ersatztv/docs/decisions/records/ffmpeg/qsv-decode-encode-split.md

key, title, status, since, supersedes, superseded-by, rule, signals, mechanics

key	title	status	since	supersedes	superseded-by	rule	signals	mechanics
ffmpeg.qsv-decode-encode-split	2026-07-20 (#498) — QSV decode is split from QSV encode via a single `QsvPreferNativeDecoder` bool	active	2026-07-20	none	none	QSV decode is decoupled from QSV encode via a single `FFmpegProfile.QsvPreferNativeDecoder` bool (default ON, Linux-only), so a QSV encode profile can decode with the more tolerant native VA-API decoder instead of the QSV decoder, mirroring Jellyfin's hybrid decode/encode toggle instead of a general decode-family enum.	QSV, native VA-API decode, Dolby Vision passthrough, hybrid decode/encode, HDR tonemap fallback · paths: `FFmpegProfile.QsvPreferNativeDecoder`, `QsvPipelineBuilder.SetTonemap`, `QsvPipelineBuilder` · issues: #498, #505	`docs/superpowers/specs/2026-07-20-qsv-native-decode-design.md`; migration `HasDefaultValue(true)` on the nullable `bool?` column; `QsvPipelineBuilder` decoder-mode branch

FFmpegProfile.HardwareAcceleration picked one pipeline builder for both decode and encode, so an Intel QSV profile decoded with the QSV decoder — which is materially less tolerant of imperfect H.264 than FFmpeg's native VA-API decoder and cannot carry Dolby Vision metadata. Jellyfin, on identical hardware, avoids this by decoding with VA-API and encoding with QSV; that combination was not expressible in ErsatzTV. Full design: docs/superpowers/specs/2026-07-20-qsv-native-decode-design.md.

• Chosen: a single, QSV-scoped FFmpegProfile.QsvPreferNativeDecoder boolean, mirroring Jellyfin's "Prefer OS native DXVA or VA-API hardware decoders" checkbox exactly — Jellyfin itself deliberately collapses this to one default-on toggle rather than a decode-family picker, and copying the reference tool's granularity avoids over-building past a problem it has already solved.
• Default ON. Native VA-API decode is strictly more tolerant of imperfect streams than the QSV decoder wrapper, and Dolby Vision passthrough requires it — so the working configuration is the default for a reliability fix. Existing QSV profiles adopt the hybrid on upgrade via the migration's HasDefaultValue(true) (SQLite INTEGER/MySQL tinyint, nullable with DEFAULT 1 — the column is bool? so an explicit false persists, while NULL/absent reads as ON via != false; a non-nullable bool + store default would make EF silently drop a create-with-false), no opt-in required.
• Rejected: a general DecodeHardwareAcceleration enum column decoupling decode and encode hardware families entirely. More configurable than Jellyfin's own control, but nothing in the codebase or the reported failure needs that generality yet — FFmpegState already separates DecoderHardwareAccelerationMode / EncoderHardwareAccelerationMode internally (used today for the software-decode-plus-hardware-encode error-loop fallback), so the seam to grow into a real enum exists if a second asymmetric-decode case ever shows up on another hardware family. Deferred rather than built now, on YAGNI grounds.
• Contained to the QSV builder. The flag does not touch PipelineBuilderFactory's single-builder dispatch per profile, and does not repurpose the existing decode/encode-mode fields into a general cross-family selector — that repurposing is exactly the rejected enum option, just introduced through the back door.
• Accepted trade-off — HDR tonemapping runs in software on the native path. With native decode ON the decoder mode is Vaapi, so QsvPipelineBuilder.SetTonemap (which only selects TonemapQsvFilter for DecoderHardwareAccelerationMode == Qsv) falls to the software zscale/tonemap chain for HDR content. Output is correct but costs CPU on the realtime path. Accepted for now because software tonemap is correct-but-slower while an unvalidated GPU-tonemap graph could be worse (needs the Intel host to verify), and the escape hatch covers it: HDR-on-QSV users who don't need the tolerant decoder set the flag OFF to keep GPU tonemap. Optimizing the native path to tonemap_qsv is tracked in #505.
• Native decode is Linux-only. Guarded with !OperatingSystem.IsWindows() in the QSV builder — FFmpeg has no vaapi hwaccel on Windows (and Windows QSV capabilities are over-reported), so on Windows a QSV profile keeps QSV decode regardless of the flag.