web.48bc338bcd5c21438abea67c0@news.povray.org...
>>   See the trace() function at
>> http://povray.org/documentation/view/3.6.1/229/#s02_02_01_04_05
>
> Thanks for this tip, but what, if the surface is not hit by a ray at the
> location, I want to plant a tree?
>
If the surface is not hit by a ray with a -z direction, there is no terrain 
at this location.

It is a good idea to test the length of the returned normal vector : it is 
null if the trace vector does not intersect the surface.

Marc

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