Say an editor asks you for a transparent clip with the person already cut out. I had one: an 8-second, 540×960 WebM from ImgIng ( https://imging.ai/ ), 2,394,232 bytes, audio included. Converting it to something an editing app accepts made the file 26 to 38 times bigger. I live on free tiers and I hate paying for egress, so I did the arithmetic before picking a format.

ImgIng's video matting is Beta, runs in desktop Chrome or Edge, processes locally, and does not work on phones. Transparent output is WebM only, which is why this question exists at all. The footage is a VOA Learning English clip from Wikimedia Commons (public domain), cut to its first 8 seconds. Everything below is that one clip, one run, FFmpeg 7.1 on an M4 Mac. MB here means 1,000,000 bytes. The ImgIng UI counts 1,048,576, so its numbers read smaller.

Three formats keep the alpha at different prices

I converted the same WebM to ProRes 4444, a PNG sequence and QuickTime Animation (qtrle), then read the alpha back from each file.…