[x264-devel] Encoding error only on some pictures

[Jonathan Rosser]

Hi,

I have encountered a possible encoding error with a particular
10 bit configuration of x264.

x264 --output outFile.h264 --fps 25 --input-res 1920x1080 --profile high422
  --preset ultrafast --tune zerolatency --threads 2 --bitrate 111820
  --vbv-maxrate 111820 --vbv-bufsize 4472 --keyint 1 --min-keyint 1
  --input-csp i422 --output-csp i422 --input-depth 10 --slice-max-mbs 816
  --tff rawVideo50.10p2

One or more slices are corrupt when decoded, but only for specific
input pictures, and then not for each individual encoding of that picture
(This is an I-frame only set up).

Decoding with either ffplay or an AVID system reproduce the same
decoding artifact. I'm attempting to test with a third decoder.

This feels like a rate control problem with this combination of
threading and slice structure, as backing off the --bitrate setting or
only using a single thread seems to clear things up. Unfortunately, two
threads are required to achieve real time performance.

So, I think I've either made a mistake in the encoding parameters or
tripped up something internal to the encoder.

I have made a test case, which can be downloaded here:

wget ftp://ftp.bbc.co.uk/incoming/x264-encode-fault.tar.bz

This contains:

encodeParams : the command line used to run the encoder
outFile.h264 : the encoded output for 50 frames
rawVideoSingle.10p2 : a single input frame, we concatenated 50 times
x264Output : the debug output from x264

The behavior is identical with the stable build and current git head.

Any advice on what is going on here would be welcome.

Regards,
Jonathan Rosser

Lead Research Engineer
BBC R&D