[MEncoder-users] Pulsating I-frames (keyint)

[Nicolas Hesler]

RC wrote:
> On Mon, 11 Jun 2007 19:21:08 -0400
> <nicolas.hesler at sheridanc.on.ca> wrote:
>
>   
>> I've noticed that on certain videos,  pulsating occurs on keyframes
>> (I-frames),and especially visible in darker picture areas.  
>>     
>
> That's common after vpass=1, but I've never noticed that after the
> second pass.
>
>   
>> 1.  Can someone help me with other parameters that might solve the
>> issue in a different way? (From what I gather it has something to do
>> with manipulating quantizers? But with so many parameters and so many
>> that seemingly affect one another, I don't know where to begin.)
>>     
>
> Try vqblur=0.2:vqcomp=0.8 OR vqblur=0.8:vqcomp=0.2, one of the two
> should improve things, and I'd bet on the first.
>
> I also recomend trying once without the *cmp=2 options to see if it's
> causing the problem.
>
>   
>> 2.  Are there drawbacks to setting vi_qfactor and vi_qoffset to the
>> values I chose? (Am I sacrificing quality?)
>>     
>
> Any time you change how the available bits are allocated, you are
> sacrificing quality in one place, for better quality in another.  It's
> all about perception, so if it looks better, it is better.
>
>   
Ok, so I have a few more details:

vqblur, vqcomp provide little in the way of fixing the pulsating.
The only settings that seem to fix it well enough to eliminate the 
pulsating are lmin=4 or higher and nr=400-600.  Unfortunately, those 
settings become really lossy.   
Increasing the min and max bitrate does nothing to eliminate the effect, 
even if i set them ridiculously high (20Mbs).
although nr seems to help (but blurs the image) the denoise video 
filters don't help eliminate the pulsating.

If I crop up to half the image (for testing purposes), the effect is 
eliminated, I'm assuming because of the way the encoder reads the entire 
image.

does any of this info offer any clues?