Collision Detection and maintaining momentum on an object

Question

I have been implementing various forms of simple collision detection with varying results. I have a fairly good working version of collision detection, but there are some odd behaviors that I can't work out.

Just for a reference, i'm making a simple pong game, and trying to refine the collision. The problems I get are when the ball collides with the paddle on either the top or bottom side. In those cases, the ball hovers above (or below) the paddle and does not move. I'm guessing this is because of how i'm checking for collision and how i'm altering the movespeed of the ball.

I would like to implement a way I differentiate between top/bottom and left/right collision but this is the only method that works decently:

static void CheckCollision(PActor object1, PActor object2, PInput pinput)
{
    if ( CheckObjectCollision( object1, object2 ) )
    {
        AdjustMoveSpeed( object1, object2, pinput );
    }
}



static bool CheckObjectCollision(PActor object1, PActor object2)
{
    int object1LeftBound = object1.position.x;
    int object1RightBound = object1.position.x + object1.actorTextureXSize;
    int object1TopBound = object1.position.y;
    int object1BottomBound = object1.position.y + object1.actorTextureYSize;

    int object2LeftBound = object2.position.x;
    int object2RightBound = object2.position.x + object1.actorTextureXSize;
    int object2TopBound = object2.position.y;
    int object2BottomBound = object2.position.y + object2.actorTextureYSize;

    if ( object1RightBound < object2LeftBound )
        return false;

    if ( object1LeftBound > object2RightBound )
        return false;

    if ( object1BottomBound < object2TopBound )
        return false;

    if ( object1TopBound > object2BottomBound )
        return false;

    return true;        
}

I am guessing that the root of some of the problems i'm having is the function AdjustMoveSpeed, here it is:

static void AdjustMoveSpeed(PActor object1, PActor object2, PInput pinput)
{
    PVector prevMouseLocation = pinput.GetPrevMouseLocation();
    PVector currMouseLocation = pinput.GetCurrMouseLocation();

    int currentMoveSpeed;
    int nextMoveSpeed;

    if (typeid(object1) == typeid(PBall))
    {

        object1.moveSpeed.x *= -1;  

        if ( typeid(object2) == typeid(PPlayer) )
        {
            currentMoveSpeed = object1.moveSpeed.y;
            nextMoveSpeed = prevMouseLocation.y  - currMouseLocation.y;

            object1.moveSpeed.y = (prevMouseLocation.y  - currMouseLocation.y) * -1; 

        }
        else
        {
            if (object1.moveSpeed.y > 0)
                object1.moveSpeed.y *= -1;
        }
    }
    else if (typeid(object2) == typeid(PBall))
    {

        object2.moveSpeed.x *= -1;

        if ( typeid(object1) == typeid(PPlayer) )
        {
            currentMoveSpeed = object1.moveSpeed.y;
            nextMoveSpeed = prevMouseLocation.y  - currMouseLocation.y;

            object2.moveSpeed.y = (prevMouseLocation.y  - currMouseLocation.y) * -1; 
        }
        else
        {
            if (object2.moveSpeed.y > 0)
                object2.moveSpeed.y *= -1;
        }
    }       
}

What I was attempting to do with AdjustMoveSpeed, is first check to see which object is the ball, after this, multiply the x move speed by -1 to reverse its direction. After this, I check to see if the other object is a player, if so I set the y move speed to the difference between the previous mouse location and current mouse location. This is here to give the player option to change the balls y speed, or add spin.

I've tried checking for intersection between objects so that I can get a specific side, and the result is the ball just flying in the middle of the screen without actually hitting either paddle.

How do I properly check for collision detection on two objects that are squares?

How can I fix AdjustMoveSpeed so that it works properly with collision detection?

Lastly, how do I keep the momentum of the ball of its current speed is greater than the difference of the mouse location before and after the hit?

I've tried taking comparing the absolute value of currentMoveSpeed and nextMoveSpeed but then the ball doesn't change y speed. Something like this:

if ( abs(currentMoveSpeed) < abs(nextMoveSpeed )
    object1.moveSpeed.y = (prevMouseLocation.y  - currMouseLocation.y) * -1;
else
  object1.moveSpeed.y *= -1

score 6 · Accepted Answer · edited May 23 '17 at 12:00

Pong is simple enough that, rather than moving the ball each frame and checking for a collision with a paddle, you can actually solve the equation for when the paddle and ball will collide - if that time is less than one frame, there is a collision.

This completely eliminates the issue of the ball moving so fast it moves through the paddle, an issue that plagues many pong-clones that use the naive method of collision-detection.

This solution is called continuous collision detection - see this answer for more information.

score 2 · Answer 2 · answered Mar 07 '12 at 15:47

2

If the ball gets stuck on the paddle instead of bouncing it is probably because it keeps changing direction back and forth. The ball should only bounce if it is heading towards the paddle.

if (sgn(object1.moveSpeed.x) == sgn(object1.x - object2.x)) {
  // Ball is already moving away from the paddle, don't bounce!
}
else {
  // Ok to bounce!
  object1.moveSpeed.x *= -1;
}

answered Mar 07 '12 at 15:47

Klas Lindbäck

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Thank you, that is correct, as far as changing direction back and forth. What does sgn(object1.moveSpeed.x) do? I haven't encountered that in D yet. Assuming its a D function. – RedShft Mar 07 '12 at 20:33
I don't know about D, but sgn is the sign function that returns +1 for positive numbers and -1 for negative numbers. – Klas Lindbäck Mar 07 '12 at 22:24

Collision Detection and maintaining momentum on an object

2 Answers2