geom/ray: make ray intersections more robust

geom/ray.hpp

@@ -90,8 +90,13 @@ namespace util::geom {
     ///////////////////////////////////////////////////////////////////////////
     // returns the distance along a ray to an aabb
     //
-    // the return value may be negative if the plane lies behind the ray.
-    // the return value may be infinity in the case the ray and plane are parallel
+    // the distance will never be negative
+    //
+    // the origin point of the ray does not provoke an intersection, so the
+    // returned distances _should_ always be strictly positive.
+    //
+    // if a forward intersection cannot be found then 'infinity' is returned
+    // instead.
     template <size_t S, typename T>
     constexpr T
     distance (const ray<S,T> r, const aabb<S,T> b)
@@ -105,12 +110,17 @@ namespace util::geom {
         const auto tmax = min (max (t1, t2));
 
         // did not intersect
-        if (tmin > tmax)
+        if (tmin >= tmax)
             return std::numeric_limits<T>::infinity ();
 
         // closest is behind us
-        if (tmin < 0)
-            return tmax;
+        if (tmin <= 0) {
+            if (tmax <= 0) {
+                return std::numeric_limits<T>::infinity ();
+            } else {
+                return tmax;
+            }
+        }
 
         // closest is in front of us
         return tmin;

test/geom/aabb.cpp

@@ -12,7 +12,7 @@ main (int, char**)
     util::TAP::logger tap;
 
     {
-
+        const auto upright = normalised (util::vector2f {1});
 
         static struct {
             util::point2f lo;
@@ -25,12 +25,24 @@ main (int, char**)
 
             const char *message;
         } const TESTS[] {
+            {
+                { 0, 0 }, { 1, 1 },
+                { 0, 0 }, upright,
+                std::sqrt (2.f),
+                "ray on aabb corner, pointing to opposite corner",
+            },
+            {
+                { 0, 0 }, { 1, 1 },
+                { 0, 1 }, upright,
+                INFINITY,
+                "ray on aabb implied corner, pointing away",
+            },
             {
                 { 1, 1 }, { 9, 9 },
-                { 2, 2 }, { 1/std::sqrt (2.f), 1/std::sqrt (2.f) },
+                { 2, 2 }, upright,
                 std::sqrt (7.f * 7 * 2),
-                "ray doesn't give negative distance to closest edge"
-            }
+                "ray inside aabb, close to one corner and pointing to far corner",
+            },
         };
 
         for (auto const &t: TESTS) {

test/geom/ray.cpp

@@ -20,34 +20,6 @@ test_intersect_plane (util::TAP::logger &tap)
 }
 
 
-//-----------------------------------------------------------------------------
-void
-test_intersect_aabb (util::TAP::logger &tap)
-{
-    using util::geom::aabb2f;
-
-    // trivial case: unit aabb at origin, ray from (0.5,-0.5) upwards
-    const aabb2f box {
-        { 0.f, 0.f },
-        { 1.f, 1.f }
-    };
-
-    const ray2f forward {
-        util::point2f  { 0.5f, -0.5f },
-        util::vector2f { 0.0f,  1.0f }
-    };
-
-    tap.expect_eq (distance (forward, box), 0.5f, "ray-aabb intersect");
-
-    const ray2f behind {
-        util::point2f  { 0.5f, 2.0f },
-        util::vector2f { 0.0f, 1.0f }
-    };
-
-    tap.expect_eq (distance (behind, box), -1.f, "ray-aabb intersect behind");
-}
-
-
 //-----------------------------------------------------------------------------
 int
 main (void)
@@ -55,7 +27,6 @@ main (void)
     util::TAP::logger tap;
 
     test_intersect_plane (tap);
-    test_intersect_aabb (tap);
 
     return tap.status ();
 }