[Ffmpeg-devel] size of encoded file not continous parameter of 'vqscale'

[Darko Veberic]

Stefan Gehrer wrote:
> Darko Veberic wrote:
>> i am usually encoding video with the following mencoder options:
>> -lavcopts vcodec=mpeg4:vqscale=$vq:vhq:v4mv:trell:autoaspect:threads=4
>>
>> i have observed strange non-continuous behavior of the resulting file
>> size with respect to the vbr quality parameter $vq.
>>
>> for example, the file size for vq=4.49 is 911M and for vq=792M (while
>> keeping the audio settings constant). i have noticed the jumps occur
>> around the x.5 values for the vbr quality... in such cases it is almost
>> impossible to get the file size right (ie, close to 700M)...
>>
>> is this a bug (or a feature) ?
>>   
> vqscale in MPEG4 is whole numbers only (2-31 I believe), and unfortunately
> the range of reasonable quality encodes is in the range somewhere like 2
> to 6.
> So a step from one vqscale to the next has a huge impact on both quality
> and
> file size. If you need to hit a certain file size, do not use vqscale
> but instead
> make use of rate control.
> 
> Regards
> Stefan Gehrer

on the contrary, the vqscale is a floating point number (reported by
mencoder with 6 decimal places). i am attaching a plot of file size vs
vqscale value for some small movie chunk. as you can clearly see, the
file size is a nice and continuous function of vqscale except around x.5
points where you get large jumps. if this jump is occuring around 700M
we are unable to tweak the file size to match the cd size unless
satisfied with substantially smaller size...

best,
-- 
darko.

Dr. Darko Veberic ------------------
Laboratory for Astroparticle Physics
University of Nova Gorica
Vipavska 13, POB 301
SI-5001 Nova Gorica, Slovenia
mobile: +386 41 860 861
 phone: +386 5 3315 255
   fax: +386 5 3315 240
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