Crop Video

No. The work happens inside this tab: the browser's own encoder for an MP4, or an engine downloaded from this site for other formats. Either way the file is read straight from your disk, and your network tab will show nothing going up.

Trimming can copy the existing frames across untouched. Cropping changes the picture, so every frame has to be decoded, cut down and encoded again. For an MP4 the browser's own encoder does it: a 30-second 1280 × 720 clip took 1.2 seconds on our test machine. Other formats go through the 10 MB engine instead, and the same clip took 19 seconds that way. A 4K clip takes considerably longer either way.

H.264, the codec in almost every MP4, stores the colour channels at half the width and half the height of the brightness channel. An odd side has no half, so the encoder refuses it. The box rounds each side down to the next even number and shows you that number, so what you read is what you get.

An MP4 with H.264 video, with the index at the front so it starts playing before it has finished downloading. When the source is an MP4 the sound track is copied across untouched, so it loses nothing; the engine path re-encodes it as 160 kbps AAC.

An MP4 is read a few frames at a time rather than all at once, so there is no 2 GB ceiling on it — we cropped a 2.2 GB, eight-minute 1080p recording here in 26 seconds. Other formats go through the video engine, which does need the whole file at once, and no browser can read more than about 2 GB in one piece; you get a plain message rather than a frozen tab.

It is a web page, so yes — Chrome, Safari, Edge and Firefox on any of them. The one real difference is memory: a phone has much less of it, so a long 4K clip that goes through on a laptop can run out on a phone. You get a message when that happens.

No — the job is the tab, so closing it ends the job. You can switch to another tab and come back; we measured a hidden tab and it ran at the same speed.