Basic Polygon Offsetting

offset.cpp

Vec2d orthogonal(Vec2d val) {
    return {-val.y, val.x};   
}

Vec2d calculateBisector(Vec2d prev, Vec2d curr, Vec2d next) {
	auto nextDir = (next - curr).normalized();
	auto prevDir = (prev - curr).normalized();
	Vec3d bisector = (nextDir + prevDir);
	Vec2d norm = orthogonal(nextDir);
	// If prev, curr and next are colinear
	if (bisector.norm() < 0.00001) {
        // bisector is orthogonal to incoming/outgoing edge
		bisector = norm;
	} else {
        // Otherwise make bisector always points in the right direction.
        bisector *= sign(Vec2d::dot(norm, bisector));
	}
	bisector.normalize();
	return bisector;
}

bool offsetPolygon(const std::vector<Vec2d>& in, double distance, std::vector<Vec2d>& out) {
	for (int i = 0; i < in.points().size(); i++) {
        int prevIndex = i == 0 ? in.points().size() - 2 : i - 1;
        int nextIndex = (i + 1) % in.points().size();
        auto curr = in[i];
        auto next = in[nextIndex];
        auto prev = in[prevIndex];
        
        Vec2d bisector = calculateBisector(prev, curr, next, up);
        Vec2d nextEdgeDir = (next - curr).normalized();
        double edgeDot = Vec2d::dot(nextEdgeDir, bisector);

        double edgeFactor = 1.0/sqrt(1.0 - edgeDot * edgeDot);
        
        auto offsetPoint = curr + bisector * edgeFactor;
    }
}