Trying to determine which side of a contour a vertex lies on.

gistfile1.cpp

ofVec2f closestPointOnLine(const ofVec2f& p1, const ofVec2f& p2, const ofVec2f& p3) {
  if(p1 == p2) {
		return p1;
	}
	float u = (p3.x - p1.x) * (p2.x - p1.x);
	u += (p3.y - p1.y) * (p2.y - p1.y);
	float len = (p2 - p1).length();
	u /= (len * len);
	if(u > 1) {
		u = 1;
	} else if(u < 0) {
		u = 0;
	}
	return p1.getInterpolated(p2, u);
}

bool isLeft(ofVec2f a, ofVec2f b, ofVec2f c){
	return ((b.x - a.x) * (c.y - a.y) - (b.y - a.y) * (c.x - a.x)) > 0;
}

bool sideTest(ofPolyline& polyline, ofVec2f position) {
	ofVec2f a, b;
	float bestDistance = 0;
	for(int i = 0; i + 1 < polyline.size(); i++) {
		ofVec2f v0 = polyline[i], v1 = polyline[i + 1];
		ofVec2f closest = closestPointOnLine(v0, v1, position);
		float distance = closest.distance(position);
		if(i == 0 || distance < bestDistance) {
			bestDistance = distance;
			a = v0, b = v1;
		}
	}
	if(bestDistance == 0) {
		return true;
	} else {
		return isLeft(a, b, position);
	}
}