Boolean Inside / Outside

While I tried to make Exact Boolean operate the same as Fast Boolean except when the latter fails due to its limitations, in the area of inside/outside testing there is a bit of a divergence, and I want to solicit opinions on what to do about it.

The difference is that Exact Boolean pays attention to normal directions, while Fast Boolean does not.

If you think of a ray shooting way of testing whether or not a point is inside the volume represented by an object, there are two choices: (1) count the number of intersections of the ray with planes (being careful to only count once if you hit a seam between two planes) and call the origin point “inside” if that count is odd; (2) similar to one, but when accumulating the count, add +1 if crossing in the same direction as the face normal, and -1 if crossing in the opposite direction; then declare the ray origin point as inside if the final count is positive.

The difference between these two shows up if you have, say, a sphere totally inside another sphere, both with their normals all facing outward. And then do a Union. With case (1), the inside of the inner sphere will be called “outside”, and thus that inner sphere’s faces will be part of the union. With (2), the inner sphere’s faces will go away because the inside of the inner sphere will be “inside”, so the union of that volume with the volume between the spheres is just the volume contained in the outer sphere. If you want the effect of a hole with (2), you can reverse the inner sphere’s normals.

In my mind, (2) is more correct. But it is clearly different from Fast.

One reason I use (2) for Exact Boolean is that the paper I followed doesn’t use ray shooting. Instead, it classifies cells in space as to their containment in other cells, using a method that depends on paying attention to the normal direction when you cross a face from one cell to another. I’m not sure I can adapt it to act like case (1), though I admit to not thinking very hard about this yet.

What do people think of this inconsistency? Is it OK to leave it? Should I try to change Exact to be like Fast? Or the reverse? Or have an option? (As I said, if the answer is to make Exact behave like current Fast, even optionally, I’m not sure I can do it without switching to ray-shooting, which can be a lot slower.)

This is coming up now because I’m looking at bugs like T89391 and wondering what to do if the object that the Boolean modifier is on has a negative transform matrix. If I keep the current behavior (actually, fixing it to reverse normals of the operand object(s) if their transform matrix has a different sign than the main object’s transform matrix), then the difference with Fast will be quite evident, since looking at normals, the main object will have normals pointing inward and thus, the volume “inside” it is actually all the space the visually looks like its outside.


I support making them consistent (and correct!) The bug report only exists because the user is relying on a side-effect of something that (probably) shouldn’t have worked in the first place IMHO. In your example of two spheres the result of unioning the smaller one with reversed normals with the larger one would create a hollow spherical shell - if they were concentric. The correct method for me would be to difference the smaller one with the correct normals. Am I missing something? Legacy support can be important, even when things are incorrect so maybe this needs to be left as-is, or marked as deprecated and removed in a future release, or covered with a preferences option to reverse the functionality. I vote for changing Fast to align with Exact.

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IMHO 2 is the way to go.

The user should know to deal with normals and if what the user is looking for is to make a hole inside a sphere, it’s logical to expect that the inner faces have the correct normal direction.

I may be wrong, but I think with 1 the result is what the user expects, what the user won’t expect is to have to go inside the sphere and flip the normal direction of the inner faces to make it correct, and I think that’s what happens now (I may be wrong because I have not tested it right away, but I think it happened to me in the past).

What I’m not sure is if 2 presents some kind of limitation for other complex operations I can’t think about right now.

With 2, what happens when the sphere is fully inside except a small extrusion that will make a hole in the exterior shell, does it matter the normal direction of the faces in that situation?

thanks for the awesome information.