[Xvid-devel] PSNR, Bitrate of Full Search and other algorithms

[Michael Militzer]

Hi,

it depends on how your full search is implemented and how you determined
your test points on the R-D curve (rate-controlled, at constant QP?). You
didn't mention about this.

In general though, it is not unusual at all that an advanced motion
estimation algorithm beats a full search strategy. It is well known that
a naive full search (so simply selecting the match that minimizes a
simple distortion metric like SAD or SSE) produces highly irregular and
erratic motion vector fields. A more advanced motion estimation algorithm
like a predictive search strategy usually produces much smoother motion
vector fields that better correlate with the underlaying "true" motion,
can be more effectively predicted and are therefore cheaper to code.

The savings in coding the motion vectors can be so large that an advanced
motion estimation algorithm produces a lower overall rate than a full
search approach (like you observed). As to PSNR: Note that PSNR typically
measures the distortion of the reconstructed pictures. In contrast, SAD or
SSE that you use during your full search capture the absolute difference
between the current block and its match in the reference frame (so before
any reconstruction). The motion vectors that minimize SAD/SSE during motion
estimation do not necessarily produce reconstructed frames with best PSNR.

Best regards,
Michael


Quoting Chien Tran <chien.study at gmail.com>:

> Hi guys,
> Sorry if this question make you feel uncomfortable because it is not
> related to XVID, but more related to Motion Estimation algorithm in video
> compression. I am implementing new search algorithm that is used in motion
> estimation. The interesting point is that the new algorithm give the bigger
> PSNR and lower bit rate in comparison with Full Search algorithm. I tested
> this new algorithm on different video sequences here
> http://trace.eas.asu.edu/yuv/ and I still got the same results (bigger PSNR
> and lower bit rate than Full Search). What do you think about this? Is it
> normal or just a bugs in my algorithm? and do you guys think it is possible
> that sometimes new algorithm can give bigger PSNR and lower bit rate than
> Full Search algorithm? Has any of you experienced this kind of strange
> result before?
> Thanks in advance!
>
> --
> Best wishes,
>
> Tran Xuan Chien
> University of Information Technology
> Phone: (+84) 1692 468 154
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