Modern codecs save space by describing most frames as "the previous frame, plus these changes". Those frames cannot be decoded without the previous frames in the same group. Keyframes are the exception: they store a complete still picture and let the decoder start fresh.
When you scrub a YouTube video, the player jumps to the nearest keyframe and decodes forward from there. This is why scrubbing in some heavily-compressed videos feels imprecise — the player can't actually start at your exact frame, only at the nearest keyframe (typically every 5 seconds in YouTube uploads).
For lossless time-range cuts, the cut points must align to keyframes. If you ask for a clip starting at 0:23 and the nearest keyframe is at 0:21, a lossless cut starts at 0:21. To start exactly at 0:23 the clip needs to be re-encoded.
Common questions
Why do video clips sometimes start a few seconds early?
Related terms
GOP (group of pictures)
A GOP (group of pictures) is a sequence of video frames that starts with a keyframe and continues with frames described as differences from earlier frames.
FPS (frames per second)
FPS (frames per second) is the number of distinct still images displayed per second of video.
Codec
A codec is the algorithm that encodes (compresses) and decodes raw audio or video into a smaller stream.
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