qa-video-timestamps

# Q&A Video Timestamps Turn a published lecture/Q&A recording into per-question `(MM:SS)` YouTube links on the course page. Works by OCR-ing the "Lambert n/N" slide counter that every colloquium deck prints in the footer — no transcript needed. macOS only (uses the Vision framework via pyobjc). ## Prerequisites - The deck's question slides and their slide numbers, from the built deck: ```bash uv run --extra teach colloquium build teach/course/<deck>.md -o /tmp/deck-build/ ``` Slide numbers equal colloquium's `#N` hash anchors, so the same numbers serve both the slide deep links and the timestamp extraction targets. ## Steps 1. **Download the video** (1080p, video-only — 480p is too small to OCR the footer counter reliably): ```bash uvx yt-dlp -f "bv*[height<=1080][ext=mp4]/bv*[height<=1080]" --no-playlist \ -o "video.mp4" "https://www.youtube.com/watch?v=<ID>" ``` 2. **Coarse sweep** — sample a frame every 5 s and OCR the counter (`ocr_slides.py` in this skill directory; crops assume the standard recording layout, slides left ~73% / webcam right): ```bash uv run --with opencv-python --with pyobjc-framework-Vision \ --with pyobjc-framework-Quartz python ocr_slides.py video.mp4 5 > times.tsv ``` 3. **Determine the recorded deck's total slide count** — check the modal value of column 3 in `times.tsv`. It may differ from today's build (e.g. colloquium later auto-appended a References slide). If it differs, do NOT assume an offset: extract probe frames at a few recorded slide numbers, view them, and compare against the current deck's titles to locate where the numbering diverges. In the qa-02 case the counts differed (26 vs 27) but all content-slide numbers were identical. 4. **Refine to the exact switch second** (no padding — links must land ON the transition, not before it; a flat pad was measurably early): ```bash uv run --with opencv-python --with pyobjc-framework-Vision \ --with pyobjc-framework-Quartz python refine_timestamps.py \ video.mp4 times.tsv <recorded_total> <slide,slide,...> ``` Prints `slide seconds MM:SS` per target — the first second the slide is confirmed on screen. 5. **Spot-check visually** — extract a frame at one computed timestamp, Read it, and confirm the question slide (and its `n/N` counter) is showing. 6. **Add the links** in `book/templates/course.html`: inside the row's `<ul class="qa-questions">`, append after each question anchor: ```html <a class="qa-time" href="https://www.youtube.com/watch?v=<ID>&t=<S>s" target="_blank" rel="noopener noreferrer">(M:SS)</a> ``` The `.qa-time` styling (red, matching the Watch action, light + dark) already exists in the template. ## Gotchas - OCR misreads at 480p look plausible ("18/28" → "16/20") — always use 1080p. - Filter readings by the expected total to drop misreads; demo segments (browser windows, etc.) legitimately return no counter. - Timestamps are per-recording: if a deck is re-recorded, re-run the sweep; if the deck markdown changes after recording, slide anchors on the course page may also need re-checking against the published build.