It will also bounce off a wall/paddle at the same angle which it hit...flipped over the normal.
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Where the left is the path from where the ball came, the underline is the wall, the vertical line is the normal of the wall (right angle to wall) and the right hand slash is the angle at which it will leave. For computing the movement from the angles...depending on which way ball_angle points when it equals 0, you'll either want to do what is stated above, or:
ball.x += cos(ball_angle);
ball.y += sin(ball_angle);
