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I have a CAD model imported into Blender in STL format. This was a STEP file exported in STL format from FreeCAD. Without making any modifications it looks like this:

enter image description here

Base model has 38, 326 faces.

So far these are the steps I have been using to retopo:

  1. Select all verts and remove doubles (face count (fc) = 34,786)
  2. Select all faces and Alt + J to convert all tris to quads (fc = 21,650)
  3. Add Decimate Planar modifier and adjust Angle Limit to get good balance between face count and "look" of the model. Usually around 10 degrees is good enough (fc = 2,755). Now the model looks like this:

enter image description here

enter image description here

So there is still alot of geometry around the holes and other finer features. Apart from making the planar angle more aggressive and manually removing verts and replacing with new faces, which is quite time consuming, are there any other tricks I can use? Ultimately this will be exported as an FBX and used in Unity.

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  • $\begingroup$ Alternatively you could try a Remesh modifier, but I don't think it will manage any better results that what you already have, and probably also require more manual cleanup work $\endgroup$ – Duarte Farrajota Ramos Mar 9 '17 at 17:27
  • $\begingroup$ You've already done much of the work. I would go in now and start cleaning up the edges in those regions will edge dissolve and the like. You should go through all of the different options in the delete menu if you don't already know how to use them. All of them will be very helpful to you when cleaning up messy topology like this. $\endgroup$ – JakeD Mar 14 '17 at 0:46
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Yeah, you've actually done a decent job.
Only trick I might add is an Edge Split modifier.

It'll crisp the boundaries a bit,
especially around the circular parts.

Believe you need to apply it before decimation,
if memory serves correctly...
so you might need to start anew to see its effect.

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So this is my final workflow:

STL Importing:

  • Set scale to 0.001
  • Tab into Edit mode:
    • Select all verts, remove doubles
    • Select all, ALT + J to convert all tris to quads
    • Mesh -> Clean Up -> Degenerate Dissolve
    • Mesh -> Clean Up -> Decimate Geometry
  • Tab into Object mode:
    • Add Decimate modifier, Planar, angle 2-10 deg
    • Check All Boundaries
    • Delimit = Normal
  • Tab into edit mode, select all faces, CTRL+N to recalc norms

Honestly you could spend hours trying to get it perfect. Depending on the complexity of the original CAD model, you may need to manually go in and merge vertices/delete faces and remake them like I am doing here:

http://imgur.com/a/TLGRm

It was so bad that it's better just to delete the bad faces and remake with basic shapes. Fortunately my application does not requite finely detailed UV unwrapping and texturing, all these shapes are one basic color. Finished product:

http://imgur.com/a/LxiXn

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