gupsammy/extract-frames

Extract Frames

Arguments

Parse the user's request for:

• video path (required): path to the video file
• --last: extract last frame of each shot instead of first
• --first --last or --all: extract both first and last frames
• --shots N,N,N: only process specific shot numbers (1-indexed)
• --threshold X: override auto-detected threshold (skip user confirmation)

Default: extract first frame of every shot.

Workflow

Phase 0: Setup

1. Validate the video file exists.
2. Get video metadata via ffprobe:

ffprobe -v error -show_entries format=duration -show_entries stream=r_frame_rate,width,height,codec_name -of default "$INPUT"

3. Parse fps (needed for last-frame calculation) and duration.
4. Create output directory next to the video: {video_basename}_frames/. If it already exists, check for scores.txt — if present, skip Phase 1.

Phase 1: Score Dump

Dump per-frame scene scores for the entire video:

ffmpeg -i "$INPUT" -vf "select='gte(scene,0)',metadata=print:file=$OUTPUT_DIR/scores.txt" -fps_mode vfr -f null - 2>&1

This is the expensive step. The output file scores.txt contains blocks like:

frame:0    pts:0       pts_time:0.000000
lavfi.scene_score=0.000000

Phase 2: Cut Detection

Parse scores.txt with this awk one-liner to get distribution + all candidate frames:

awk '
/pts_time/ { split($0, a, "pts_time:"); ts=a[2]+0 }
/scene_score/ { split($0, a, "="); score=a[2]+0;
  if (score < 0.01) b1++;
  else if (score < 0.05) b2++;
  else if (score < 0.10) b3++;
  else if (score < 0.20) b4++;
  else b5++;
  total++;
  if (score > max) max=score;
  if (score > 0.05) printf "  ts=%.3fs  score=%.6f\n", ts, score;
}
END {
  print "\n--- Distribution ---";
  printf "< 0.01:      %d (%.1f%%)\n", b1, b1/total*100;
  printf "0.01 - 0.05: %d (%.1f%%)\n", b2, b2/total*100;
  printf "0.05 - 0.10: %d (%.1f%%)\n", b3, b3/total*100;
  printf "0.10 - 0.20: %d (%.1f%%)\n", b4, b4/total*100;
  printf "0.20+:       %d (%.1f%%)\n", b5, b5/total*100;
  printf "Max score:   %.6f\n", max;
}' "$OUTPUT_DIR/scores.txt"

Step 2a — Startup artifact filter: Discard any frames in the first 0.5s where score > 0.05 (a fixed preliminary value — the final user-confirmed threshold is not known yet). Fade-ins from black or codec initialization commonly produce score=1.0 spikes at t=0.03-0.08s that are not real cuts. After discarding these, recompute max score from the remaining frames.

Step 2b — Branch on max score. If max score (after startup filter) < 0.05, the video has no cuts:

• Report: "No shot boundaries detected — single continuous shot."
• Extract only frame at t=0 (or t=1.0s if t=0 is a black frame — check file size, <50KB indicates black).
• Skip threshold confirmation.

Step 2c — Gap analysis and threshold. If max score >= 0.05, use the raw candidates (all frames with score > 0.05, after startup filter) to find the noise ceiling and the lowest-scoring candidate. Place the proposed threshold at the midpoint of the gap. Present to user via AskUserQuestion:

• Score distribution summary
• Gap analysis (noise ceiling → lowest cut, gap width)
• Proposed threshold and resulting shot count
• Options: Accept proposed (Recommended), Lower threshold, Higher threshold, Custom value

If --threshold was provided, skip confirmation and set $THRESHOLD_VALUE directly. Otherwise set $THRESHOLD_VALUE to the user-confirmed value before proceeding.

Step 2d — Run-based dedup. Using $THRESHOLD_VALUE from Step 2c, identify cut points. A "run" is a sequence of consecutive frames where every frame scores above threshold. The principle: an aftershock immediately follows its parent cut (consecutive frames both above threshold), while a real cut always rises from the noise floor (preceded by at least one below-threshold frame).

awk -v THRESHOLD="$THRESHOLD_VALUE" '
/pts_time/ { split($0, a, "pts_time:"); ts=a[2]+0 }
/scene_score/ { split($0, a, "="); score=a[2]+0;
  if (score > THRESHOLD) {
    if (!in_run) { run_best_ts = ts; run_best_score = score; in_run = 1; }
    else if (score > run_best_score) { run_best_ts = ts; run_best_score = score; }
  } else {
    if (in_run) { printf "%.3f %.6f\n", run_best_ts, run_best_score; in_run = 0; }
  }
}
END { if (in_run) printf "%.3f %.6f\n", run_best_ts, run_best_score }
' "$OUTPUT_DIR/scores.txt"

This keeps the peak frame of each run and discards aftershocks within the same run. It correctly handles:

• Standard cuts: isolated spikes → each kept (run length 1)
• Cuts with aftershocks: 2-3 consecutive high frames → peak kept, echoes discarded
• Rapid montages: each cut separated by noise frames → all kept, even at 0.12s intervals

Phase 3: Frame Extraction

For each detected cut point (plus t=0 for shot 1):

First frame (default):

ffmpeg -y -ss $TIMESTAMP -i "$INPUT" -frames:v 1 -update 1 "$OUTPUT_DIR/shot_${NUM}_${TIME}s.png"

Last frame (--last):

• For shots 1 through N-1: last_time = next_shot_timestamp - (1/fps)
• For final shot: last_time = duration - (2/fps) (use 2 frames back, not 1 — seeking to duration - 1/fps can produce empty files near the end of some videos)

ffmpeg -y -ss $LAST_TIME -i "$INPUT" -frames:v 1 -update 1 "$OUTPUT_DIR/shot_${NUM}_last_${TIME}s.png"

If a last-frame extraction produces an empty file (0 bytes), back off by another frame and retry.

Both (--first --last or --all): extract both per shot.

Filtered (--shots 3,5,7): only extract for the specified shot numbers.

Output

• Directory: {video_basename}_frames/ next to the input video
• First frames: shot_01_0.00s.png, shot_02_1.63s.png, ...
• Last frames: shot_01_last_1.60s.png, shot_02_last_3.77s.png, ...
• scores.txt always retained for re-runs at different thresholds

Report to user: total shots detected, shot list with timestamps, output directory path.

Re-run Behavior

If scores.txt already exists in the output directory, skip Phase 1 entirely and go straight to Phase 2 analysis. This makes threshold iteration instant — the user can re-run with --threshold 0.15 without waiting for the score dump again.

Shell Portability

Use pipe-based loops for frame extraction instead of array syntax (zsh handles for over arrays differently than bash):

echo "0.000 2.480 4.280" | tr ' ' '\n' | awk '{printf "%02d %s\n", NR, $1}' | while read LABEL TS; do
  TS_FMT=$(printf "%.2f" "$TS")
  ffmpeg -y -ss "$TS" -i "$INPUT" -frames:v 1 -update 1 "$OUTPUT_DIR/shot_${LABEL}_${TS_FMT}s.png" 2>/dev/null
done

Edge Cases

• Single continuous shots: Handled by step 2b. Common in fashion videos with rack-focus reveals, slow wardrobe progression, or single-take lifestyle shots.
• Startup artifacts: Fade-ins from black produce score=1.0 at t=0.03-0.08s. Step 2a discards frames with score > 0.05 in the first 0.5s. If t=0 itself is a black frame (PNG < 50KB), extract at t=1.0s instead.
• Aftershock spikes: Consecutive above-threshold frames (a cut + its echo). Run-based dedup in step 2d keeps only the peak of each run — no temporal window needed.
• Rapid montages: Videos where shots are 2-4 frames long (0.08-0.16s). Each cut rises from the noise floor with 1-2 noise frames between spikes. Run-based dedup correctly preserves every cut because no two spikes are frame-adjacent. Report montage segments to the user: "Detected rapid montage from Xs-Ys with N shots."
• Dissolves/fades: Score gradually ramps over multiple frames — forms a single run. Run-based dedup takes the peak frame as the cut point.
• Empty file on seek: If ffmpeg produces a 0-byte PNG (common near video end), back off by one frame interval and retry.