Hi sneather, definitely Joining is the key thing for avoiding those kinds of cracks - if 2 surfaces are meshed totally independent from one another it's quite easy for them to have different vertex structures along their edges and that shows up as exactly those kinds of cracks that you are showing. When edges are joined to one another MoI's mesher will do extra work to ensure a unified "water tight" mesh structure along those edges, so you have to get things joined in order to avoid that problem.

There is a script which will highlight all unjoined edges, it can be useful to see areas of a model that might need repair. To set it up, go to Options > Shortcut keys and push the Add button to add in a new entry at the top of the list. In the key column put in N and for the command column paste in the following:

script: /* Highlight naked edges */ var gd = moi.geometryDatabase; gd.deselectAll(); var breps = gd.getObjects().getBreps(); for ( var i = 0; i < breps.length; ++i ) breps.item(i).getNakedEdges().setProperty( 'selected', true );


Then after that if you push N any naked (unjoined) edges in the model will highlight, it can help you to zero in on joining issues.


> Finally, the offending seams look to be perfectly aligned inside MoI. The surfaces seem to meet up at
> precisely the same poly-lines. So, it's not like they are (obviously) further than that .005 threshold
> number for "join" to work.

It's often difficult to eyeball small distances - I was able to use the "CrvDeviation" command in Rhino to get a numeric value and the maximum gap between those edges you are asking about is 0.02 units, so it's quite a ways above the join tolerance.

You can't easily see the gap from some directions, in this case you would see it most easily by looking downwards on it, here is a screenshot to show you:





In the typical way things work a 0.02 unit gap is pretty large, the model may have been fairly roughly modeled in that area and was probably not originally joined together in its originating application either.


You could try a trick of shrinking everything down in size - when you scale models down gap sizes will also shrink as well, so you could try using Edit > Separate and then scale downwards by 1/10 in size (select all and then run Transform > Scale, type in 0 and push enter to specify 0,0,0 as the scaling origin, and then type in 0.1 and push enter for the scale factor) and then do the join. That can sometimes by an easy solution. Otherwise if some areas of the model are of particularly poor quality they may need to be reconstructed.


In your case here it looks like doing the scale down will get a bunch more things joined, so try that since it is the easiest.


Basically if the geometry in the actual file is of somewhat low or rough quality that can make things more difficult for you when trying to process it. You won't generally get nice results in an automatic fashion from poor quality geometry, you'll likely need to do some repairs and fix up modeling work as well.

Hope this helps!

- Michael