Decal rendering #2: Clipping

Sledge.DataStructures/MapObjects/Entity.cs

@@ -137,11 +137,12 @@ public void CalculateDecalGeometry()
             // Get the faces that intersect with the decal's radius
             var faces = root.GetAllNodesIntersectingWith(box).OfType<Solid>()
                 .SelectMany(x => x.Faces).Where(x => x.IntersectsWithBox(box));
+            var idg = new IDGenerator(); // Dummy generator
             foreach (var face in faces)
             {
                 // Project the decal onto the face
                 var center = face.Plane.Project(Origin);
-                var decalFace = new Face(int.MinValue) // Use a dummy ID
+                var decalFace = new Face(idg.GetNextFaceID())
                                     {
                                         Colour = Colour,
                                         IsSelected = IsSelected,
@@ -160,20 +161,75 @@ public void CalculateDecalGeometry()
                                             }
                                     };
                 // Re-project the vertices in case the texture axes are not on the face plane
-                // Also add a tiny bit to the normal axis to ensure the decal is rendered in front of the face
-                var normalAdd = face.Plane.Normal * 0.2m;
                 var xShift = face.Texture.UAxis * face.Texture.XScale * Decal.Width / 2;
                 var yShift = face.Texture.VAxis * face.Texture.YScale * Decal.Height / 2;
-                decalFace.Vertices.Add(new Vertex(face.Plane.Project(center + xShift - yShift) + normalAdd, decalFace)); // Bottom Right
-                decalFace.Vertices.Add(new Vertex(face.Plane.Project(center + xShift + yShift) + normalAdd, decalFace)); // Top Right
-                decalFace.Vertices.Add(new Vertex(face.Plane.Project(center - xShift + yShift) + normalAdd, decalFace)); // Top Left
-                decalFace.Vertices.Add(new Vertex(face.Plane.Project(center - xShift - yShift) + normalAdd, decalFace)); // Bottom Left
+                decalFace.Vertices.Add(new Vertex(face.Plane.Project(center + xShift - yShift), decalFace)); // Bottom Right
+                decalFace.Vertices.Add(new Vertex(face.Plane.Project(center + xShift + yShift), decalFace)); // Top Right
+                decalFace.Vertices.Add(new Vertex(face.Plane.Project(center - xShift + yShift), decalFace)); // Top Left
+                decalFace.Vertices.Add(new Vertex(face.Plane.Project(center - xShift - yShift), decalFace)); // Bottom Left
+                decalFace.UpdateBoundingBox();
                 // TODO: verify this covers all situations and I don't have to manually calculate the texture coordinates
                 decalFace.FitTextureToPointCloud(new Cloud(decalFace.Vertices.Select(x => x.Location)));
+
+                // Next, the decal geometry needs to be clipped to the face so it doesn't spill into the void
+                // Create a fake solid out of the decal geometry and clip it against all the brush planes
+                var fake = CreateFakeDecalSolid(decalFace);
+                foreach (var f in face.Parent.Faces.Except(new[] { face }))
+                {
+                    Solid back, front;
+                    fake.Split(f.Plane, out back, out front, idg);
+                    fake = back ?? fake;
+                }
+
+                // Extract out the original face
+                decalFace = fake.Faces.First(x => x.Plane.EquivalentTo(face.Plane));
+
+                // Add a tiny bit to the normal axis to ensure the decal is rendered in front of the face
+                var normalAdd = face.Plane.Normal * 0.2m;
+                decalFace.Transform(new UnitTranslate(normalAdd));
+
                 _decalGeometry.Add(decalFace);
             }
         }
 
+        private static Solid CreateFakeDecalSolid(Face face)
+        {
+            var s = new Solid(0)
+                        {
+                            Colour = face.Colour,
+                            IsVisgroupHidden = face.IsHidden,
+                            IsSelected = face.IsSelected
+                        };
+            s.Faces.Add(face);
+            var p = face.BoundingBox.Center - face.Plane.Normal * 10; // create a new point underneath the face
+            var p1 = face.Vertices[0].Location;
+            var p2 = face.Vertices[1].Location;
+            var p3 = face.Vertices[2].Location;
+            var p4 = face.Vertices[3].Location;
+            var faces = new[]
+                            {
+                                new[] { p2, p1, p},
+                                new[] { p3, p2, p},
+                                new[] { p4, p3, p},
+                                new[] { p1, p4, p}
+                            };
+            foreach (var ff in faces)
+            {
+                var f = new Face(-1)
+                            {
+                                Colour = face.Colour,
+                                IsSelected = face.IsSelected,
+                                IsHidden = face.IsHidden,
+                                Plane = new Plane(ff[0], ff[1], ff[2])
+                            };
+                f.Vertices.AddRange(ff.Select(x => new Vertex(x, f)));
+                f.UpdateBoundingBox();
+                s.Faces.Add(f);
+            }
+            s.UpdateBoundingBox(false);
+            return s;
+        }
+
         public override void Transform(IUnitTransformation transform)
         {
             Origin = transform.Transform(Origin);