Thread: Collision Detection Problem

I am trying to program a pong game, and I am using openGL and Glut for the graphics.

In my main loop I draw the sprites and then test to see if they have collided, if so then I alter the velocites, either way I add to the coords of the ball and then redraw (to move the ball). Now, when I comment out the collide condition the ball moves across the screen fine, except it goes through the paddle. When I uncomment out the collide test, however, the ball just stays in the center, yet I can still move the paddles so the redraw portion is being exectuted properly. It seems as though the code after the test is not being executed. I am also lead to believe there is a problem with my collide function. Everything in the game is rectangluar even the ball (for now) and are initialize in a struct fyi. Here are my functions:

Code:

bool Collide(Rect *object1, Rect *object2) {
  
    float left1, left2;
    float right1, right2;
    float top1, top2;
    float bottom1, bottom2;

    left1 = object1->x1 + object1->col_x_offset;
    left2 = object2->x1 + object2->col_x_offset;
    right1 =object1->x2 + object1->col_width;
    right2 =object2->x2 + object2->col_width;
    top1 = object1->y1+ object1->col_y_offset;
    top2 = object2->y1 + object1->col_y_offset;
    bottom1 = object2->y1 + object1->col_height;
    bottom2 = object2->y2 + object2->col_height;

    if (bottom1 < top2) return true;
    if (top1 > bottom2) return true;
  
    if (right1 < left2) return true;
    if (left1 > right2) return true;

    return false;

}

Code:

void display(void)
{ 
     

        
  glClear(GL_COLOR_BUFFER_BIT);
  glClearColor(0.0,0.0,0.0,1.0);   
     
    glPushMatrix(); 
      glBegin (GL_LINES);
        glVertex2f(-3.0, 1.3);
        glVertex2f(3.0, 1.3);
        glVertex2f(-3.0,-1.3);
        glVertex2f(3.0,-1.3);
      glEnd ();
    glPopMatrix();
    
   glPushMatrix();  
    glBegin(GL_QUADS);
        glVertex3f(Paddle1.x1,Paddle1.y1,0.0);
        glVertex3f(Paddle1.x1,Paddle1.y2,0.0);
        glVertex3f(Paddle1.x2,Paddle1.y2,0.0);
        glVertex3f(Paddle1.x2,Paddle1.y1,0.0);
     glEnd();
    glPopMatrix();
             
   glPushMatrix();           
    glBegin(GL_QUADS);
        glVertex3f(Paddle2.x1,Paddle2.y1,0.0);
        glVertex3f(Paddle2.x1,Paddle2.y2,0.0);
        glVertex3f(Paddle2.x2,Paddle2.y2,0.0);
        glVertex3f(Paddle2.x2,Paddle2.y1,0.0);
     glEnd();
    glPopMatrix();         
             
    glPushMatrix(); 
     glBegin(GL_QUADS);
        glVertex3f(ball.x1,ball.y1,0.0);
        glVertex3f(ball.x1,ball.y2,0.0);
        glVertex3f(ball.x2,ball.y2,0.0);
        glVertex3f(ball.x2,ball.y1,0.0);
     glEnd();
    glPopMatrix();
   
   if (Collide(&ball,&Paddle1)){vx=-vx;vy=-vy;}
      
   ball.x1=ball.x1+vx;
   ball.x2=ball.x2+vx;
   ball.y1=ball.y1+vy;
   ball.y2=ball.y2+vy;
     
     glutSwapBuffers();

}

If anyone could provide me with any ideas that would be a huge help.

Thanks

Code:

    top1 = object1->y1+ object1->col_y_offset;
    top2 = object2->y1 + object1->col_y_offset;
    bottom1 = object2->y1 + object1->col_height;
    bottom2 = object2->y2 + object2->col_height;

    if (bottom1 < top2) return true;

I think you mean

Code:

bottom1 = object1->y1 + object1->col_height;

As is, the if statement is always true, unless the top of object2 is below its bottom (not necessarily a nonsensical statement if the collision box rotates).

Hope that helps.

EDIT: Also, use more parentheses to make reading your code easier. For example, in this line

Code:

if (Collide(&ball,&Paddle1)){vx=-vx;vy=-vy;}

It's hard to tell if you mean "assign value of inverted (hope that's right) vx to vx", or if you meant to say "calculate value of vx minus vx, then assign the result to vx" and got the '-' and '=' the wrong way round.