Graphics Reference
In-Depth Information
at this. The mapping of data values to appearance
(i.e., color and transparency) is known as a color
mapping , and in the visualization world, the function
that applies this mapping is more generally called a
transfer function , color map , or color ramp . The appro-
priate use of transfer functions is a very important
issue. By using different transfer functions, you can
create very diferent mental models of paterns in
the data.
Figure 15.20 . Red, green, blue, and alpha
transfer function with the colors applied to
a data histogram.
Same data + different mapping = different insights.
For example, in the tsunami wave simulation
display in Figure 15.19, the paterns in the data come
across quite differently, depending on the color
mapping.
Often, visualization programs have a user inter-
face for sculpting a transfer function. An example of
such a sculpted function is shown in Figure 15.20. In
this figure, the horizontal axis represents the range
of scalar values. The red, green, and blue lines show
how those color components change with respect
to scalar value (in this case implementing a heated
object scale). The white line is how alpha changes
with respect to scalar value. The background shows
a histogram of the data value frequency. 3
Figure 15.21 shows how the temperature dis-
tribution point cloud of Figure 15.8 looks with the
heated object transfer function instead of the rain-
bow scale.
If you do not have a user interface for sculpting a transfer function, you
can easily write your own transfer function. The sources for functions that
implement the rainbow transfer function and the heated-object transfer func-
tion are shown here:
3.   Actually, we usually use the logarithm of the frequency, because often some of the data values, espe-
cially the lowest values, occur much more often than all the others. Without the log function, the
other values would scarcely be visible on the same set of axes. Many visualization programs have a
similar looking user interface for sculpting a transfer function.
Figure 15.21. Previously seen volume with
a different transfer function.
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