[Utils/Ray] Ray-triangle intersection gives the triangle's own normal

- If the normal always remains in the ray's opposite direction, there is no way knowing if we hit the triangle from behind, which can be useful in some cases
  - Another solution, that might be used later, would be to give a negative distance in this case

- Added a Geogebra link for the ray-triangle intersection unit test

src/RaZ/Utils/Ray.cpp

@@ -139,13 +139,7 @@ bool Ray::intersects(const Triangle& triangle, RayHit* hit) const {
 
   if (hit) {
     hit->position = m_origin + m_direction * hitDist;
-
-    const Vec3f normal = firstEdge.cross(secondEdge).normalize();
-
-    // We want the normal facing the ray, not the opposite direction (no culling)
-    // This may not be the ideal behavior; this may change when a real use case will be available
-    hit->normal = (normal.dot(m_direction) > 0.f ? -normal : normal);
-
+    hit->normal   = firstEdge.cross(secondEdge).normalize(); // Directly computing the normal using the already calculated triangle's edges
     hit->distance = hitDist;
   }
 

tests/src/RaZ/Data/BvhSystem.cpp

@@ -240,7 +240,7 @@ TEST_CASE("BvhSystem query") {
   CHECK(entity == &mesh1);
   CHECK(hit.position == Raz::Vec3f(0.f, 0.f, 0.25f));
   CHECK_THROWS(triangle1.contains(hit.position)); // TODO: wait for the triangle's point containment check to be available
-  CHECK(hit.normal == -triangle1.computeNormal());
+  CHECK(hit.normal == triangle1.computeNormal());
   CHECK(hit.distance == 1.414213538f);
 
   // Same as the above, but in the opposite direction
@@ -249,7 +249,7 @@ TEST_CASE("BvhSystem query") {
   CHECK(entity == &mesh2);
   CHECK(hit.position == Raz::Vec3f(0.f, 0.375f, 0.f));
   CHECK_THROWS(triangle2.contains(hit.position)); // TODO: wait for the triangle's point containment check to be available
-  CHECK(hit.normal == -triangle2.computeNormal());
+  CHECK(hit.normal == triangle2.computeNormal());
   CHECK(hit.distance == 1.414213538f);
 
   // If the ray hits nothing, a null pointer is returned

tests/src/RaZ/Utils/Ray.cpp

@@ -156,6 +156,8 @@ TEST_CASE("Ray-sphere intersection") {
 }
 
 TEST_CASE("Ray-triangle intersection") {
+  // See: https://www.geogebra.org/m/e8uqvjwh
+
   // These triangles are defined so that:
   //  - triangle1 is laying flat slightly above 0
   //  - triangle2 is standing, parallel to the Y/Z plane (facing the X direction)
@@ -168,12 +170,12 @@ TEST_CASE("Ray-triangle intersection") {
 
   CHECK(ray1.intersects(triangle1, &hit));
   CHECK(hit.position == Raz::Vec3f(0.f, 0.5f, 0.f));
-  CHECK(hit.normal   == -Raz::Axis::Y);
+  CHECK(hit.normal   == Raz::Axis::Y);
   CHECK(hit.distance == 0.5f);
 
   CHECK(ray2.intersects(triangle1, &hit));
   CHECK(hit.position == Raz::Vec3f(0.5f, 0.5f, 0.f));
-  CHECK(hit.normal   == -Raz::Axis::Y);
+  CHECK(hit.normal   == Raz::Axis::Y);
   CHECK_THAT(hit.distance, IsNearlyEqualTo(2.1213205f));
 
   CHECK(ray3.intersects(triangle1, &hit));
@@ -185,7 +187,7 @@ TEST_CASE("Ray-triangle intersection") {
 
   CHECK(ray2.intersects(triangle2, &hit));
   CHECK(hit.position == Raz::Vec3f(0.5f, 0.5f, 0.f));
-  CHECK(hit.normal   == -Raz::Axis::X);
+  CHECK(hit.normal   == Raz::Axis::X);
   CHECK_THAT(hit.distance, IsNearlyEqualTo(2.1213202f));
 
   CHECK(ray3.intersects(triangle2, &hit));
@@ -200,7 +202,7 @@ TEST_CASE("Ray-triangle intersection") {
   CHECK(ray3.intersects(triangle3, &hit));
   // The second point is almost aligned with the ray; see https://www.geogebra.org/m/g4pumzwu
   CHECK_THAT(hit.position, IsNearlyEqualToVector(Raz::Vec3f(-1.5000002f, -1.5000002f, 0.f)));
-  CHECK_THAT(hit.normal, IsNearlyEqualToVector(Raz::Vec3f(-0.077791f, 0.9334918f, -0.3500594f)));
+  CHECK_THAT(hit.normal, IsNearlyEqualToVector(Raz::Vec3f(0.077791f, -0.9334918f, 0.3500594f)));
   CHECK_THAT(hit.distance, IsNearlyEqualTo(3.5355341f));
 }