name: deck-to-video
description: Use when turning a slide deck plus per-slide narration audio into an mp4 video — a self-playing/self-presenting deck video, "slides to video", "presentation to mp4", a shareable WhatsApp/Slack/Drive video of a narrated deck, or when a slide video needs a per-slide progress bar / countdown / slide counter. Also use when an ffmpeg-drawn progress bar renders full/static from the first frame, or ffmpeg errors "No such filter: 'drawtext'".
Deck to Video (slides + narration → self-playing mp4)
Overview
Turn N slide images (or a PDF) + N per-slide audio clips into one mp4: each slide holds for exactly its narration's length plus a short pause, then hard-cuts to the next. Overlays: a filling progress bar + seconds-remaining countdown (bottom-right) and an "N / M" slide counter (top-right).
The one command (the script ships in this skill's own scripts/ directory — call it by its full path there):
python3 <this-skill-dir>/scripts/build_video.py SLIDES AUDIO_DIR out.mp4
On success it prints one line per slide — that per-slide length already includes the pad — and ends with duration matches segment sum (it exits non-zero otherwise).
Getting the inputs right
- Slides: if starting from a pptx, rasterize from real PowerPoint, not LibreOffice. Export the pptx to PDF via actual PowerPoint (the office-render skill does this on macOS), then hand the PDF to the script — LibreOffice substitutes fonts and re-flows layouts, so the video would not match what the deck's author sees. 200 dpi keeps dense slide text crisp at 1080p. (Already have per-slide images? Use them directly — skip this step.)
- Audio: one clip per slide, named
slideNN.mp3. If the deck was narrated with the narrating-pptx skill, its narration/ directory is already in this exact shape — reuse it so the pptx and the video sound identical.
- If narration pace was adjusted (e.g.
ffmpeg -filter:a atempo=1.1), build the video from the adjusted files.
Why the overlays are baked with PIL, not drawn by ffmpeg (paid for in blood)
drawbox with a t-based width expression renders a FULL bar from frame 0 in several ffmpeg builds — the expression is evaluated once at init, not per frame. The failure is silent: the video encodes fine and the bar simply never moves. drawtext expressions do update per frame in the same builds, which makes the broken bar easy to miss next to a working countdown.- Homebrew's ffmpeg bottles (2026+) are built without libfreetype —
No such filter: 'drawtext' — and the keg versions (ffmpeg@7, ffmpeg@8) are the same. Don't spend an install cycle discovering this; if a full build is needed for anything else, uv tool install static-ffmpeg ships one (the script auto-falls-back to it).
So the script bakes one frame per second with PIL (bar fill, countdown, counter) and feeds ffmpeg a 1 fps image sequence encoded at -r 30. Deterministic, font-controlled, and cheaper to encode than filtering a 30 fps stream — the slide is static anyway. Second-granularity updates read naturally for a countdown.
Encoding choices the script already makes (don't undo them)
| Choice | Why |
|---|
apad=pad_dur=PAD + -t seg_dur | exact tail padding; naive -shortest cuts the pause after speech |
yuv420p + even dimensions | anything else fails to play in QuickTime/WhatsApp/browsers |
-tune stillimage, CRF 20 | static slides compress well without text shimmer |
concat demuxer + -c copy | lossless join of identically-encoded segments |
-movflags +faststart | video streams before it finishes downloading |
Validate — assume the overlay is broken until frames prove otherwise
The script asserts final duration ≈ sum of segments. The overlay needs eyes:
ffmpeg -y -ss T1 -i out.mp4 -frames:v 1 a.jpg
ffmpeg -y -ss T2 -i out.mp4 -frames:v 1 b.jpg
Pick T1/T2 from the script's printed per-slide lengths (cumulative-sum them for slide boundaries; remember they include the pad). Read both images. The bar must be visibly fuller in b.jpg and the countdown lower — and it should reset at each new slide. A full-looking bar at an early timestamp = the static-bar bug — this exact check is what caught it originally. You cannot verify audio headlessly — say so; never claim you heard it.
Common mistakes
| Mistake | Consequence | Fix |
|---|
ffmpeg drawbox animated by t | bar full from frame 0, silently | PIL-baked frames (the script) |
Assuming drawtext exists | No such filter on Homebrew builds | script falls back to static-ffmpeg / PIL text |
| LibreOffice rasterization | fonts/layout differ from the real deck | real PowerPoint PDF via office-render |
-shortest for segment length | eats the pause after narration | apad + explicit -t |
| Audio count ≠ slide count | desynced or missing slides | script hard-fails; fix inputs, don't skip slides |
| Judging output by "it encoded fine" | static bar / wrong slide slips through | extract early/late frames and look |
| Deck prints its own slide numbers | number appears twice (deck footer + counter) | render the video from a numberless deck variant, or pass --no-counter |
| ffmpeg hangs at 0% CPU after writing a segment | build stalls forever; a killed attempt leaves an INVALID partial file | the script now times out each segment (300s) and retries once, deleting the partial first |
Piping the build through | tail inside an && chain | pipeline exit = tail's 0 — a failed build "succeeds" and stale artifacts get delivered | run the script unpiped (or set -o pipefail); keep delivery/sync in a separate step gated on verified output |
Caveats
- Hard cuts between slides by design — crossfades break the lossless
-c copy concat and add little.
- ~13–17 min of 1080p mostly-static slides ≈ 40–50 MB.
- The video is a third artifact beside the clean pptx and the narrated pptx — it forces the narration's pace on the viewer, so keep the pptx variants for people who prefer reading.
