Short answer: if every clip has identical stream parameters, use the concat demuxer with -c copy . It is instant and lossless. If anything differs (resolution, frame rate, SAR, pixel format, audio sample rate or channel count), use the concat filter and normalize each input first. The trap is that the demuxer does not refuse mismatched clips: it writes a file whose header looks fine while half the frames have a different size. This post shows that with real clips, gives a small Python wrapper that picks the right path, and pins the behaviour down with 15 pytest checks.

The test material is six real clips generated with PixVerse V6 (public examples from Voor's PixVerse V6 page): a 1920×1080 night market, a 720×1280 vertical underwater shot, a 1280×560 multi-shot bike ride and two 960×960 fox clips. All are H.264 High + AAC at 24 fps. That sounds compatible. It is not.

Real clips in, one geometry out: the vertical clip is pillarboxed, the 21:9 clip letterboxed, and the durations add up exactly.

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