On Mon, Sep 21, 2009 at 6:26 PM, Noel Stoutenburg <mjol...@ticnet.com> wrote:
> Friends,
> I'm trying to figure the best way to use GIMP blend colors in a
> particular application.
> I have a topographical map, and I want to apply a color gradient which
> follows the contour lines of the map, and blends from a darker hue at

BTW: I don't know what 'a color gradient that follows the contour
lines' means, I could only guess.
If my advice is not suitable for what you mean, I suggest clarifying.

> the lower contour to a lighter hue at the next higher contour. The
> contour lines are not parallel. In the final image, I want a uniform
> darker hue right next to the contour line, and a uniform ligher hue at
> the higher one. A further complication is that the contours do not have
> a uniform direction. In one part of the image the gradient from 10 units
> to 20 units will be right to left, in another part of the image going
> from 10 units to 20 will go from left to right, and in still others, the
> contour representing 20 units will be a smaller irregular shape inside
> the larger, different, but still irregular shape representing an
> elevation of 10 units.
> I've thought of a number of ways to do this manually, for example,
> divide the map into different layers at the contour lines, and using the
> airbrush tool to overlay the colors; another is to leave the different
> contour levels in one layer, and use the smudge tool to blend across the
> contours. But are there filters of plug-ins which might automate at
> least part of the process?

The built in edge detection filters may help you in creating a
suitable selection to apply darkening to.

For the basic colorizing , I think you want the Gradient Map filter
(Colors->Map->Gradient Map)

Hope that helps
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