C++ Collision Detection doesn't work on last check?

Question

Over the last few days I have been trying to implement simple collision detection of objects drawn using OpenGL. With the aid of the Pearson, Computer Graphics with OpenGL I have managed to write the following function:

void move(){
    if(check_collision(sprite,platform1) || check_collision(sprite,platform2)){ //if colliding...
        if     (downKeyPressed ){ y_Vel += speed; downKeyPressed  = false;} //going down
        else if(upKeyPressed   ){ y_Vel -= speed; upKeyPressed    = false;} //going up
        else if(rightKeyPressed){ x_Vel -= speed; rightKeyPressed = false;} //going right
        else if(leftKeyPressed ){ x_Vel += speed; leftKeyPressed  = false;} //going left
    } // always glitches on whatever is last else if above?!?!
    else{ glTranslatef(sprite.x+x_Vel, sprite.y+y_Vel, 0.0); }
}

My sprite moves in accordance to keyboard inputs (the arrow keys). If it collides with a stationary object it stops moving and stays in its position.

So far, it works when colliding with the top, left side and bottom of the stationary object. Unfortunately (even though I use the same logic) the right hand side doesn't work and upon a collision the sprite is redrawn at its original x/y coordinates. I'm baffled.

As it turns out, which-ever is the last check in the move() function (the last else-if) is the one that doesn't work... I have swapped the left with the right and sure enough when left is then the last one and the one that plays up :(

Any advice or ideas on how I can improve this and stop it glitching?

Please excuse my naivety and amateur coding. I'm merely a self-taught beginning. Thanks.

I'd try to change float speed to int speed along with x_vel and y_vel, to rule off the rounding error issues first of all... — vines
– vines, Commented Mar 21, 2013 at 18:06
also, it's not clear what you mean by redrawn at original x/y coordinates — vines
– vines, Commented Mar 21, 2013 at 18:10
Thanks i'll change the type now. Well before any keys are pressed, the sprite is drawn in the bottom left of the screen and when it collides with the right hand side of the stationary object it basically deletes the sprite and moves/redraws it back in the bottom left corner... — Reanimation
– Reanimation, Commented Mar 21, 2013 at 18:11

DiegoNolan · Accepted Answer · 2013-03-21 18:10:09Z

2

You should not use an else if. There is a possibility that it is hitting a side and the top or the bottom in the same frame. Trying changing those to all ifs because you want to check each one. or at the least change it to this.

if( /* check top */)
{

}
else if( /* check bot */)
{

}

if( /* check right */ )
{

}
else if( /* check left */)
{

}

Also, you should avoid declaring global variables like Y_VEL and X_VEL as this creates confusion. You may just be doing this to get your feet wet but I would avoid it. Create a class for each object and then have the velocities as members of that class.

answered Mar 21, 2013 at 18:10

DiegoNolan

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2 Comments

Reanimation Over a year ago

Ok, thanks for letting me know. That makes perfect sense. I've now changed it. Unfortunately it doesn't solve the glitch

DiegoNolan Over a year ago

Did you ever figure this out? I'm curious.

Serge · Accepted Answer · 2013-03-21 18:17:03Z

1

Well, it seems to me that you have an issue when it comes to translating the actual object. Your move code states

if(there is a collision)
{ 
    //do stuff
}
else
{
    glTranslateF( );
}

SO, whenever there is a collision, the translate function never gets called.

My opinion is that you should pull the glTranslateF() call out of the else {...}, just have it get called every time. However, it seems you're using the exact same 'draw' function for every rectangle, not just the player sprite. You'll probably have to ind a way to distinguish between regular rectangles (such as the platforms) and the player rectangle. Perhaps the simplest way for you to implement this, would be to have two different drawSprite functions: one for regular platforms, and the other for the player. Only call the move() function from within the player's draw function (perhaps called drawPlayer()?)

I don't quite have the time to look over all of your code to make a more educated and specific suggestion at the moment; however, I'll be free later this evening if this question is still open and needing help.

answered Mar 21, 2013 at 18:17

Serge

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7 Comments

Reanimation Over a year ago

I think I understand what you're saying. I do however have two functions to do the drawing. One for the Sprite and a second for 2 individual platforms. If you get time later that would be great, I appreciate any help I can get.

vines Over a year ago

@Serge By the way, he draws sprites in a way which doesn't require translation at all: glVertex3f(rect.x, rect.y+rect.h, 0.0); -- he moves the very vertices themselves.

Reanimation Over a year ago

The thing is, if i remove the glTranslate, the sprite doesnt move when a key is pressed...

Reanimation Over a year ago

It finally twigged what you meant. I've taken the glTranslate() call out of the else{...} (having it being called every time) and it now collides will all edges and not just the top, left and bottom. Thanks for taking the time to post your answer. I'll do a few more checked, but i think thats the problem solved. :)

Serge Over a year ago

@Reanimation just posted and delete two stupid comments. I'll come back when I have more time to focus, sorry!

|

vines · Accepted Answer · 2013-03-21 18:41:47Z

As you've already figured out, the problem is related to glTranslate(). Since you operate on both sprite's position and velocity, you should repeatedly update the positions using the velocities. That is:

sprite.x += x_vel;
sprite.y += y_vel;

and do it simply all the time (i.e. by some timer or every frame, if the scne is redrawn repeatedly). A collision then is equivalent to changing the velocity vector (x_vel, y_vel) in some way to simulate the collision effect: either zero it to stop any movement at all, or change the velocity component sign (x_vel = -x_vel) to make it rebound in an absolutely elastic manner, or do something else that fits.

What happens now with glTranslate() is that x_vel, y_vel actually hold offsets from the starting position, but not velocities of movement.

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