world view ray casting

internal/util/itype/direction.go

@@ -35,3 +35,54 @@ var DirectionVecd = [6]Vec3d{
 	ZPlus:  {0, 0, 1},
 	ZMinus: {0, 0, -1},
 }
+var DirectionName = [6]string{
+	"X+",
+	"X-",
+	"Y+",
+	"Y-",
+	"Z+",
+	"Z-",
+}
+
+func (d Direction) Opposite() Direction {
+	switch d {
+	case XPlus:
+		return XMinus
+	case XMinus:
+		return XPlus
+	case YPlus:
+		return YMinus
+	case YMinus:
+		return YPlus
+	case ZPlus:
+		return ZMinus
+	case ZMinus:
+		return ZPlus
+	}
+	return 0
+}
+
+// DirectionIs returns X/Y/Z for 0/1/2, and +/- for #pos/neg.
+func DirectionIs(index int, positive bool) Direction {
+	switch index {
+	case 0:
+		if positive {
+			return XPlus
+		} else {
+			return XMinus
+		}
+	case 1:
+		if positive {
+			return YPlus
+		} else {
+			return YMinus
+		}
+	case 2:
+		if positive {
+			return ZPlus
+		} else {
+			return ZMinus
+		}
+	}
+	return 0
+}

internal/util/itype/rect.go

@@ -155,3 +155,51 @@ func (box1 Boxd) Intersect(box2 Boxd) (ok bool, intersect Boxd) {
 		}
 	}
 }
+
+func between(val, min, max float64) bool {
+	return min <= val && val <= max
+}
+
+// IntersectRay computes if a Box intersects with a ray.
+// #dir must be normalized, otherwise it's garbage-in garbage-out.
+func (box Boxd) IntersectRay(start, dir Vec3d, length float64) (ok bool, hitface Direction, where Vec3d, dist float64) {
+	// https://gamedev.stackexchange.com/questions/18436/most-efficient-aabb-vs-ray-collision-algorithms
+	if box.Contains(start) {
+		return true, XPlus, start, 0
+	}
+
+	min, max := box.MinPoint(), box.MaxPoint()
+
+	var t Vec3d
+	for i := 0; i < 3; i++ {
+		if dir[i] > 0 {
+			t[i] = (min[i] - start[i]) / dir[i]
+		} else {
+			t[i] = (max[i] - start[i]) / dir[i]
+		}
+	}
+
+	maxi := 0
+	for i, ti := range t {
+		if ti > t[maxi] {
+			maxi = i
+		}
+	}
+
+	if between(t[maxi], 0, length) {
+		pt := start.Add(dir.Multiply(t[maxi]))
+
+		o1 := (maxi + 1) % 3
+		o2 := (maxi + 1) % 3
+
+		if between(pt[o1], min[o1], max[o1]) && between(pt[o2], min[o2], max[o2]) {
+			ok = true
+			hitface = DirectionIs(maxi, dir[maxi] < 0)
+			where = pt
+			dist = t[maxi]
+			return
+		}
+	}
+
+	return
+}