Hello,
I have written some functionality in combination with vectors and angles. The more I use them, the more I think the way how SFML handles angles is not consistent. Relative to the screen, sf::Drawable::Rotate() rotates in counter-clockwise direction. But since the Y axis points downwards, the mathematical correct rotation would be clockwise.
This is not only a matter of taste. The status quo complicates the correct computation of angles. In screen coordinates, a vector with polar angle 90° points downwards, but rotating a drawable by 90° makes it "point" upwards (given angle 0° corresponds right). This is a problem when interacting with other objects on the screen. For example, a particle emitter that has the direction of the mentioned vector, emits the particles in a "wrong" direction (compared to sf::Drawable). The angle of sf::Drawable is also inconsistent with sf::View. Rotating both the view and the drawable by 1° should have no effects, however the drawable appears rotated by 2°.
Generally, the user has to flip the sign of the rotation before drawing, as soon as he computes screen objects like projectiles which require more complex vector algebra. While this is certainly possible, it is counter-intuitive and probably confusing for beginners, once they delve into trigonometry.
Therefore, my proposal is to change the angle sign in the sf::Drawable methods. I know it's not an easy decision to modify such a thing, but you should take the opportunity of SFML 2 before it is released
