iii. Encoding is performed in this step. After encoding each frame, the future QPs are
updated to account for mispredictions in size (this is referred to as long-term
compensation). If the second pass is consistently off from the predicted size, then
offset =
/6
2
(real filesize/predicted filesize)
(2)
is added to all future QPs. Apart from long-term compensation, there is short-term
compensation (using real filesize - predicted filesize instead of the ratio) to prevent x264 from
deviating too far from the desired file size near the beginning (when there is less data for
long-term compensation) and near the end (when long-term compensation does not have
time to react). Short-term compensation is based on the absolute difference between
encoded size and target size, rather than the ratio.
2.1.2 Average bitrate (ABR)
This is a one-pass scheme where the rate control must be done without knowledge of
future frames. Therefore, ABR can not exactly achieve the target file size. The steps
given below are numbered to match the corresponding steps in 2pass.
i. This is the same as in 2pass, except that instead of estimating complexity from a
previous encode, we run a fast motion estimation algorithm over a half-resolution
version of the frame, and use the Sum of Absolute Hadamard Transformed
Differences (SATD) of the residuals as the complexity. Also, the complexity of the
following GOP is unknown, so I-frame QPs are based on the past.
ii. We do not know the complexities of future frames, so we can only scale based on
the past. The scaling factor is chosen to be the one that would have resulted in the
desired bitrate if it had been applied to all frames so far.
iii. Long and short term compensation is the same as in 2pass. By tuning the strength
of compensation, it is possible to obtain quality ranging from close to 2pass (but
with file size error of ±10%) to lower quality with reasonably strict file size.
3
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