High!

Together with a friend who is re-discovering PoV-Ray after several years 
with Blender, I'm programming a fictitious (but at least halfway 
scientifically accurate) extrasolar planetary system. Currently, I'm 
trying to model asteroid-like small moons; as we want their surfaces to 
be realistic (and keep the memory used low), we turned to isosurfaces 
rather than meshes.

The first image attached shows a moon before applying any craters, it's 
a standard ellipsoid function with added large-scale noise:
f_ellipsoid (x, y, z, 1, 1/(9/20), 1/(14/20))+f_noise3d(x, y, z)/1.7-1

In the second image I added a "craters" function consisting of four 
crackle (form <n, 0, 0>, the x component is varied to get different 
crater densities) functions multiplied together, each representing a 
"layer" of craters of different sizes:

f_ellipsoid (x, y, z, 1, 1/(9/20), 1/(14/20))+f_noise3d(x, y, 
z)/1.7-1-craters(x*2,y*2,z*2)

(the intended radii of the asteroidal moon are 20 by 9 by 14 kilometres)

with "craters" being

#declare craters=
function { crater_layer_1(x, y, z).gray*crater_layer_2(x, y, 
z).gray*crater_layer_3(x,y,z).gray*crater_layer_4(x,y,z).gray*crater_layer_5(x, 
y, z).gray }

Strangely, the subtracted craters function not only added craters, but 
also doubled the overall size of the asteroid...

So I decided to cut the scale factor by half, and as the craters 
appeared to be bottomless dark holes, I made them much shallower by 
setting the darkest part of the crackle color_maps to 0.95 instead of 0 
or 0.5. Finally, I added a fifth layer of many very small craters, so 
that the whole isosurface formula complex is now:

#include "functions.inc"

#declare crater_layer_1=
function
{
   pigment
     {
     crackle
     form <1.5, 0, 0>
     color_map
     {
       [0.05  rgb <0, 0, 0.95>]
       [0.2  rgb <1, 1, 0>]
     }
   }
}

#declare crater_layer_2=function
{
   pigment
   {
     crackle form <1.5, 0, 0>
     color_map
     {
       [0.05 rgb 0.95]
       [0.2  rgb 1]
     }
     translate y*2
   }
}

#declare crater_layer_3=function
{
   pigment
   {
     crackle form <1.2, 0, 0>
     color_map
     {
       [0.05 rgb 0.95]
       [0.2  rgb 1]
     }
     translate <4, 51, 4>
     scale 0.4
   }
}

#declare crater_layer_4=function
{
   pigment
   {
     crackle form <1.8, 0, 0>
     color_map
     {
       [0.05 rgb 0.95]
       [0.2  rgb 1]
     }
     translate <4, 51, 4>
     scale 1.6
   }
}

#declare crater_layer_5=function
{
   pigment
   {
     crackle form <1.45, 0, 0>
     color_map
     {
       [0.05 rgb 0.95]
       [0.2  rgb 1]
     }
     rotate x*20
     scale 0.1
   }
}


#declare craters=
function { crater_layer_1(x, y, z).gray*crater_layer_2(x, y, 
z).gray*crater_layer_3(x,y,z).gray*crater_layer_4(x,y,z).gray*crater_layer_5(x, 
y, z).gray }

isosurface  // Moon #4j; irregularly shaped asteroidal body
{
   function { f_ellipsoid (x, y, z, 1, 1/(9/20), 1/(14/20))+f_noise3d(x, 
y, z)/1.7-1-craters(x*2,y*2,z*2) }
   threshold 1
   contained_by { box { -100, 100 } }
   accuracy 0.0001
   max_gradient 20
   texture
   {
     pigment { color rgb <0.6, 0.6, 0.6> }
     finish { Standard }
   }
   scale 10
   translate Pos_4j
}

But the resulting scene is still pretty far from a real asteroid, I 
think... for comparison, I added an image from the 2000/01 NASA NEAR 
mission to asteroid Eros.

On a real asteroid, you find crater formations in all stages of 
degradation by micro-impact erosion, with younger craters obliterating 
the older ones. And when looked upon close, even young craters don't 
have razor-sharp rims, but are softened.

I doubt whether simply adding small- to medium-scale noise will do the 
trick... is there a way to modify the crackle function itself (which, 
unfortunately, is not documented in the PoV-Ray on-screen manual) to 
produce less sharp-rimmed craters? And how do I get the craters really 
obliterating older ones instead of just adding up to the existing relief?

See you in Khyberspace!

Yadgar

Now playing: Whipping Boy (Elton John)

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Attachments:
Download '4j_flight000.png' (25 KB) Download '2006-06-21_michatopia_moon_.jpg' (14 KB) Download '2006-06-22_michatopia_moon_.jpg' (9 KB) Download '20000317.jpg' (93 KB)

Preview of image '4j_flight000.png'

Preview of image '2006-06-21_michatopia_moon_.jpg'

Preview of image '2006-06-22_michatopia_moon_.jpg'

Preview of image '20000317.jpg'