What does forcing key frames mean?

Question

What does forcing key frames mean?

As per the doc

-force_key_frames[:stream_specifier] expr:expr (output,per-stream) Force key frames at the specified timestamps, more precisely at the first frames after each specified time. If the argument is prefixed with expr:, the string expr is interpreted like an expression and is evaluated for each frame. A key frame is forced in case the evaluation is non-zero.

Still I am not able to understand what does forcing key frames at specified timestamp means and what is its use?I can see this command is used while segmenting video.What is its purpose there?

Answer 1

A typical video codec uses temporal compression i.e. most frames only store the difference with respect to earlier (and in some cases, future) frames. So, in order to decode these frames, earlier frames have to be referenced, in order to generate a full image. In short, keyframes are frames which don't rely on other frames for decoding, and which other frames rely on in order to get decoded.

If a video has to be cut or segmented, without transcoding (recompression), then the segmenting can only occur at keyframes, so that the first frame of a segment is a keyframe. If this were not the case, then the frames of a segment till the next keyframe could not be played.

A encoder like x264 typically generates keyframes only if it detects that a scene change has occurred*. This isn't conducive for segmentation, as the keyframes may be generated at irregular intervals. In order to ensure that segments of identical and predictable lengths can be made, the force_key_frames option can be used to ensure desired keyframe placement.

-force_key_frames expr:gte(t,n_forced*5) forces a keyframe at t=5,10,15 seconds...

The GOP size option g is another method to ensure keyframe placement, e.g. -g 50 forces a keyframe every 50 frames.

*subject to minimum and maximum keyframe distance parameters.