Databases Reference
In-Depth Information
our description will be rather cursory. The reason for including it here is the great promise it
holds for the future.
19.5 ITU-T Recommendation H.261
The earliest DCT-based video coding standard is the ITU-T H.261 standard. This algorithm
assumes one of two formats, CIF and QCIF. A block diagram of the H.261 video coder is
shown in Figure
19.10
. The basic idea is simple. An input image is divided into blocks of
8
8 block, we subtract the prediction generated using the previous
frame. (If there is no previous frame or the previous frame is very different from the current
frame, the prediction might be zero.) The difference between the block being encoded and
the prediction is transformed using a DCT. The transform coefficients are quantized and the
quantization label encoded using a variable-length code. In the following discussion, we will
take a more detailed look at the various components of the compression algorithm.
×
8 pixels. For a given 8
×
19.5.1 Motion Compensation
Motion compensation requires a large amount of computation. Consider finding a matching
block for an 8
8 block. Each comparison requires taking 64 differences and then computing
the sum of the absolute value of the differences. If we assume that the closest block in the
previous frame is located within 20 pixels in either the horizontal or vertical direction of the
block to be encoded, we need to perform 1681 comparisons. There are several ways we can
reduce the total number of computations.
×
Discrete
cosine
transform
Quantizer
Inverse
quantization
Inverse
transform
+
+
+
Loop
filter
M
o
tion-
compensated
prediction
Motion vector
Loop filter status
F I GU R E 19 . 10
Block diagram of the ITU-T H.261 encoder.
