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design: §9 open questions → Phase 1 decisions (user confirmed 2026-05-24) #1

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marfrit merged 1 commits from noether/design-decisions into main 2026-05-24 19:58:41 +00:00
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**Phase 4 — Production-ready deblock + perf optimization + libva integration** (+4 weeks). Real-world stream conformance. Plug into daedalus-v4l2 daemon as the actual decode backend.
**Total budget:** 4-6 months.
**Total H.264 budget:** 4-6 months.
**Phase 5+ (future codec scope, not committed):** VP9 and AV1 reuse the same frame-level dispatch architecture, daedalus-fourier kernel pack, and DPB plumbing. Per §9.7, they are deferred but *not firmly out-of-scope*. HEVC stays firmly out (Pi 5 has `rpi-hevc-dec` for that).
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## 9. Open questions
## 9. Phase 1 decisions
1. **Intra prediction strategy:** GPU wavefront (~187 dispatches, more complex) vs CPU speculative (simpler, slower). Plan: wavefront in Phase 1; revisit if it's the perf bottleneck.
User-confirmed 2026-05-24. All seven questions from the initial
draft are now decided; this section preserves the original wording
of each item for traceability.
2. **libavcodec intercept granularity:** macroblock-level (substitution-arc evolution) vs slice-level (cleaner rewrite). Plan: macroblock-level for Phase 1; consider slice-level later if buffer accumulation overhead is non-trivial.
1. **Intra prediction strategy:** GPU wavefront (~187 dispatches, more complex) vs CPU speculative (simpler, slower). **Decision: wavefront in Phase 1; revisit if it's the perf bottleneck.**
3. **Shader parameterization:** 16 qpel variants as 16 shaders, or one parameterized shader with switch on mc_position? V3D's compiler might inline-optimize either; needs measurement.
2. **libavcodec intercept granularity:** macroblock-level (substitution-arc evolution) vs slice-level (cleaner rewrite). **Decision: macroblock-level for Phase 1; consider slice-level later if buffer accumulation overhead is non-trivial.**
4. **DPB allocation:** Vulkan-native VkImage with dmabuf export, vs CPU-allocated dma_buf imported into Vulkan. Affects V4L2 integration story. Plan: Vulkan-native with `VK_KHR_external_memory_dma_buf` export; daedalus-v4l2 daemon imports.
3. **Shader parameterization:** 16 qpel variants as 16 shaders, or one parameterized shader with switch on mc_position? **Decision: measure both during Phase 2 (the MC phase) and pick the winner. No commit ahead of measurement.**
5. **Daemon integration shape:** does daedalus-decoder ship as a static library the daemon links, or as a separate process the daemon talks to? Library, almost certainly — process boundary would multiply IPC cost.
4. **DPB allocation:** Vulkan-native VkImage with dmabuf export, vs CPU-allocated dma_buf imported into Vulkan. **Decision: Vulkan-native with `VK_KHR_external_memory_dma_buf` export; daedalus-v4l2 daemon imports.**
6. **Build dependency on daedalus-fourier:** as a CMake `find_package`, or vendored? `find_package`, pinned to a tagged release. daedalus-fourier becomes the "kernel pack" upstream library.
5. **Daemon integration shape:** static library the daemon links, or separate process. **Decision: library link.**
7. **Out-of-scope for daedalus-decoder (firmly):** VP9, AV1, HEVC (Pi 5 has rpi-hevc-dec for that), 10-bit, interlaced, FMO/ASO.
6. **Build dependency on daedalus-fourier:** CMake `find_package`, or vendored? **Decision: `find_package`, pinned to a tagged release. daedalus-fourier becomes the "kernel pack" upstream library.**
7. **Codec scope.** **Decision: firmly out-of-scope for daedalus-decoder are HEVC (Pi 5 has `rpi-hevc-dec` for that), 10-bit, interlaced, and FMO/ASO.** VP9 and AV1 are *not* firmly out — they're future codec scope for the same framework after H.264 lands. This is a scope expansion from the initial draft, which had grouped them with HEVC under "firmly out".
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