From e7d51a38387f1a3ce815be6ef34eb22b09ec53df Mon Sep 17 00:00:00 2001
From: Stefan Maass <sma-github@bitplane.de>
Date: Sat, 14 Sep 2019 23:38:44 +0200
Subject: [PATCH] bugfix intersection ray/plane, added related tests

---
 glm/gtx/intersect.inl      |  9 ++++++---
 test/gtx/gtx_intersect.cpp | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+), 3 deletions(-)

diff --git a/glm/gtx/intersect.inl b/glm/gtx/intersect.inl
index e76fd628..54ecb4d9 100644
--- a/glm/gtx/intersect.inl
+++ b/glm/gtx/intersect.inl
@@ -13,10 +13,13 @@ namespace glm
 		typename genType::value_type d = glm::dot(dir, planeNormal);
 		typename genType::value_type Epsilon = std::numeric_limits<typename genType::value_type>::epsilon();
 
-		if(d < -Epsilon)
+		if(glm::abs(d) > Epsilon)  // if dir and planeNormal are not perpendicular
 		{
-			intersectionDistance = glm::dot(planeOrig - orig, planeNormal) / d;
-			return true;
+			typename genType::value_type const tmp_intersectionDistance = 	glm::dot(planeOrig - orig, planeNormal) / d;
+			if (tmp_intersectionDistance > static_cast<typename genType::value_type>(0)) { // allow only intersections
+				intersectionDistance = tmp_intersectionDistance;
+				return true;
+			}
 		}
 
 		return false;
diff --git a/test/gtx/gtx_intersect.cpp b/test/gtx/gtx_intersect.cpp
index 5a575490..c4a1b2a5 100644
--- a/test/gtx/gtx_intersect.cpp
+++ b/test/gtx/gtx_intersect.cpp
@@ -3,6 +3,39 @@
 #include <glm/gtc/epsilon.hpp>
 #include <glm/gtx/intersect.hpp>
 
+int test_intersectRayPlane()
+{
+	int Error = 0;
+	glm::vec3 const PlaneOrigin(0, 0, 1);
+	glm::vec3 const PlaneNormal(0, 0, -1);
+	glm::vec3 const RayOrigin(0, 0, 0);
+	glm::vec3 const RayDir(0, 0, 1);
+
+	// check that inversion of the plane normal has no effect
+	{
+		float Distance = 0;
+		bool const Result = glm::intersectRayPlane(RayOrigin, RayDir, PlaneOrigin, PlaneNormal, Distance);
+		Error += glm::abs(Distance - 1.f) <= std::numeric_limits<float>::epsilon() ? 0 : 1;
+		Error += Result ? 0 : 1;
+	}
+	{
+		float Distance = 0;
+		bool const Result = glm::intersectRayPlane(RayOrigin, RayDir, PlaneOrigin, -1.f * PlaneNormal, Distance);
+		Error += glm::abs(Distance - 1.f) <= std::numeric_limits<float>::epsilon() ? 0 : 1;
+		Error += Result ? 0 : 1;
+	}
+
+	// check if plane is before of behind the ray origin
+	{
+		float Distance = 9.9999f; // value should not be changed
+		bool const Result = glm::intersectRayPlane(RayOrigin, RayDir, -1.f * PlaneOrigin, PlaneNormal, Distance);
+		Error += glm::abs(Distance - 9.9999f) <= std::numeric_limits<float>::epsilon() ? 0 : 1;
+		Error += Result ? 1 : 0; // there is no intersection in front of the ray origin, only behind
+	}
+
+	return Error;
+}
+
 int test_intersectRayTriangle()
 {
 	int Error = 0;
@@ -47,6 +80,7 @@ int main()
 {
 	int Error = 0;
 
+	Error += test_intersectRayPlane();
 	Error += test_intersectRayTriangle();
 	Error += test_intersectLineTriangle();