I want to apply a velocity vector to a dynamic body in the cursor direction:

void Game::mousePressEvent(QMouseEvent *e){

    double angle = atan2(realBall->GetPosition().y - e->pos().y(), realBall->GetPosition().x - e->pos().x());
    realBall->SetLinearVelocity(b2Vec2(-cos(angle) * 50, -sin(angle) * 50));
 }

But the dynamic body has an incorrect direction, so i think that the cursor position it's wrong.

Thank you for the help!

CodePudding user response：

First, you must know that in order for your code to work, the coordinates of your screen and the coordinates of box2d must match. Be aware that if you use screen coordinates in pixels, it means that the size of one pixel matches an 1 meter in box2d. But let’s assume that you have already taken all this into account. Then I would not advise you to use trigonometry for calculations. So you can easily make a mistake. In this case, simple vector operations will be enough for you: substraction, scaling and normalizing a vector. You can try this: velocity = (cursor_position - real_ball_position).normalize().scale(50f). In box2d there is a b2Vec class for vector operations. You can read about it in detail in the documentation.