Databases Reference
In-Depth Information
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F I GU R E 7 . 16
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F I GU R E 7 . 17
The background layer.
of writing in two “colors.” Notice that the heights of the two slices are not the same, and
the complexity of the information contained in the two slices is not the same. The top slice
contains multicolored text, and a continuous tone image whereas the bottom slice contains
only bi-level text. Let us take the upper slice first, and see how to divide it into the three layers.
We will discuss how to code these layers later. The background layer consists of the cake and
nothing else. The default color for the background layer is white (though this can be changed).
Therefore, we do not need to send the left half of this layer, which contains only white pixels
(Figure
7.17
).
The mask layer (Figure
7.18
) consists of a bi-level representation of the textual information,
while the foreground layer contains the colors used in the text. To reassemble the slice, we
begin with the background layer. We then add to it pixels from the foreground layer using the
mask layer as the guide. Wherever the mask layer pixel is black (1), we pick the corresponding
pixel from the foreground layer. Wherever the mask pixel is white (0), we use the pixel from
the background layer. Because of its role in selecting pixels, the mask layer is also known as
the selector layer. During transmission, the mask layer is transmitted first, followed by the
background and the foreground layers. During the rendering process, the background layer is
rendered first (Figure
7.19
).
When we look at the lower slice, we notice that it contains only bi-level information. In
this case, we only need the mask layer because the other two layers would be superfluous. In
order to deal with this kind of situation, the standard defines three different kinds of stripes.
Three-layer stripes (3LS) contain all three layers and are useful when there is both image and
