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Problem with beginning of 2-pass encoded video

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William Lipinski
Problem with beginning of 2-pass encoded video
on Jul 4, 2011 at 5:50:26 am

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.


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