Basic 2d platform collision detection algorithm has a slight bug

Question

I'm trying to implement collision detection between Player (the main character) and a Platform. They are both modelled as rectangles; when player hits the side of a platform he should bounce off it and when player hits the top of a platform he should glide (move) along it. The problem I'm facing is that sometimes the player bounces when he lands on top of the platform (near an edge) rather than glide along it.

For the collision detection, I'm using the Intersector class (LibGDX). If you are unfamiliar with this class, all it does is create the new rectangle that occurs when two rectangles intersect (i.e. the rectangle shaded green below).

Code:

for (int i = 0; i < list.size(); i++) {

    // An object from a list of objects that the player can collide with
    GameObject go1 = list.get(i);

    // True if the object is of type platform
    boolean playerPlatformCollision = (go1.getType() == ObjectType.PLATFORM);

    // If the two objects being compared are Player and Platform
    if (playerPlatformCollision) {
        Rectangle objectRect = go1.getRect();

        // If the Player and Platform overlap
        if (player.getRect().overlaps(objectRect)) {

            // The resulting rectangle from the overlap (as shaded green in the above pic)
            Rectangle intersection = new Rectangle();
            Intersector.intersectRectangles(player.getRect(), objectRect, intersection);

            // If the Player collides with bottom side of platform
            if (intersection.y > player.getRect().y) {
                player.setySpeed(-getySpeed()); // Make him bounce downwards 
            }

            // If the Player collides with left side of platform
            if (intersection.x > player.getRect().x) {
                player.bounce(); // Make him bounce
            }

            // If the player collides with the top side of platform
            if (intersection.y + intersection.height
                < player.getRect().y + player.getRect().height)
            {
                player.setGravity(false);
                player.onPlatform(true);
            }
            else {
                player.onPlatform(false);
            }
        }
    }
}

Is there any sort of trick I can use for this? Haven't been able to find anything online that works.

Answer 1

Yes, there is a trick you can use here. Based off of your glitch and your code sample, I'm assuming that you're moving both the player's X and Y position at the same time, then checking for collision.

Instead, move the player by its X velocity, then check for collision. If it has collided with any platform, then it must have hit the side of a platform. Take any action necessary for this collision, such as backing the player out of the platform and negating its X velocity.

After X velocity has been handled, you can now move the player by its Y velocity. If there is a collision, then it must have hit either the ceiling or the floor of a platform. Back the player out of the floor/ceiling, then set Y velocity to zero.

Hope this helps!

Answer 2

Found an easy solution to the problem. I just compared the midpoint x-coordinate of the player to the x-coordinate of the intersection to see where the player was relative to the platform (i.e. left or right). If the x midpoint of the player's rectangle > x point of intersection rectangle, then he should be on top - otherwise, he's on the side of the platform. Changed the if statements for left side and top side to:

// left side of platform
if (intersection.x > player.getRect().x
    && player.rectangleMidPointX() < intersection.x) {
    // ...
}

// top side of platform
if (intersection.y + intersection.height < player.getRect().y + player.getRect().height
    && player.rectangleMidPointX() > intersection.x) {
    // ...
}