Trim a bounded reference-video time window and verify the kept frames

01Prepare environment and inputs

1. Use Node.js plus FFmpeg and set FFMPEG_BIN if needed. The fixture builds a small 320×180, ten-FPS, three-second original reference clip.
2. Read window.json: source duration is measured as 30 decoded frames at the fixture’s declared constant rate; selected start is 0.5 and end is 1.5 seconds.
3. Choose a new output directory. Use the provided CFR fixture first; deriving the usable end of an arbitrary VFR or nonzero-start reference requires its own timestamp inventory.

02Run the recipe

Run command

node run-recipe.mjs --out ./my-run

1. Establish the available source interval

   Fully decode the original fixture and confirm thirty frames at ten FPS, establishing its three-second controlled interval. The raw construction buffer is temporary and removed in finally. Do not use this controlled CFR duration rule as an unqualified VFR duration estimator.

2. Reject invalid boundaries before encoding

   The local window validator requires finite numbers, start ≥ 0, end > start and end ≤ measured source duration. It executes separate negative cases for a negative start, equal boundaries, reversed boundaries, a past-end request and NaN.

3. Trim and reset the output time origin

   Apply trim=start=0.5:end=1.5,setpts=PTS-STARTPTS and re-encode the video. The trim filter keeps the continuous source interval, while setpts makes the excerpt start at zero. The end boundary excludes the source frame beginning at 1.5 seconds.

4. Compare both endpoints and every output timestamp

   Decode the result to ten frames and compare its first/last RGB frames against source indices 5 and 14. Parse actual showinfo output to verify PTS 0.0 through 0.9 in 0.1-second steps. The executed fixture measured zero endpoint channel difference.

03Check the result

• The source actually decodes to thirty 320×180 frames at the controlled ten-FPS rate.
• The 0.5–1.5-second window passes the finite-number and interval rules.
• The excerpt decodes to exactly ten frames, corresponding to one second at ten FPS.
• First and last output frames match source indices 5 and 14 within four channel values; the recorded differences are zero.
• The observed output timestamps begin at zero and finish at 0.9 seconds.
• All five invalid-window cases are rejected and the eleven-check receipt retains actual showinfo evidence.

04Troubleshoot and deliver

Keep the requested window, measured source interval and decoded endpoint checks together. The small CFR fixture proves this trim operation and does not establish every real reference clip’s timestamp behaviour.

05Download scripts and fixtures

• Original task-specific local-fixture diagram ↓
• Complete runnable kit with original inputs and real receipts ↓
• Task-specific standard-library fixture runner ↓
• Bounded local execution and receipt helpers ↓
• English execution and acceptance guide ↓
• English machine-readable task brief ↓
• Chinese execution and acceptance guide ↓
• Chinese machine-readable task brief ↓
• Actual complete passing execution receipt ↓
• Actual expected-rejection receipt ↓
• Named task-specific check results ↓
• SHA-256 inventory for distributed files ↓
• Recorded fixture: window.json ↓
• Recorded fixture: three-second-reference.mp4 ↓
• Recorded fixture: reference-0.5-to-1.5.mp4 ↓
• Recorded fixture: trim-showinfo.txt ↓