Render Engines (ffmpeg vs MoviePy)

Two renderers accept the same plan/cut_plan.json and produce the same kind of file, so they can be compared on identical input.

render_reel.py is the default. Only reach for MoviePy when the user asks to compare, or when a plan needs per-clip compositing that the flat concat model cannot express.

Running the comparison

Same plan, same grade, same resolution — change only the script:

python3 scripts/render_reel.py    plan/cut_plan.json --preview --grade rapha --out preview/ab_ffmpeg.mp4
python3 scripts/render_moviepy.py plan/cut_plan.json --preview --grade rapha --out preview/ab_moviepy.mp4

render_moviepy.py prints its wall-clock time so the two are directly comparable. Grading is not reimplemented — MoviePy handles cut, concat, and audio, then the identical GRADES chain from render_reel.py runs as a second ffmpeg pass. That keeps the assembly engine as the only variable.

Ask before switching

Which engine to use is a user-facing choice, not an implementation detail: it changes render time and, slightly, the audio mix. Offer the comparison rather than silently swapping engines mid-project.

Known differences

• Duration can differ by a frame or two. Concat butts segments together; MoviePy's compose recomputes the timeline.
• Audio level differs slightly — measured about 2.7dB quieter from MoviePy on the same plan with the same gains. Check loudness before publishing a master rendered by the non-default engine.
• Preview size is 540x960 from both, ignoring --resolution.