Agree overall, but a couple nits: > The "slow way" actually decodes the frames, that is, it takes the blobs of compressed data and turns them into actual pixel

Agree overall, but a couple nits: > The "slow way" actually decodes the frames, that is, it takes the blobs of compressed data and turns them into actual pixels, which especially in the case of 4K video, is very slow. The "slow way" decodes and re-encodes the frames. Decoding is a little slow. Encoding is very very slow if done at high quality. (And even high quality is still lossy.) > The reason the "fast way" might be less accurate is that the frame you're asking for might not be possible to obtain without decoding the video. This is often possible to solve with an .mp4 "edit list". You include more data than is expected to be displayed along with instructions for the player to skip part of it. One obvious caveat is that the person you send the video from can remove the edit list, so the hidden frames shouldn't be anything you want to redact for privacy.