don't understand the Avidemux Wiki about I-frames option

[T-buch]

Trying to understand the Wiki-section about Frame -> i-frames (https://www.avidemux.org/admWiki/doku.php?id=tutorial:h.264)


If I want to recode a 25 fps videofile and sets the GOP size to max 25 and min. 25, can I then count on the result to be 25 key-frames per second?

[eumagga0x2a]

This would set the GOP size (the interval between two IDR frames) to 25 = 1 second between two seek points (1 keyframe per second) resulting in a significantly worse compression rate than with more moderate settings (longer GOPs).

[T-buch]

This would set the GOP size (the interval between two IDR frames) to 25 = 1 second between two seek points (1 keyframe per second) resulting in a significantly worse compression rate than with more moderate settings (longer GOPs).

Thanks - It makes sense. So the figure represents a fps?

[eumagga0x2a]

No, fps is number of pictures per second (ideally, the time increment between frames is a constant), GOP size is the number of frames between two keyframes plus one.

[T-buch]

This would set the GOP size (the interval between two IDR frames) to 25 = 1 second between two seek points (1 keyframe per second) resulting in a significantly worse compression rate than with more moderate settings (longer GOPs).

Why is there 1 key frame / I-frame per second?

[eumagga0x2a]

Because you specified for some reason a fixed (mix=max) number of frames between keyframes equal 25. At FPS = 25, it means you want 1 keyframe each second. At FPS = 50 it would result in a keyframe every 0.5 seconds. Normally, the encoder decides based on the content of pictures when a keyframe should be used.