boolean with protected faces

[czc98]

Hi, I have two mesh models, as shown in the picture. The red faces on the blue model are protected areas. When performing a boolean operation between the blue model - the white model, how can I make sure the red faces stay unchanged?

[Fedr]

Hi,

boolean only re-triangulates the triangles crossed by the surface of the other mesh. Every other triangle of mesh A goes into the result unchanged; it only gets a new FaceId. So the red triangles stay exactly as they are as long as the white model does not cross them. To find them in the result, pass a BooleanResultMapper:

BooleanResultMapper mapper;
auto res = boolean( blue, white, BooleanOperation::DifferenceAB, { .mapper = &mapper } );
FaceBitSet redInResult = mapper.map( red, BooleanResultMapper::MapObject::A );

In your picture, though, the cube overlaps the red region. blue - white removes everything inside the cube, so the red triangles under the cube have to be cut. No boolean option can both subtract the cube there and keep those triangles. You can check this before the operation:

auto [redHit, whiteHit] = findCollidingTriangleBitsets( { blue, &red }, white );
// redHit.any() == true -> the operation will change the protected area

If what you want is "subtract the white model everywhere except near the protected area", cut the protected zone out of the white model first:

Mesh redPatch = blue.cloneRegion( red );
Mesh guard = makeThickMesh( redPatch, { .outsideOffset = 0.1f, .insideOffset = depth } );
Mesh whiteTrimmed = *boolean( white, guard, BooleanOperation::DifferenceAB );
auto res = boolean( blue, whiteTrimmed, BooleanOperation::DifferenceAB, { .mapper = &mapper } );

Then the red surface, and the material up to depth below it, stays untouched. Pick outsideOffset and depth to suit your model's scale.

If your intended result is different, could you describe or sketch it?