[FFmpeg-user] Size of cropped video segments

[Chao Zhou]

Hi Moritz,

Thank you!

I do have some tests that reencoding source videos with same ffmpeg/libx264
setting first, then cropped the video to small segments. The problem
remains the same. However, your opinion about less motion or animation
enlightens me; If the moving object of the source video just stays within
one of the small segments, others of segments may have better encoding
efficiency since they are more static.

Hopefully, this guess can be proven by some experts ~ :-)

Chao


On Thu, Feb 9, 2017 at 11:25 AM, Moritz Barsnick <barsnick at gmx.net> wrote:

> 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. ;-)
>
> Moritz
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