Every elementary patch of the surface is tested for interior location of the neighbour primitive/object. This query is usually referred as point/solid classification. If the patch is inside other object then the type of boolean operation determines if the patch will be used for result set of patches. Some patches are partially occluded ant cannot be transferred to result set. For such patches trimming is necessary with intersection line. When doing regularized subtraction, patches previously owned by neighbour object must change the surface normal from outside to inside.