# Strategy to reduce polygon count of Comet 67P shape model using python

I've imported an obj file of Comet 67P here (newly released) and found it has over 4 million polygons. I'd like to reduce that number, but using somewhat "scientific" method/strategy using an algorithm I understand. For example - I would want to avoid alter heights or shadow positions, volume, etc.

So instead of "Just use decimate", could I have a strategy, or some ideas how to approach creating an algorithm which reduces the polygons - by a factor of say between 2 and 10, which maintains peaks, valleys, volume, etc. Maybe something like a "it's pretty flat around here" detector?

Note - monkey added for scale reference.

• The decimate modifier supports "pretty flat around here" style decimation. Set the decimate mode to Planar. Note that this is similar to Delete > Limited Dissolve, but non-destructive. That said, you might actually get more uniform/nicer looking results by using the ratio decimation mode (default) and applying normal and bump maps to get the tiny details. Perhaps try a few different methods and compare them Aug 20, 2015 at 7:04
• Thanks. So I need to understand the algorithm well to be sure I know the impact on the position of shadows - even a little bit, and other physical properties. One way would be to write the algorithm myself, another might be to read and understand exactly how decimate works. I'd prefer the former, hands-on approach. - Blender has an amazing capability to be realistic and correct, I don't want to compromise that.
– uhoh
Aug 20, 2015 at 7:10
• Of course, I don't know what your use-case is (so I don't really know what to suggest), but if the accuracy of the model is that important you may want to leave it undecimated in the render and only use a lower-poly proxy in the viewport. Aug 20, 2015 at 7:14
• I'll try the Planar mode and see what happens anyway, and read about what Limited Dissolve means. I'll leave good looking results to those with better artistic skills than I have. I'm just trying to understand how the comet rotates, and how the shadows change as it swings around the sun. Thanks @gandalf3!
– uhoh
Aug 20, 2015 at 10:38
• Sounds like a c++ job ;) You could also try to ask here: computergraphics.stackexchange.com for a strategy, ...maybe that's interesting for the guys there.
– p2or
Aug 20, 2015 at 10:58