Honestly, I have no idea of what bitrate could be comparable to an encoding with x264. I don't use x265 myself. I have added the possibility to use it in BD3D2MK3D at the request of an user. (Is it you? I don't remember.)
I did only a few tests in CRF mode. I have noticed that CRF 23 is the default for x264, and CRF 28 for x265. Therefore, I did a test with the two encoders with their respective default CRF values. I did my first test with x264 CRF 23, preset slower. (Usually, an encoding of a full movie with these settings lasts 7 or 8 hours on my computer.) Then, I launched a second test with x265 CRF 28 and preset slower too. But after about 30 minutes, x264 displayed a remaining time of more than 24 hours, and I have interrupted it. I relaunched it with preset medium, and this time it was reasonably fast. Finally, the size of the h265 stream was a little bit more than 1/3 of the size of the x264 stream. Good result, given the fact that the preset was "worse" for x265! I suppose that the two renderings were comparable in quality, and I was unable to see any difference, but I have not tried to compare the two renderings scientifically.
So, currently, I assume that 1/3 to 1/2 of the bitrate of x264 should be sufficient for x265. But I can't be sure. Anyway, I don't like to use 1 or 2-pass modes with specific bitrate values. CRF mode is MUCH better, because it "adapts itself" to the complexity of the movie. But I'm not the best person to reply to your question. I suppose you'll have more elaborated replies in the x264/x265 forums.
Last edited by r0lZ; 1st April 2015 at 18:02.
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