The position of the paddle when the ball hits it is wholely irrelevant - the paddle is the same all over, flat, and moves in a constrained direction ( in its own plane ), so any moving point on the paddle is 'identical' to any other point in terms of any calculation you perform. The only areas of the paddle that ARE different are the corners, do you model that accurately?
The quick trick you describe ( inverting one of the components of the moving object's velocity on a bounce with a plane ) works irrespective of the initial angle that the ball is travelling at; if you start the ball moving at any angle other than 45 deg, then bounce angles will be a little more 'varied', because only 45 deg will invert as itself. Any other angle will have slightly more possible results.
But.. I get what you're saying. Given any initial starting angle, and assuming that the player is 'perfect', and assuming that you don't handle corners of the paddle in any special way; the movement of the ball will always be exactly the same; the game is deterministic with a perfect player.
I am not sure if that's how breakout always is.. Alot of basic collision detection code assumes that a collision happens instantanously, and I believe ( although I may be completely wrong on this! ) that if a perfect sphere bounces off a perfect plane, that the velocity of the plane that is
in the plane doesn't affect the bounce angle of the sphere. Like I say, I may be wrong
For a quick solution, instead of doing this with the ball velocity after a bounce:
velocity2[x] = velocity1[x];
velocity2[y] = velocity1[y] * -1;
try this:
velocity2[x] = velocity1[x] + ( paddle_velocity[x] * some_constant );
velocity2[y] = velocity1[y] * -1;
[[ some_constant should be between 0 = original behaviour and 1 = lots of sideways effect ]]
the effect here, is that if the paddle moves in the horizontal direction that the ball is moving, the ball will bounce 'flatter', if the paddle moves in the opposite horizontal direction, the ball bounces upwards more sharply. this is sorta like the paddle being rough, the collision being non-instanteanous, and 'pushing' the ball sideways slightly if the paddle moves fast enough.
bear in mind, that this will add energy to your system ( or remove it ) over time. normalize the post-collision velocity and multiply it by the length of the pre-collision velocity to prevent that.. [[ in a system that doesnt have a player adding forces, you'd normally conserve the energy by pushing the paddle backwards ( or starting it spinning ) after the bounce.. but.. that just wouldn't be breakout would it. ]]
