> Here is a fly by of Valles Marineris on Mars. I think the relief is a bit
to
> high.

It looks about right to me. The min_alt to max_alt distance is about 30 or
so, and the radius of the sphere is about 3400, so the depth should be about
3/340 or about 1/110 the radius of the sphere, which is about what it looks
like it is.

There *is* one thing slightly wrong: you're subtracting your image depths
from the sphere, which means the sphere's radius at a given point is
pl_radius+max_alt + the height of the image at that point. (Remember that
subtracting something from a function makes it more likely to be below zero,
which *increases* the surface height.) So I think what you wanted was

function { f_sphere(x,y,z,pl_radius+min_alt) -
alt_fct*f_relief(x,y,z).gray }

(I simply replaced max_alt with min_alt.)

To test this, you can always make a normal (non-isosurface) sphere of radius
pl_radius and check that it cuts the terrain properly.

 - Slime
 [ http://www.slimeland.com/ ]

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