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:

  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):

   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):

   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):

   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:

   <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.