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DIGITAL SIGNAL PROCESSING
Input
Video
Signal
Split into
Macroblocks
16x16 pixels
Coder
Control
Transform
Scal./Quant.
Decoder Scaling & Inv.
Transform
Intra-frame
Prediction
Motion-
Compensation
Motion
Estimation
Improved Prediction Methods
Let’s highlight some of the main features of
the H.264/AVC video coding standard
design that enable its enhanced coding efficiency,
evaluating these functional modules
based on the design criteria discussed in the
previous section.
• Quarter-pixel-accurate motion compensation.
Prior standards use half-pixel
motion vector accuracy. The new design
improves on this by providing quarterpixel
motion vector accuracy. The prediction
values at half-pixel positions are
calculated by applying a one-dimensional
six-tap FIR filter [1, -5, 20, 20, -5,
1]/32 horizontally and vertically.
Prediction values at quarter-pixel positions
are generated by averaging samples
at the full- and half-pixel positions.
These sub-sampling interpolation operations
can be efficiently implemented
in hardware inside the FPGA fabric.
• Variable block-sized motion compensation
with small block size. The standard
provides more flexibility for the
tiling structure in a macroblock size of
16 x 16 pixels. It allows the use of 16 x
16, 16 x 8, 8 x 16, 8 x 8, 8 x 4, 4 x 8,
and 4 x 4 sub-macroblock sizes.
Because of the increasing combinations
of tiling geometry with a given
De-blocking
Filter
Output
Video
Signal
Control
Data
Quant.
Transf. Coeffs
Motion
Data
16 x 16 macroblock, to find a rate
distortion optimal tiling solution is
extremely computationally intensive.
This additional feature places an
enormous burden on the computational
engines used in motion estimation,
refinement, and mode decision
process.
• In-the-loop adaptive deblocking filtering.
The deblocking filter has been successfully
applied in H.263+ and
MPEG-4 part 2 implementations as a
post-processing filter. In H.264/AVC,
the deblocking filter is moved inside
the motion-compensated loop to filter
block edges resulting from the prediction
and residual difference coding
stages of the decoding process. The filtering
is applied on both 4 x 4 block
and 16 x 16 macroblock boundaries, in
which two pixels on either side of the
boundary may be updated using a
three-tap filter. The filter coefficients or
“strength” are governed by a contentadaptive
non-linear filtering scheme.
• Directional spatial prediction for intra
coding. In cases where motion estimation
cannot be exploited, intra-directional
spatial prediction is used to
eliminate spatial redundancies. This
technique attempts to predict the current
block by extrapolating the neigh-
boring pixels from adjacent blocks in a
defined set of directions. The difference
between the predicted block and
the actual block is then coded.
This approach is particularly useful in
flat backgrounds where spatial redundancies
exist. There are a total of nine
prediction directions for Intra_4x4
prediction, and four prediction directions
for Intra_16x16 prediction.
Note that the data causality imposes
quick memory access to the neighboring
13 pixel values to the above and
left of the current block in the case of
Intra_4x4. For the Intra_16x16, 16
neighboring pixels on each side are
used to predict a 16 x 16 block.
• Multiple reference picture motion compensation.
The H.264/AVC standard
offers the option for multiple reference
frames in the inter-frame coding. Unless
the number of the referenced pictures is
one, the index at which the reference
picture is located inside the multi-picture
buffer has to be signaled. The
multi-picture buffer size determines the
memory usage in the encoder and
decoder. These reference frame buffers
must be addressed correspondingly during
the motion estimation and compensation
stages in the encoder.
• Weighted prediction. The JVT recognizes
that in encoding certain video
scenes that involve fades, having a
weighted motion-compensated prediction
dramatically improves the coding
efficiency.
Improved Coding Efficiency
In addition to improved prediction methods,
other parts of the standard design were
also enhanced for improved coding efficiency.
Two additional features are most likely to
impact the overall system architecture based
on our design criteria for software and hardware
partitioning:
• Small block size, hierarchical, exactmatch
inverse, and short word-length
transform. The H.264/AVC, like other
standards, also applies transform coding
to the motion-compensated prediction
42 Xcell Journal Winter 2004
Entropy
Coding
Figure 1 – H.264/AVC macroblock encoder with functional blocks and data flows