Mike Smith wrote:
> 
> Hi
> 
> I'm using a Digital Elevation Model,converted to a  POV
> heigthfield, to try to produce a more realistic simulation of a
> satellite image than simple shading. I've modified a little the
> following script which was used for other DEM work, however I
> am particularly interested in simulating the effects of changes in
> solar elevation (ie y value of the illuminator). If this angle is low
> then terrain undulations are crisply picked out. Greater than
> 20degrees and they become virtually unidentifiable.
> 
> In general shaders (and the current script) are unable to cope
> with the low levels of ambient light and, I assume, a greater
> degree of diffusion. I have posted on my web site a couple of
> graphics; the first shows a "real" satellite image
> (http://www.geocities.com/Yosemite/Rapids/5855/satellite_im
> age.jpg) which shows small linear landforms clearly depicted.
> The second
> (http://www.geocities.com/Yosemite/Rapids/5855/rendered.jp
> g) shows my rendering so far (both using the same solar
> illumination angle). This is much darker however similar linear
> landforms are visible in the centre (although its a different area).
> Does anyone have any suggestions for simulating the results in
> the image??
> 
> Cheers
> 
> Mike

   A couple of things come to mind. It looks like the sun has the
 advantage of scattering light through the atmosphere where you are
 using a single point light as you source of illumination. The are a
 couple of things you can try. First and easiest would be something
 like a sky sphere, fog with a high offset so it is way above the
 ground, or the simple background color option. Each of these will
 give you a uniform global illumination without causing shadows.
 Where your light source hits the terrain it's intensity will be
 much more than the feeble light the virtual background light sources
 put out and you should still get the contrasting shadows you are
 looking for.
   Another possibility would be to use a cylindrical light source so
 your light location is spread out more than from a single point.
 It also has the advantage of casting light rays in a parallel path
 which the omni directional point lights you are using now do not.

-- 
Ken Tyler

mailto://tylereng@pacbell.net

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