Problem with beginning of 2-pass encoded video

So I'm trying to use 2-pass encoding on an uncompressed avi to produce a good-looking mp4. The presets are read correctly and everything goes fine during encoding. Then, once the video is uploaded to youtube, I find out that the first few seconds are a blocky mess, as here:





I've isolated the problem to the 2-pass aspect because single-pass encoding doesn't cause the issue:



is the same video but done in single-pass (qscale 5). The curious thing is that I've used these exact settings to make 2-pass encoded videos in the past, and those looked fine once uploaded, but now I can try over and over again without any luck. I don't see the effect when I run the files in media player classic; possibly it isn't introduced until youtube processes it, but there must be some reason why these particular videos won't process properly.

Here's the script:

ffmpeg -i C:ffmpegmovie%1 -bf 16 -pass 1 -vcodec libx264 -vpre libx264-%3 -b %2k -bt %2k -threads 0 -f mp4 -y NUL
ffmpeg -i C:ffmpegmovie%1 -bf 16 -vcodec libx264 -pass 2 -acodec libfaac -ab 128k -ac 2 -bt %2k -b %2k -threads 0 -vpre libx264-%3 C:ffmpegmovie%1%4_encoded.mp4

And the preset details for good measure (typically I use 6000 for %2, so the bitrate target is 6000k):

coder=1
flags=+loop
cmp=+chroma
partitions=+parti8x8+parti4x4+partp8x8+partb8x8
me_method=umh
subq=8
me_range=16
g=250
keyint_min=25
sc_threshold=40
i_qfactor=0.71
b_strategy=2
qcomp=0.6
qmin=10
qmax=51
qdiff=4
bf=4
refs=4
directpred=3
trellis=1
flags2=+bpyramid+wpred+mixed_refs+dct8x8+fastpskip

I was previously trying to use a fastfirstpass preset for the first pass, but that gave me an error so that the encode would stop at the beginning of the second pass. Ultimately I decided that the speed of the encode wasn't so big a deal for me, so I gave the first pass the same preset as the second and it ran after that.

Can anyone diagnose the cause of this problem at the beginning of the first video? I would love to switch to 2-pass, but I'm stuck on single-pass until I find out what I'm doing wrong.

Thanks for the advice! I switched to a single-pass script and am getting good results with -crf 12. For whatever reason, my media player even seems to like these products better; with qscale, MPC would always lag on the video while audio played through at normal rate, although it was moot because it was all going into youtube anyhow.

Regardless, problem solved. Cheers!

OK. did some more digging and testing and got some solid results and answers.

To use a football metaphor, this is a case of "it is the ball, not the player"

Check out this post:
http://doom10.org/index.php?topic=1343.0

Dark Shikari (aka Jason Garrett-Glaser - developer of x264) states that youtube has a broken mp4 demuxer.

I followed his suggestions and rewrapped the file to an mkv wrapper.
$ffmpeg -i badfile.mp4 -vcodec copy goodfile.mkv

This worked (no grey mess at the start) with the B-frames intact.
My other test (without B-frames) also worked but who wants to be using a baseline profile for HD video.... no one;-)

Michael