[x265] x265 misses target bitrate in 2-pass VBR; extreme sample: unable to converge

[Mario *LigH* Rohkrämer]

http://forum.doom9.org/showthread.php?p=1758217#post1758217

n808 wrote: "Trying to encode a standard test sequence (BasketballDrill)  
using 2-pass and the latest 1.9 build (as well as earlier builds), the 2nd  
pass creates a bitrate just a fraction of the requested."

I did not yet find the raw YUV source of this clip, only a compressed  
encode (CRF 10 AVC in MKV), but I could confirm that x265 is unable to  
converge the bitrate curve in the 2nd pass with a target bitrate of 1000  
kbps. Testing other sources, I also noticed that the resulting bitrate was  
often close to only 75-80% of the desired target.

I will try to recreate this result with some older versions to narrow down  
a possible reason...

Sources of the MKV version of BasketballDrill:

https://www.dropbox.com/s/6f8ssh22viycs2s/BasketballDrill_832x480_50_crf10.mkv?dl=1

https://www.mediafire.com/download/l6arqxn7kac8ar7/BasketballDrill_832x480_50_crf10.mkv

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