Collision Problems

Question

I'm having some troubles with a 2D platformer type game (there's more to it, but that's all that matters for this question). I had collision with the platforms working fine, but if the character hit the side of the platform, they would be automatically be placed on top of the platform. This is bad, for obvious reasons. I want the player to hit the side and fall, like one would expect. So what I'm trying to do is simply keep track of the previous position of the player, and if the player's bounding box overlaps the platform's bounding box, move the player back to their previous position. The problem is, when the player is on the platform, they cannot move left or right. They can jump, they can move while they're in the air, and their falls are stopped by the platforms. I just can't figure out why they won't move anymore. Here's some relevant code:

// previous position is stored
previousPos = new Vector2(playerPosition.x, playerPosition.y);

....
// game checks input and calculates next position and
// stores it in playerPosition. only changes the Vector2
// playerPosition, does not display the change via the sprite yet,
// so it doesn't look like it's stuttering to the player
....

if (!isJumping) {
    // loop through the platform list
    for (Platform platform : platformList) {
        // if the player is touching a platform, then they are on a platform
        // and they are not falling
        if (platform.getBoundingBox().contains(playerBoundingBox)) {
            playerPosition = new Vector2(previousPos.x, previousPos.y);
            boundingBoxPositionLower = new Vector3(playerPosition.x, playerPosition.y, 0);
            boundingBoxPositionUpper = new Vector3(playerPosition.x+playerSprite.getWidth(), playerPosition.y+playerSprite.getHeight(), 0);
            playerBoundingBox.set(boundingBoxPositionLower, boundingBoxPositionUpper);
            isOnPlatform = true;
            isFalling = false;
            playerDirection.y = 0;
            break;
        }
    }
}

I'm not sure where to go from here. Maybe ideas on how to debug? Or maybe it's just a simple problem I'm overlooking. If you need any other code snippets, let me know.

Thanks guys!

PS - Sorry I don't follow the 80 characters per line standard, it all fits on my 1080p monitor fine in my IDE :P .

Answer 1

A little late, but posted as an answer as requested.

They wont move any more because they 'hit' the platform, so you're moving them back to their previous position. You need to check their new position against the old position. If they moved down into the platform from the top (as with gravity), you just need to use their old Y position, not the X.

To solve the 'sticking to the side' issue, you'd do the same as the 'sticking to the top' solution, but just revert the X and leave the Y in tact.

In the case where you move the character too far back, you could just calculate the exact X or Y position using the width/height of the bounding boxes.

Eg. If you're using the centroid as your position, and the player is moving left-to-right and hits a wall:

player.X = wall.X - (wall.Width / 2) - (player.width / 2);

Answer 2

You really just got to do the inverse for side collisions, go back to the old X and let the Y go on to continue the fall. Beware of corners where you hit two different blocks, there you got to make the full stop.

And by the way, ideally you shouldn't move all the way back to the old position, but just far enough that you don't collide.