When NVENC is available the HLS transcode encoded on the GPU but still
decoded on the CPU, since the command had no -hwaccel. Rebuilding the
4K homemade videos ran three ffmpegs at 210-240% CPU each with the
machine at 0% idle, while NVDEC sat at 0%.
Pass -hwaccel cuda when transcoding with NVENC, as generate_preview_clip
already does. Frames return to system memory, so autorotate and the
scale filter work unchanged; NVDEC-unsupported codecs fall back to
software decode.
10 s of 4K H.264 (2025-USAF-MuseumRace.mp4): 27.3 s -> 6.1 s CPU,
9.6 s -> 3.6 s wall, identical output (1920x1080, 245 frames, 3 s
segments). Also checked a 90-degree-rotated phone clip (comes out
1080x1440 upright, 233/233 frames) and ProRes (software fallback,
correct output).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Cameron CordesandClaude Opus 5.5