Mpg4/h264 settings - GoP (Group of pics) - how far can/should you go?

Hi all,

i was investigating how much the GOP-setting in the export setting (advanced - video) matters in respect to resulting file size. I tried 15, 30, 60, 120, 300 and 900 GOP. The decrease in file size from 15 to 30 or 30 to 60 is about 4…5% (in my case). From 60 to 120 its around 4% and from 120 to 300 less than 3%. From 300 to 900 its only around 1% or less.
From the visual result to my eyes i hardly could see any difference and i tried several quality settings as well. Reduction in quality pays much more to size reduction than GOP but also starts to insert visual artefacts below around 50.

What are reasonable values for the GOP setting? I would guess 15…60, or in some cases even 300 but it probably makes any later edits more difficult resulting in additional artefacts when GOP increases much. Is my understanding roughly correct?

In general small GOP sizes create larger files that are of poorer quality. However they are more resilient to errors. Often GOP sizes are in the 1 to 2 second range e.g. 30 to 60 for 30fps. Small GOPs are fine for fast moving content, where there are large changes from frame to frame.

For a good description of GOPs and the reasoning behind them see:

2 Likes

Thanks a lot, Elusien!
One of the best articles about compression regarding GOP so far!
I didn’t know there is such a big difference in size for B and P-frames - amazing.
Still an open question: how many B’s should be inbetween the P’s for a good compromise between resulting file size and video quality, given a fixed bitrate (Mbps)?