[pbivens]

I am working on a pong game using c++ and dx9.I want the ball to bounce off the screen walls.here is the code I am working on.Right now the ball is going off the screen.

		ball_move_x += ball_vel;

		if(ball_move_x>=screen_width)
		{
		ball_move_x -= ball_vel;
		}

I think I have to reverse the velocity of the ball when it hits the screen side.

[Nano511]

yeah when the ball hits the wall just do "ball_vel = -ball_vel;"

[LittleGyros]

Yes it's right to invert the x-velocity, but i'd also go one step back before doing this.

xPos += xVel;
if(xPos >= screenWidth)
{
   xPos -= xVel;
   xVel = -xVel;
}

this way you're avoiding some strange ball/wall interaction..