x264: how affect b/r 1st pass?

[[)370|\|470!2]

Following string is from 1st pass (x264-1.stats)

#options: 720x400 fps=1000000/40000 cabac=1 ref=1 deblock=1:0:0 analyse=0x3:0x13 me=hex subme=3 brdo=0
mixed_ref=0 me_range=16 chroma_me=0 trellis=0 8x8dct=1 cqm=0 chroma_qp_offset=0 slices=1 bframes=2
b_pyramid=1 b_adapt=1 b_bias=0 direct=2 wpredb=1 bime=0 keyint=200 keyint_min=25 scenecut=40 pass=1
bitrate=1163 ratetol=4.0 rceq='blurCplx^(1-qComp)' qcomp=0.60 qpmin=10 qpmax=51 qpstep=4 ip_ratio=1.40 pb_ratio=1.30

Q is: what happens if i change bitrate for n-th pass?

[foxyshadis]

It uses your new bitrate. You can run a first pass with any bitrate and as many 2nd passes with different bitrates as you want.

[kotrtim]

I have the same question too, in XviD 1st pass, we don't have to specify bitrate, it will constant q2 for 1st pass

Why would we have to set the bitrate for 1st pass for x264, what do x264 use for the 1st pass, quality mode? ABR? Constant Quantizer?

[foxyshadis]

I'd actually be rather interested in x264 supporting --crf while writing a stats file. Similar to xvid's q2 full first pass (if it's small enough, just keep it, otherwise run a 2nd pass), but with a little more control over size/quality. Downside is that you wouldn't get the huge speedup of turbo if you do want 2pass.

[[)370|\|470!2]

True about xvid - it only gathers basic frame information during 1st pass.
But x264 collects much more data in its stat file, thats why i thought that it
could also calculate quantization parameters depending on b/r set. It's also been
said, that x264 optimal results are gained with 3 passes, while with xvid 2 is more
than enough.