the skimming thing is this piece here:



In the upper area, that surface skims along the main body part up here:



And it also skims along the very same shape along that needle area - for things like this it's usually better if a cutting surface more sharply cuts through the other surface. 2 surfaces that follow along a largely similar shape across a large region will tend to produce complex intersections, a more sharply pushing through one would be shaped more like this maybe:



- Michael

This is so great.

After posting, I was starting to figure as much about the way that I created the booleans, so I started from scratch with clean geometry and got something much closer to what I was imagining. You are absolutely correct about the fact that my two profiles - the clean one and the one that I used to boolean - were very close to each other. I will definitely make the next one more like your sketch.

Thank you so much for the detailed explanation. Understanding the underlying structure, where the various geometries have trouble interfacing and such, is really helpful.

I created some elaborate surfaces to construct a better boolean


and it worked! Sort of. The first 11 worked, the twelfth created what must be a bug.

I've attached the .3dm, and as you'll see, the area where the lobes are have developed a pixelated half-covering that thinks it's part of the surface of my cup. I don't know where it came from, or have any idea how to remove it. Thoughts?

Hi Sophie things like that usually means that a trimming boundary has become malformed in some way, like too large of gaps between segments or also if it has trim curves that criss-cross back over top of themselves instead of making an orderly boundary.

Usually such things have to be "untrimmed" and retrimmed again in order to repair it, I'll take a look at your model and see if I can help you with that. I haven't taken a look at your model yet but if you also have the step _before_ you did the boolean that got the bad result if you could post that as well that might help to see what's going on.

- Michael

Hi Sophie, it's probably related to some of these same type of "needle thin" type areas such as here:



Those edges come down to a fair amount of area where the edges on opposite side are very very close to one another:



That type of structure where 2 different edges from the same boundary are so close to overlapping can make some mechanisms consider the trim boundary to have self-intersections, and a trim boundary that intersects itself basically messes up the sense of inside and outside of the boundary, you can get stuff that sort of appears to "leak out" from the boundary since it doesn't know which side is supposed to be the active side anymore.

However in this particular case the confusion seems fairly limited to the display mesher, that's used for preparing triangles for the shaded viewport screen display. If you go to do an export to a polygon mesh format a different export mesher will be used and it's somewhat less sensitive to certain things at a cost of taking somewhat more time to calculate stuff.

If you do a file export like to .obj format for example you'll get what looks like an ok result like this:



So it's possible if you just ignore the messy on screen display and do the file export you may be able to get a good result.

Other than that those needly thin type structures can just tend to cause various problems.

- Michael

looks good! I found some workarounds for that strange bug, and the next iteration I will work harder on eliminating those needle thin spots.

Thanks again for your help :)

I've never seen a developer so dedicated to his users as you are, Michael, kudos.