Databases Reference
In-Depth Information
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F I GU R E 7 . 2
The HINT scheme for hierarchical prediction.
images (MRI), and mammograms). It is not enough to have information. We also need to
make these images accessible to individuals who can make use of them. There are many issues
involved with making large amounts of information accessible to a large number of people. In
this section, we will look at one particular issue—transmitting these images to remote users.
(For a more general look at the problem of managing large amounts of information, see [
92
].)
Suppose a user wants to browse through a number of images in a remote database. The user
is connected to the database via a 1 Mbps link. Suppose the size of the images is 2048
1536,
and on average, users have to look through 30 images before finding the image for which
they are looking. If these images were monochrome with 8 bits per pixel, this process would
take close to 15 minutes, which is not very practical. Even if we compressed these images
before transmission, lossless compression, on average, gives us about a 2:1 compression. This
would only cut the transmission in half, which still makes the approach cumbersome. A better
alternative is to send an approximation of each image first, which does not require too many
bits but is still sufficiently accurate to give users an idea of what the image looks like. If users
find the image to be of interest, they can request a further refinement of the approximation or
the complete image. This approach is called
progressive image transmission
.
×
Example7.6.1:
A simple progressive transmission scheme is to divide the image into blocks and then send
a representative pixel for the block. The receiver replaces each pixel in the block with the
representative value. In this example, the representative value is the value of the pixel in the
top-left corner. Depending on the size of the block, the amount of data that would need to be
transmitted could be substantially reduced. For example, to transmit a 2048
1536 image at 8
bits per pixel over a 1 Mbps link takes about 25 seconds. Using a block size of 8
×
8 and using
the top-left pixel in each block as the representative value, we can approximate the 1024
×
×
1024
image with a 128
128 subsampled image. Using 8 bits per pixel and a 1 Mbps link, the
time required to transmit this approximation to the image takes less than a third of a second.
×
