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[Zack Bell]

Typically, when starting a platformer prototype in Game Maker, I whip out a few scripts that I've come to love from Matt Thorsen's Grandma Engine.

One of the scripts checks to see if the player is on the ground, as well if the player is on top of a platform, but not colliding with a platform. This handles both solid ground and jump-throughs (to a certain extent).

Code:

// Original code with names changed to suit my project
return place_meeting(x, y + 1, oParentFloor) 
    || (place_meeting(x, y + 1, oParentPlatform) && !place_meeting(x, y, oParentPlatform));

Simple, but you can't stand on a platform while colliding with a different platform. Say you
use them as stepping stones and the player or an enemy is taller than two platforms that are stacked. Hopefully that makes sense?

So basically, I need to check if you're standing on a platform and not colliding with that same platform. I have come up with several ways to do it that work in 'most' cases. I also came up with a way to do it that seems very slow and troublesome (but it works).
 

Code:

// Slow, but useful
with (oParentPlatform) {
    if (place_meeting(x, y - 1, other) && !place_meeting(x, y, other))
        with (other)
            if (place_meeting(x, y + 1, other))
                return true;
}

return place_meeting(x, y + 1, oParentFloor);

Any advice on ways to do it differently or speed it up?

EDIT: For example, I can use instance_place() to return the ID of the platform I'm standing on and compare that to the platform I am colliding with. There are issues with that though; the big one being that instance_place() returns the first instance and you could be colliding with several. That is where I got the idea to step through the platforms with with().

[ThemsAllTook]

I'm not sure how this would work in Game Maker, but the way I normally do this is to keep an onGround boolean for each moving game object affected by gravity, set it to false right before collision detection, and set it to true when a downward collision is detected. When you're standing on the ground, you're continuously colliding with it every frame due to gravity, so it gets set back to true right away in that case. If you walk off the edge of a platform, since it's assumed to be false before collision detection is run, it stays false since there's no longer solid ground under you. When you jump, you'd of course manually set it to false.

[Zack Bell]

Sorry, maybe I wasn't super clear. I do have an onGround variable. Every frame I do the following:

Code:

onGround = CheckOnGround();

// CheckOnGround() being the code snippets that I posted in the OP

This shitty paint art demonstrates the abilities and disadvantages of the first way of doing things (Grandma Engine).



The third player on the right is standing on a platform, but also colliding with a separate platform. This causes him to fall through the bottom platform as well.

[ஒழுக்கின்மை (Paul Eres)]

i'd use collision_line -- imagine an invisible line under the player, like right at his feet. check that line's collision with platforms. the code you are using checks the *player's* collision with platforms

[Zack Bell]

Lol, dammit. That is why I asked you. This works perfectly.

Code:

var platformBelow, platformCollide;

platformBelow   = collision_line(bbox_left, bbox_bottom + 1, bbox_right, bbox_bottom + 1, oParentPlatform, true, false);
platformCollide = instance_place(x, y, platformBelow);

return place_meeting(x, y + 1, oParentSolid) || (platformBelow && (platformCollide != platformBelow));

Thank you, sir 

[robageejammin]

Hey there!

Having the same issue here where walking into a platform while standing on a different one drops me down, but having trouble applying the above solution to my current setup. Here's what I currently have for vertical platform collisions (based on a Shaun Spalding tutorial).

Code:

if vsp >0 && place_meeting(x,y+vsp,plat) && !place_meeting(x,y,plat)
{
    while(!place_meeting(x,y+sign(vsp),plat))
    {
        y += sign(vsp);
    }
    vsp = 0;
}

y += vsp;

Sorry if it is obvious, but do you think the same solution would apply here, and how do you think it would fit? Any help would be greatly appreciated, thanks!

[robageejammin]

Scratch that! Came up with a slightly different solution that I really like! Here it is below if it can help anyone.

Code:

//Collision with Platform
//Checks for collision from player's feet through where player's feet will be next frame
if vsp >0 && collision_rectangle(bbox_left,bbox_bottom+1,bbox_right,bbox_bottom+1+vsp,plat,0,1)
{
    //while loop: If there is no platform 1 pixel underneath, player will move down 1 pixel
    while(!collision_line(bbox_left,bbox_bottom+sign(vsp),bbox_right,bbox_bottom+sign(vsp),plat,0,1))
    {
        y += sign(vsp);
    }
    vsp = 0;
}
y += vsp;

//IMPORTANT: Platform mask must be 1 pixel in height for this method to work

Put it through rigorous testing and seems very reliable so far. As an added bonus, it also won't let your player fall through a platform if its falling too fast since it is taking vsp (or whatever your vertical speed variable is) into account.