Stack two videos: match events before resetting their origins
Resetting two video timestamp origins makes their local starts equal; it does not prove that those starts depict the same event. Before stacking recordings, identify matching content and choose deliberate geometry and cadence. Verify the saved comparison’s action, ending and resource scope rather than accepting a spatially plausible preview alone.
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Identify matching source events and intervals
Retain both authorized originals and record which version and viewpoint each represents. Find a shared visible or audible landmark, then check another later event. Similar starts or equal durations are weaker evidence than actual matching action. If one camera missed the event, document that absence instead of inventing coverage through a timestamp reset.
Choose the source-relative intervals and required output purpose. A side-by-side review movie is a composite, not two independent retained masters. Decide whether the task needs sound from one source, a reviewed mix or a silent diagnostic. Keep the originals and their time context even when the composite uses local zero.
Normalize only the intended geometry and cadence
For a documented shared event at twelve seconds in one source and eighteen in the other, a bounded square-pixel eight-bit SDR test can use ffmpeg -n -ss 12 -t 5 -i left.mp4 -ss 18 -t 5 -i right.mp4 -filter_complex "[0:v:0] setpts=PTS-STARTPTS, scale=-2: 360, setsar=1, fps=25, format=yuv420p[l]; [1:v:0] setpts=PTS-STARTPTS, scale=-2: 360, setsar=1, fps=25, format=yuv420p[r]; [l] [r] hstack=inputs=2: shortest=1[v]" -map "[v]" -an -c:v libx264 review-stack.mp4. Confirm those actual source assumptions first.
The example matches heights and compatible pixel representations while deliberately producing a 25-frame-per-second video-only derivative. Its rate filtering may duplicate or drop pictures; it does not add captured detail. Do not apply the same representation conversion blindly to HDR, anamorphic or higher-precision input. The selected times are illustrative matching-event assumptions, not evidence that arbitrary sources align at those values.
Verify the composite and any complete delivery
Open the saved output and compare the shared action across both panels, including a later landmark and the ending. Inspect text, aspect ratio and the actual resulting dimensions. A shortest-input ending policy can omit later coverage from the other source; record that scope instead of calling the composite a complete backup of both recordings.
If the accepted movie requires sound, choose and map the actual permitted soundtrack deliberately and verify its alignment. A composite’s picture timing does not certify every audio or caption resource. Keep the originals and accepted graph settings, and label the scale and cadence changes. SocialVidDownloader does not expose arbitrary multi-input stack graphs or guarantee that independently recorded events synchronize automatically. This external technical handoff succeeds only when the actual matching content and receiving artifact have been checked.
Worked example
Example: two cameras begin at different times but record the same clap. The editor selects the corresponding source positions, resets the local origins and checks a later action before accepting the stack. Equal local zeroes without the shared-event evidence are never reported as verified synchronization.
Related guidance
Sources and verification
Reviewed 2026-10-13. Product behavior is limited to available source formats and provider access; it is not a promise that every public link works.