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supermule 05-04-2007 09:44 PM

HCMatic: Scene change detection
 
Why is the HCmatic not using the scene change detection in its prediction routine and then the encodes ???

KVCD specifications suggests usage of scene change detection, if the encoder permits.

Dialhot 05-05-2007 03:40 AM

Because SCD completely screw the prediction and I always said that it's nut to use it. It was the case with CQMatic, and it's still the case.

Prediction is based on slices (in case of HCMatic : 1 second each minute of movie). At each slice end there is a very big chance that the SCD algorythm detects a scene change and insert an I frame. The result is that the % of I frames in the encoded sample is much more important than the reality in the whole movie. Thus the sample size is bigger that it should be, and in the end the Q value found is way under the ideal value.

rds_correia 05-05-2007 06:24 AM

Quote:

Originally Posted by Dialhot
Because SCD completely screw the prediction and I always said that it's nut to use it. It was the case with CQMatic, and it's still the case.

Exactly!
It doesn't matter if you're using HCMatic, CQMatic or even a manual prediction approach.
If you are running a prediction routine you have to disable the scene change detection otherwise you'll get mistaken output figures for Q.
As for enabling SCD for final encoding I just don't know...
Sometimes I do that and I get a bit oversized movies that I have to run through DVDShrink or then reencode but sometimes I do that and the outcome is pretty close to the final predicted sizes.
So IMHO there's no sense in using SCD at all.
Cheers

supermule 05-05-2007 08:24 AM

Quote:

Originally Posted by Dialhot
Because SCD completely screw the prediction and I always said that it's nut to use it. It was the case with CQMatic, and it's still the case.

Prediction is based on slices (in case of HCMatic : 1 second each minute of movie). At each slice end there is a very big chance that the SCD algorythm detects a scene change and insert an I frame. The result is that the % of I frames in the encoded sample is much more important than the reality in the whole movie. Thus the sample size is bigger that it should be, and in the end the Q value found is way under the ideal value.

Thanks for the info!!

Boulder 05-11-2007 12:07 AM

Quote:

Originally Posted by rds_correia
As for enabling SCD for final encoding I just don't know...
Sometimes I do that and I get a bit oversized movies that I have to run through DVDShrink or then reencode but sometimes I do that and the outcome is pretty close to the final predicted sizes.
So IMHO there's no sense in using SCD at all.
Cheers

It's strongly recommended that scene change detection is enabled. HC has a very good algorithm which will catch them very well. Otherwise you might end up in situations where a scene change is P-B which means that there will be severe degradation of the image quality before the next I-frame arrives.

Dialhot 05-11-2007 02:27 AM

Quote:

Originally Posted by Boulder
Otherwise you might end up in situations where a scene change is P-B which means that there will be severe degradation of the image quality before the next I-frame arrives.

You are right. I never think about that. Not sure this is noticiable, at least, I never have been chocked by that ((with my eyes, while watching at the video I mean), but I will think about activating it.

Boulder 05-11-2007 03:09 AM

In prediction, it is better to disable it as you already had figured. But in the regular encode, there's really no good reason not to use it..I recently did a test in which I noticed what happens when scd is not used or doesn't work. There was a fast action scene followed by a lo-motion scene, the scene change was P-B as CCE didn't for some reason put an I-frame there. The first few frames of the latter scene looked really terrible as they were predicted from incorrect motion vectors due to the missing I-frame. It was quite noticable during playback, in fact it was why I started looking at the scene more thoroughly.

rds_correia 05-11-2007 11:32 PM

Thanks for your comments Boulder :).
Cheers

Dialhot 05-19-2007 08:21 AM

I'm currently regarding a video that I did not encode where SCD is obviously off, and the result is awfull. Each SC the frame is at best pixellated, at worst awfullyblended.
I will add the SCD to final encode in HCMatic once I verify that does not screw up the final filesize too much.


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