Collision Detection and responding to it

Collision Detection and responding to it

NickyAtfat

Member #8,918

August 2007

Hey guys, I'm working on an engine for a 2D action RPG using C++ and the Allegro library. I just discovered this forum so I decided to put forward a problem I have...

I have the code written to find out if one sprite is colliding with another sprite. But if one sprite goes inside something it shouldn't, like the ground, I want it to stop moving.

The problem: the sprites x and y positions are updated based on its velocity x and velocity y valuables. The code to stop the sprite going through the ground looks like this a the moment:

if (onSurface())
{
vY = 0;
}

However if the sprite is traveling too fast (for example after jumping or falling off a ledge) it can end up being stuck inside the ground. Any ideas? I have thought about using an old x and old y value and if the sprite collides, set the x and y values back to the old ones. However I don't think this would work because of the sprite traveling in incrementations of velocity x and y.

I would be really interested to hear how other people solve this and appreciate some help

Thanks,
Nicky

Jonatan Hedborg

Member #4,886

July 2004

The easiest would probably be to check for the future location. That is x+vX, not just x, and then backtrack (back away until they no longer overlap, then stop).

NickyAtfat

Member #8,918

August 2007

Thanks for you response!

Hmm I've been trying to implement that and I'm having trouble with the "backtracking" part... I'm not sure I understand what you mean... Maybe you have an example?

Jonatan Hedborg

Member #4,886

July 2004

Not really. But a non-optimal solution could be something like this (pseudo code)
(This is when colliding with non-moving objects, moving objects is a bit harder.)

Is movingObject.position+movingObject+velocity colliding with staticObject.position?
(
  Do for each point in a line going from movingObject.position to movingObject.position+velocity
    (
       Is currentPosition colliding with staticobject.position?
       (
          //This is the final position of the moving object. Set speed to 0 etc
       )
    )
)

You may want to do this on a per-axis basis oslt, not sure

NickyAtfat

Member #8,918

August 2007

Aha I was forgetting to set the velocity to 0, works fine now.

Thanks very much!