fix wrong indices in OBJ normal computation

src/ACGL/OpenGL/Data/GeometryDataLoadStoreOBJ.cc

@@ -306,9 +306,9 @@ SharedGeometryData loadGeometryDataFromOBJ(const std::string& _filename, bool _c
             if(_computeNormals)
             {
                 size_t triangleIndex = i / 3;
-                glm::vec3 v0 = (glm::vec3)positionData[indices[triangleIndex+0].position];
-                glm::vec3 v1 = (glm::vec3)positionData[indices[triangleIndex+1].position];
-                glm::vec3 v2 = (glm::vec3)positionData[indices[triangleIndex+2].position];
+                glm::vec3 v0 = (glm::vec3)positionData[indices[3 * triangleIndex + 0].position];
+                glm::vec3 v1 = (glm::vec3)positionData[indices[3 * triangleIndex + 1].position];
+                glm::vec3 v2 = (glm::vec3)positionData[indices[3 * triangleIndex + 2].position];
                 glm::vec3 normal = glm::normalize(glm::cross(v1 - v0, v2 - v0));
                 abData[pos++] = normal.x;
                 abData[pos++] = normal.y;