Databases Reference
In-Depth Information
T A B L E 7 . 2
Comparison of the file sizes
obtained using JPEG lossless
compression,GIF,andPNG.
Image
Best JPEG
GIF
PNG
Sena
31,055
51,085
31,577
Sensin
32,429
60,649
34,488
Earth
32,137
34,276
26,995
Omaha
48,818
61,341
50,185
7.3 CALIC
The Context Adaptive Lossless Image Compression (CALIC) scheme, which came into being
in response to a call for proposals for a new lossless image compression scheme in 1994
[
83
,
84
], uses both context and prediction of the pixel values. The CALIC scheme actually
functions in two modes, one for grayscale images and an
o
ther for bi-level images.
In this
section, we will concentrate on the compression of grayscale images.
In an image, a given pixel generally has a value close to one of its neighbors. Which
neighbor has the closest value depends on the local structure of the image. Depending on
whether there is a horizontal or vertical edge in the neighborhood of the pixel being encoded,
the pixel above, or the pixel to the left, or some weighted average of neighboring pixels may
give the best prediction. How close the prediction is to the pixel being encoded depends on
the surrounding texture. In a region of the image with a great deal of variability, the prediction
is likely to be further from the pixel being encoded than in the regions with less variability.
In order to take into account all of these factors, the algorithm has to make a determina-
tion about the environment of the pixel to be encoded. The information used to make this
determination has to be available to both the encoder and decoder.
Let's take up the question of the presence of vertical or horizontal edges in the neighborhood
of the pixel being encoded. To help our discussion, we will refer to Figure
7.1
. In this figure,
the pixel to be encoded has been marked with an
X
. The pixel above is called the north pixel,
the pixel to the left is the west pixel, and so on. Note that when pixel
X
is being encoded, all
other marked pixels (
N, W, NW, NE, WW, NN, NE,
and
NNE
) are available to both the encoder
and decoder.
NN
NNE
NW
N
NE
WW
W
X
F I GU R E 7 . 1
Labeling the neighbors of pixel X.
