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Fig. 17
Overlapping blocks. The darker-shades areas show overlapping areas.
Each block that the pixel p is part of has a predicting block within the reference
frame selected by motion estimation. The predictor
p
for p is the weighted sum of
all the corresponding pixels in the predicting blocks in frame
t
'
, given by p(x-V
I
,
y - W
i
, t
'
) for motion vectors (V
i
, W
i
). The Raised-Cosine mask has the necessary
property that the sum of the weights will always be 1:
This may seem complicated but in implementation the only additional complexity
over standard block-based motion compensation is to apply the weighting mask to
a predicting block before subtracting it from the frame. The fact that the weights
sum to 1 automatically takes care of splicing the predictors together across the
overlaps.
As explained, Dirac provides motion vectors to sub-pixel accuracy, the
software supports accuracy up to 1/8th pixel, although 1/4 pixel is the default.
This means up converting the reference frame components by a factor of up to 8 in
each dimension. The area corresponding to the matching block in the up converted
reference then consists of 64 times more points. These can be thought of as 64
reference blocks on different sub-lattices of points separated by a step of 8 sub-
pixels, each one corresponding to different sub-pixel offsets.
Sub-pixel motion compensation places a huge load on memory bandwidth if
done naively, i.e. by up converting the reference by a factor 8 in each dimension. In
Dirac, however, we just up convert the reference by a factor of 2 in each dimension
and compute the other offsets by linear interpolation on the fly. In other words we
throw the load from the bus to the CPU. The 2x2 up conversion filter has been
designed to get the best prediction error across all the possible sub-pixel offsets.
5.4 Entropy Coding
Entropy coding is applied after wavelet transform to minimize the number of bits
used. There are three stages in entropy coding used by Dirac in wavelet subband
coefficient coding, that is as follows:
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