The code below is my attempt at triangulation. It outputs the wrong angles (it read a square's angles as 90, 90. 90, 176) and draws the wrong shapes. What am I doing wrong? //use earclipping to generate a list of triangles to draw std::vector<vec> calcTriDraw(std::vector<vec> poly) { std::vector<double> polyAngles; //get angles for(unsigned int i = 0;i < poly.size();i++) { int p1 = i - 1; int p2 = i; int p3 = i + 1; if(p3 > int(poly.size())) p3 -= poly.size(); if(p1 < 0) p1 += poly.size(); //get the angle from 3 points double dx, dy; dx = poly[p2].x - poly[p1].x; dy = poly[p2].y - poly[p1].y; double a = atan2(dy,dx); dx = poly[p3].x - poly[p2].x; dy = poly[p3].y - poly[p2].y; double b = atan2(dy,dx); polyAngles.push_back((a-b)*180/PI); } std::vector<vec> triList; for(unsigned int i = 0;i < poly.size() && poly.size() > 2;i++) { int p1 = i - 1; int p2 = i; int p3 = i + 1; if(p3 > int(poly.size())) p3 -= poly.size(); if(p1 < 0) p1 += poly.size(); if(polyAngles[p2] >= 180) { continue; } else { triList.push_back(poly[p1]); triList.push_back(poly[p2]); triList.push_back(poly[p3]); poly.erase(poly.begin()+p2); std::vector<vec> add = calcTriDraw(poly); triList.insert(triList.end(), add.begin(), add.end()); break; } } return triList; }