void FlowField::drawVectorAt( const ofVec2f &vec, const ofVec2f &pos, const float &strength ) {
    ofVec2f np = pos;
    drawVector( np, np + vec.normalized() * strength, 5, 3 );
}
void FlowField::drawVectorAt( const ofVec2f &vec, const ofVec2f &pos ) {
    ofVec2f np = pos - (vec.normalized()) * 5.0;
    drawVector( np, np + vec.normalized() * 10.0, 5, 3 );
}