[x265] Under VBV rc mode , increasing the vbv-maxrate or vbv-bufsize has no or very little effect on the final result

[Mario *LigH* Rohkrämer]

I believe the reason is the relation between the three values. You can  
fill the buffer faster than the video target bitrate, so it needs only few  
seconds to fill a previously empty buffer, and even less if the buffer was  
already pre-filled. And then you compare it to a case where you can fill  
it even faster than before, so there is even less problem; the encoder is  
almost unrestricted by the VBV, only restricted by the target bitrate.

You will certainly see a difference in cases where filling the buffer  
would take longer, and the video bitrate would exceed this fill rate for a  
longer duration if it was not limited by the fill rate: Then the encoder  
needs to reduce its target bitrate and spend less than the optimum for the  
desired quality.

 From my experience as DVD Video author, we had cases of close to optimum  
utilizations of videos just even rejected by the authoring tool, where the  
analysis of the scene in question revealed per GOP spikes of 13-14 Mbps  
(VBV max fill rate of DVD Video is 9.8 Mbps) which the VBV simulator of  
the encoder found acceptable, but the VBV simulator in the authoring tool  
could not accept (possibly due to an unfortunate burst of subtitles).  
Agerage bitrates tell little, their distribution over the VBV filling  
duration makes the difference.

One of the most demanding scenes ever was the beginning of "Cruel  
Intentions" with a flight over a graveyard, filmed downwards: Several  
seconds of heavy motion right at the start. It revealed a few rare DVD  
player models which possibly had less than the specified 224 KB of  
decoding buffer built in.


Am 20.10.2015, 04:59 Uhr, schrieb fangzhen <wintelx264 at 163.com>:

> Recently, I am testing the x265 codes  and mostly on VBV rc mode.\
> But, I find an strange problem. For instance, I set up the following  
> test circumstance:
> resolution: 1920x1080
> bitrate=1000kbps, vbv-bufsize=3000kbit
> when I set vbv-maxbitrate>1250kbps, I get almost the same result, like  
> the same psnr, the almost same bitrate.
> I get the same result for vbv-bufsize where the vbv-maxbitrate is fixed,  
> like 3000kbps.
> So I wonder why this situation exists? Can anybody give me some  
> explanation? Thanks so much in advance.


-- 

Fun and success!
Mario *LigH* Rohkrämer
mailto:contact at ligh.de