When there are multiple objects of the same type, how do I know which one I am colliding with?

Question

I'm working on a platformer game using Gamemaker Studio 2. , and currently I'm dealing with implementing platforms (those thin tiles that are able to pass through from the bottom, but remains solid when landing on).

To use some more advanced collision detection, I'm using place_meeting() instead of the build-in Collision Event.

It's working good and well for a single platform, but when I add multiple, then only the lowest placed platforms will work properly. I'm already aware that the problem lies on the second if-statement in the code below:

if (place_meeting(x, y+mySpeed[1], o_platform) &&       //If the player collides with a platform
    mySpeed[1] > 0  )                                   //if the player is falling
{
    if ((y + 29) < o_platform.y)                        //if the player's feets are located higher than a platform
    {
        while !place_meeting(x, y+sign(mySpeed[1]), o_platform)
        {
            y += sign(mySpeed[1]);
        }
        mySpeed[@ 1] = 0;
    }
}

The problem is likely because it's reading information from any platform. but not the higher platform I'm colliding with. So I should need a check to make sure the program knows it's the same object I'm colliding with.

If this was the build-in Collision Event, I could use the other statement to get the right instance, but currently I'm using place_meeting in a step method to collide, and other would be too broad to use properly. So how would I be able to archieve this?

Answer 1

Instance_place did the trick.
If you want to use the object ID outside the collision event, then you can use Instance_Place instead of Place_Meeting.

var myObject = instance_place(x, y, object)
if (myObject != noone)
{
    //...you can use myObject as the object it's colliding with.
}

Answer 2

• Have an array objects (or a dict of )
• Give your objects a ulong ID field
• Whenever you create an object, assign it an ID incrementally and then index it into the array with that id

Now whenever you collide with an object you just called:

var myCollider = arrayOfObjects[object.ID]

NOTE: It'd be better to do this with a Dictionary if you're in C#, but since I don't know what language you're in I gave you a starting point of using an array. An array in this implementation may well waste a lot of memory if you have lots of destructable objects.

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The other way to do this is usually done by a Delegate (or a callback depending on your language.) I would imagine if you're in game-maker that you can probably find something out there on this; but the above will work either way.