Mimas bump map

This is an experimental bump map of Saturn's satellite Mimas that I created by running a texture map of Mimas through my "bump map generator". The texture map I used is a slightly modified version of the one at David Seal's site. Creating this map was made easier by the fact that the Mimas texture map is an airbrushed one created from spacecraft photographs and shows the entire surface with the sun coming from the left at a 30° elevation angle. This is in contrast to the actual spacecraft images where the lighting varies a lot. Also Mimas has no albedo features (i.e. brightness variations) to speak of; their presence would greatly complicate matters. The main problems are "noise" in the source map and "self shadowing" within the craters. This resulted in lots of ugly, horizontal lines in the first version of the map but interestingly that version of the map looked spectacular when rendered as a heightfield in POV-Ray (lots of "valleys" and "ridges"). I was able to clean up the majority of this using a combination of manual and "semi-automatic" methods but some horizontal lines still exist and show up as spurious features when the map is rendered. Perhaps this could also be fixed using Leveller but I haven't tried doing that. In any case I intend to improve the map. Since this map is experimental I would like to know if anyone manages to improve it (and also get the improved version :-). One thing to note is that in my experience, trying to fix this in something like Photoshop ruins the map; other "laws" apply to these maps than ordinary texture maps.

The smooth areas in the map were not photographed by the Voyager spacecraft, they imaged only a part of the surface. This should improve greatly in 2004 when the Cassini spacecraft reaches Saturn.

Click the map below to get the full size version (1440x720 pixel 97 KB JPG).

The map is in simple cylindrical projection with latitudes running with a uniform interval from -90 (bottom) to 90 (top) and longitude 0 at the left edge of the map. It should be rendered by projecting it onto a sphere of 195 km radius or some equivalent unit. The lowest areas (intensity 0 in the map) should be about 6 km below "sea level" and the highest (intensity 255 in the map) ones about 2 km above.

There are some renderings done in my renderer on my experimental renderings page.

This can also be rendered in POV-Ray as a bump map on a sphere but I have not been too happy with the result, possibly because I'm not a POV expert. The floor of the big crater looks too smooth and in fact there seems to be a serious bug in the spherical bump map rendering code in the most recent version of POV (older versions work better).

The code I use is like this:

sphere  {
   <0,0,0>,195
   texture  {
      pigment {color White}
      normal  {
         bump_map {
            jpg "mimas_bump.jpg"
            map_type 1
            bump_size 16.0
         }
      }
   }
   finish { ambient 0.05 }
}

You may want some other color than white/gray for Mimas as above. A slightly brownish-grayish one might be appropriate but I don't know Mimas' true color except that the color in the Voyager spacecraft photos is exaggerated. An important thing here is that Mimas is largely made of ice so it is fairly bright. Adding a colored lightsource may be a good idea, e.g. one colored with red=1.2, green=1.0 and blue=0.8.

In contrast to the above example, I get great results if I render this map as a heightfield in POV-Ray. The rendering below is an example (click it to view the full size version) but of course it looks even better as a heightfield on a sphere.

There are two such "spherical heightfield renderings" on my experimental renderings page. They were rendered in POV using a smooth triangle mesh created by wrapping the bump map around a sphere. Messy but works.

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