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While modelling an object in blender it's not unusual to have overlapping vertices that are exactly on top of each other. This happens easily when I extrude and manipulate the mesh and then change my mind later on.

So removing doubles tends to be a common operation. However, wouldn't it make more sense to have blender automatically do this to help improve workflow? Are there any situations where I wouldn't want to remove double vertices?

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    $\begingroup$ If your geometry is good, double vertices removal is okay; however, if you do some lazy stuff when modeling you can have issues. For instance, I worked on a bookshelf once where I'd just extruded up the frame; when I got to the top when I started merging vertices I tore off my faces, meaning that I had to go through and manually redo the whole thing (I wound up taking out every vertex but the ones on the edges, then made faces between the remaining outer edges to get a single smooth surface). $\endgroup$ – Kyle Willey May 23 '13 at 1:19
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    $\begingroup$ Are you aware of 'Auto Merge' ?, this is a toggle that removes duplicates as you edit. Access on the right hand side of the 3d view header, next to render buttons. $\endgroup$ – ideasman42 May 23 '13 at 4:23
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    $\begingroup$ @ideasman42, how is the Merge Distance handled with this toggled feature that you mention? $\endgroup$ – MarcClintDion Feb 11 '14 at 17:16
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    $\begingroup$ @MarcClintDion it uses a very low threshold (effectively zero), since this is meant to be used with snapping - where you don't just manually move it close-enough. $\endgroup$ – ideasman42 Dec 26 '16 at 15:36
  • $\begingroup$ I started using it on a regular basis after you mentioned it here. The auto-merge is excellent in combination with edge/vertex slide after a model has its textures baked since these tools allow for quick polygon reduction without altering the UV layout. You do nice work. :) $\endgroup$ – MarcClintDion Dec 27 '16 at 19:24
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One case would be if you didn't want to lose sharp edges.

When you apply the edge-split modifier, it will be splitting off faces based upon the angle. It will actually be splitting them individually so there will be doubled vertices. If you remove doubles, you'll lose the sharp edges.

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    $\begingroup$ "When you apply the edge-split modifier" exactly. My suggestion is "never ever apply the edge split modifier." $\endgroup$ – wchargin Jun 9 '13 at 23:23
  • $\begingroup$ Agreed. I never apply it, even when I'm exporting a model. $\endgroup$ – CharlesL Jun 9 '13 at 23:24
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Yes when using Blender for exporting to other applications. Not all exporters handle the "hard edges" attribute in Blender meshes in the same way and, often, the only way to have said hard edges is to use the edge-split modifier.

Not as bad when everything is being done in Blender, but that is not possible for commercial indie game development, external renderers (other than Cycles), etc.

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There are a few reasons, but to list some that aren't already mentioned.

  • When doing subsurf modeling (or modeling with deform modifiers for that matter). Its possible that the cage overlaps at points that don't show up on the final output, but removing doubles will adversely effect the subsurf result.

  • Sometimes when your modeling you want the model to be manifold (water tight) with obvious inside and outside. In this case you may want to preserver a solid mesh shell and not worry about accidentally overlapping vertices while you edit.

  • Removing doubles can give unusual geometry (an edge with 3+ faces using it for example), such meshes are supported but often won't give good results with blenders tools.

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You should also keep in mind that removing doubles always involves a merge distance, and that merge distance is relative to the object space, not the global space. So if you accidentally scaled the object to be really big and are then working on small detail, this feature might actually kill geometry that you carefully modeled because the distance between two points in your geometry is, relative to the object size, below the merge value.

On the other hand, the tool can sometimes actually be used to reduce complexity quite effectively, for example if you want to delete all loops that are close together, and without creating triangles where you wanted to preserve quads.

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