[FFmpeg-user] Size of cropped video segments

Moritz Barsnick barsnick at gmx.net
Thu Feb 9 18:25:09 EET 2017

On Thu, Feb 09, 2017 at 09:39:13 -0500, Chao Zhou wrote:
> > I expect the size of total 9 segments will be larger than the source video
> When encoding source video, the P/B frame can search previous full size I
> frame to reduce their encoding size. After cropping, each of these video
> segments has a smaller size of I frame. Then, the following P/B frame have
> to encode more redundant data for correct recovery on the decoder side. Is
> it right? I didn't change any bitrate or video quality parameters during
> cropping process.

You did change them, because you decode the original video and then
reencode it with ffmpeg/libx264. The resulting bitrate depends on
various parameters, which are not related to the original video:
encoding profile and level, CRF value, various other encoding
parameters, and last not least encoding efficiency of the utilized
encoder. You can probably vary the bitrate by about a factor of 10x
with little variation in the visual output quality. (YMMV)

If you use a different H.264 encoder, you will most likely get
different results as well!

To get back to your original question as to why your observed relation
no longer applied to "larger scale" videos: Perhaps the originals were
*less* efficiently encoded, so the sum of your segment encodings became
relatively smaller. Or they were different material (less motion,
animation, or so) leading to better encoding efficiency. Or - long
shot, and I doubt it - something else made the encoder more efficient
at the higher resolutions.

> [libx264 @ 0x332d380] frame I:4     Avg QP:18.31  size: 33702
> [libx264 @ 0x332d380] frame P:261   Avg QP:20.60  size:  3323
> [libx264 @ 0x332d380] frame B:709   Avg QP:25.26  size:   268
> [libx264 @ 0x332d380] consecutive B-frames:  2.8%  0.4%  0.3% 96.5%

You might want to compare some of these statistics. On the other hand,
they may be misleading - I don't really know. ;-)


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