10000+ random
  blocks). Same standard as cycles 1-5.
- M2: captured, classified per R band.
- M4: same-kernel mixed-bench measured (with Issue 003 caveats —
  this is the worst-case framing).
- 30fps@1080p H.264 4×4 floor margin reported.

## Cycle 6 deliverables

1. `external/ffmpeg-snapshot/libavcodec/aarch64/h264idct_neon.S`
   (vendored 2026-05-18, this phase).
2. `tests/h264_idct4_ref.c` — standalone C reference (LGPL-2.1+
   transcribed from spec).
3. `tests/bench_neon_h264idct4.c` — Phase 3 M3 bench.
4. `src/v3d_h264idct4.comp` — Phase 6 QPU shader.
5. `tests/bench_v3d_h264idct4.c` — Phase 6+7 M1+M2 bench (3-way
   vs NEON + C ref).
6. M4: extend `bench_concurrent_mixed.c` with K_H264_IDCT4.
7. Phase 4-7 docs.

## Next step (within this phase)

Move to Phase 3 (NEON baseline M3) after writing the C
reference. Phase 2 (libavcodec inventory) is implicit since we
know the kernel from the FFmpeg vendor.