Intersection boolen in triangle mesh mode is properly implemented

In addition the union operation in tiangle mesh mode is changed
to keep coplanar faces the same way as the NURBS union does.

Finalizes: https://github.com/solvespace/solvespace/issues/35

src/bsp.cpp

@@ -62,14 +62,17 @@ void SBsp3::InsertInPlane(bool pos2, STriangle *tr, SMesh *m) {
         ll = ll->more;
     }
 
-    if(m->flipNormal && ((!pos2 && !onFace) ||
+    if((!onFace && ((m->keepInsideOtherShell && !pos2) ||
-                                   (onFace && !sameNormal && m->keepCoplanar)))
+                    (!m->keepInsideOtherShell && pos2))) ||
-    {
+       (onFace && ((m->keepCoplanar && m->flipNormal && !sameNormal) ||
-        m->AddTriangle(tr->meta, tr->c, tr->b, tr->a);
+                   (m->keepCoplanar && !m->flipNormal && sameNormal)))) {
-    } else if(!(m->flipNormal) && ((pos2 && !onFace) ||
+        // We have decided that we need to keep a triangle either inside,
-                                   (onFace && sameNormal && m->keepCoplanar)))
+        // outside or on the other shell. So add it and flip it if requested.
-    {
+        if(!(m->flipNormal)) {
-        m->AddTriangle(tr->meta, tr->a, tr->b, tr->c);
+            m->AddTriangle(tr->meta, tr->a, tr->b, tr->c);
+        } else {
+            m->AddTriangle(tr->meta, tr->c, tr->b, tr->a);
+        }
     } else {
         m->atLeastOneDiscarded = true;
     }
@@ -101,10 +104,15 @@ void SBsp3::InsertHow(BspClass how, STriangle *tr, SMesh *instead) {
     return;
 
 alt:
-    if(how == BspClass::POS && !(instead->flipNormal)) {
+    if(((BspClass::POS == how) && !instead->keepInsideOtherShell) ||
-        instead->AddTriangle(tr->meta, tr->a, tr->b, tr->c);
+       ((BspClass::NEG == how) && instead->keepInsideOtherShell)) {
-    } else if(how == BspClass::NEG && instead->flipNormal) {
+        // We have decided that we need to keep a triangle (either inside or
-        instead->AddTriangle(tr->meta, tr->c, tr->b, tr->a);
+        // outside the other shell. So add it and flip it if requested.
+        if(!(instead->flipNormal)) {
+            instead->AddTriangle(tr->meta, tr->a, tr->b, tr->c);
+        } else {
+            instead->AddTriangle(tr->meta, tr->c, tr->b, tr->a);
+        }
     } else if(how == BspClass::COPLANAR) {
         if(edges) {
             edges->InsertTriangle(tr, instead, this);
@@ -431,6 +439,9 @@ SBsp3 *SBsp3::InsertConvex(STriMeta meta, Vector *vertex, size_t cnt, SMesh *ins
 SBsp3 *SBsp3::InsertOrCreate(SBsp3 *where, STriangle *tr, SMesh *instead) {
     if(where == NULL) {
         if(instead) {
+            // ruevs: I do not think this code is reachable, but in
+            // principle should we use instead->keepInsideOtherShell
+            // in place of instead->flipNormal ?
             if(instead->flipNormal) {
                 instead->atLeastOneDiscarded = true;
             } else {

src/mesh.cpp

@@ -265,11 +265,12 @@ void SMesh::MakeFromUnionOf(SMesh *a, SMesh *b) {
     SBsp3 *bspb = SBsp3::FromMesh(b);
 
     flipNormal = false;
-    keepCoplanar = false;
+    keepInsideOtherShell = false;
+
+    keepCoplanar = true;
     AddAgainstBsp(b, bspa);
 
-    flipNormal = false;
+    keepCoplanar = false;
-    keepCoplanar = true;
     AddAgainstBsp(a, bspb);
 }
 
@@ -279,28 +280,27 @@ void SMesh::MakeFromDifferenceOf(SMesh *a, SMesh *b) {
 
     flipNormal = true;
     keepCoplanar = true;
+    keepInsideOtherShell = true;
     AddAgainstBsp(b, bspa);
 
     flipNormal = false;
     keepCoplanar = false;
+    keepInsideOtherShell = false;
     AddAgainstBsp(a, bspb);
 }
 
 void SMesh::MakeFromIntersectionOf(SMesh *a, SMesh *b) {
-    // Emulate triangle mesh intersection with difference
+    SBsp3 *bspa = SBsp3::FromMesh(a);
-    // by doing C=A-(A-B). Figure out how to do it properly later.
-	SMesh c = {};
-	c.MakeFromDifferenceOf(a, b);
-	MakeFromDifferenceOf(a, &c);
-
-	c.Clear();
-
-/*  SBsp3 *bspa = SBsp3::FromMesh(a);
     SBsp3 *bspb = SBsp3::FromMesh(b);
 
+    keepInsideOtherShell = true;
+    flipNormal = false;
+
+    keepCoplanar = false;
     AddAgainstBsp(a, bspb);
+
+    keepCoplanar = true;
     AddAgainstBsp(b, bspa);
-*/
 }
 
 void SMesh::MakeFromCopyOf(SMesh *a) {

src/polygon.h

@@ -252,6 +252,7 @@ public:
     List<STriangle>     l;
 
     bool    flipNormal;
+    bool    keepInsideOtherShell;
     bool    keepCoplanar;
     bool    atLeastOneDiscarded;
     bool    isTransparent;