mathlib: add GLM_PolyArea

common/mathlib.cc

@@ -440,6 +440,24 @@ glm::vec3 GLM_ProjectPointOntoPlane(const glm::vec4 &plane, const glm::vec3 &poi
     return point + move;
 }
 
+float GLM_PolyArea(const std::vector<glm::vec3> &points)
+{
+    Q_assert(points.size() >= 3);
+    
+    float poly_area = 0;
+    
+    const vec3 v0 = points.at(0);
+    for (int i = 2; i < points.size(); i++) {
+        const vec3 v1 = points.at(i-1);
+        const vec3 v2 = points.at(i);
+        
+        const float triarea = GLM_TriangleArea(v0, v1, v2);
+        
+        poly_area += triarea;
+    }
+    
+    return poly_area;
+}
 
 glm::vec3 GLM_PolyCentroid(const std::vector<glm::vec3> &points)
 {

include/common/mathlib.hh

@@ -332,6 +332,7 @@ glm::vec3 GLM_TriangleCentroid(const glm::vec3 &v0, const glm::vec3 &v1, const g
 float GLM_TriangleArea(const glm::vec3 &v0, const glm::vec3 &v1, const glm::vec3 &v2);
 float GLM_DistAbovePlane(const glm::vec4 &plane, const glm::vec3 &point);
 glm::vec3 GLM_ProjectPointOntoPlane(const glm::vec4 &plane, const glm::vec3 &point);
+float GLM_PolyArea(const std::vector<glm::vec3> &points);
 glm::vec3 GLM_PolyCentroid(const std::vector<glm::vec3> &points);
 glm::vec4 GLM_PolyPlane(const std::vector<glm::vec3> &points);
 /// Returns the index of the polygon edge, and the closest point on that edge, to the given point

light/test_light.cc

@@ -187,6 +187,19 @@ TEST(mathlib, PolygonCentroid) {
     EXPECT_EQ(vec3(32,32,0), GLM_PolyCentroid(poly));
 }
 
+TEST(mathlib, PolygonArea) {
+    // poor test.. but at least checks that the colinear point is treated correctly
+    const vector<vec3> poly {
+        { 0,0,0 },
+        { 0,32,0 }, // colinear
+        { 0,64,0 },
+        { 64,64,0 },
+        { 64,0,0 }
+    };
+    
+    EXPECT_EQ(64.0f * 64.0f, GLM_PolyArea(poly));
+}
+
 TEST(mathlib, BarycentricFromPoint) {
     const tri_t tri = make_tuple<vec3,vec3,vec3>( // clockwise
                                                  { 0,0,0 },