Keyframes relation to file size

[Ben Waggoner]

Keyframes take up more relative space, but if you have a fixed bitrate, what happens is that the keyframes get more bits and the delta frames get fewer. So the file size will be the same, but you’ll get lower compression efficiency.

As to why it’s getting LARGER, not sure. Is it a significant amount? Are you using 2-pass encoding (you should be as long as you’re not doing real-time streaming via RTSP).

My Book: https://www.benwaggoner.com/books.htm
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Compression Class at Stanford: https://www.digitalmediaacademy.org/compression.html

[Travis Dao]

Ben,

So if I have a fixed bitrate than the file size should remain the same regardless of the number(frequency) of keyframes?
I’m using 2 pass VBR and the same bitate. These are my test results:

FLV 10(fps)
1,626,514 bytes — keyframe every 10 frames
1,626,955 bytes — keyframe every 20 frames
1,627,166 bytes — keyframe every 50 frames

QT 15 (fps)
2,955,391 bytes — keyframe every 15 frames
2,712,897 bytes — keyframe every 30 frames
2,831,045 bytes — keyframe every 75 frames
2,871,357 bytes — keyframe every 150 frames

T

[Charles Simonson]

Those numbers sound about right and what Ben stated is a good explanation of how keyframes are supposed to work. It is almost impossible to get the same file-size for two encodes at even the exact same settings. And as long as your results aren’t producing sizes that are drastically different (over 10%), then I would say you are doing Ok and not to worry about it.