equal area colour distribution

[Hal Mueller]

At 10:09 AM +0000 12/3/99, Marc Pelletier wrote:
>Hello,
>
>I would like to do an equal area colour distribution using a blue to red
>color map. The default color map is fine, but seems to be linear.

There's a nice concise description in Ballard and Brown's book 
"Computer Vision".  The computer graphics literature refers to the 
technique as histogram equalization.

You need to keep track of two histograms.  One is the hist of values 
in your original image layer.  The other is the histogram of color 
indices in your displayed image.  Since you know the size of your 
layer, you know how many pixels (or areal units of your choice) are 
in the image.  Just divide number of pixels in the image by number of 
color levels.

Build your color transformation map from lowest to highest value of 
the original image.  Keep a running total of how many pixels are 
represented by the current output value.

----
Hal Mueller                hal at seanet.com
Mobile Geographics LLC
Seattle, Washington        (206) 297-9575