[x264-devel] rate control for static macroblocks

[Anne Aaron]

I'm running x264 with CBR rate control under a few seconds of VBV buffer constraint,  on content where some rows of macroblocks are perfectly static (zero distortion from the previous frame co-located macroblocks). For these macroblocks,  the satd is zero so their statistics are not included when update_predictor(...) is called. Aside from divide by zero, any reason for when (var <  10), update_predictor() returns right away?

This may be the cause of one artifact that I am observing --  After a row of static macroblocks, the rate control tends to think that it will overproduce for the remaining rows and therefore increase the QP for the remaining rows. In fact, when the entire frame is static, the QP of the frame is very high. Since the frame is static, this has no detrimental effect on the frame itself. However, since the QP of the  I frames are based on the average of the previous P frames, the I frames tend to have low quality and beating is observed . Anyone has had similar observations or explanations?

Anne
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