Platforms collide-able on one side only

Question

I have frequently tried to make some sort of a platformer, but when I actually code it, I run into a bit of a snag. How would I be able to make platforms in the air stop objects if they hit the sides or bottom, but allow them to sit on top of them?

I have tried many different methods, and some work slightly, but have many bugs, or some don't even work at all. Like this one I tried made it so that if you hit the side of the platform at the right angle, it forced you up on top of it.

Answer 1

Without knowing much about how your code works it's hard to give an exact answer, but something like this should work:

if (collision(platform) && pos.y - height - vel.y < platform.y && vel.y > 0) {
    // Player collides, was over the platform the last update and is falling
    vel.y = 0;                      // Stop player
    pos.y = platform.y - height;    // Position player on platform
    onGround = true;
}

If the registration point of the player is centered you would use height/2 instead of height, and you might have to take the platforms height into account as well if it's registration point isn't at the top.