Zremesher is without a doubt the best automatic retopology algorithm I've tried so far, and yes I've used Instant Meshes interactive field alignment algorithm and 3dCoat's AutoTopo as well. Both of those are fine in their own right, but Zbrush's method and double algorithms are much more precise and result in a much cleaner topology which can actually be used on its own in production for non-animated objects, unlike some of the results from Instant Meshes. Of course, there is no automatic retopology process which will be perfect in the end. There is no way to automatically retopologize a mesh and expect results to be as good as manual retopology by someone who knows what they're doing. However, in terms of a clean, usable result with only minor issues in edge flow, Zbrush wins that comparison hands down as long as the result is a static object.

Topogun does not have automatic retopology features as far as I am aware. It is an older, although more cost effective, solution to manual retopology. Please do not hesitate to correct me with some links to show the automatic retopology examples in Topogun if I am wrong in that observation.

I am curious when you say that the AutoTopo feature in 3dCoat is taking hours for you, though. I know it's not as fast as Zremesher or Instant Meshes in some cases, but I've never seen it take longer than maybe 5 minutes on even the most challenging mesh. I wonder why it's behaving that way for you?

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