Question

I’m making a 2D game, nothing fancy, with objects (called entities from now on) that should collide with eachother and terrain. Now, detecting collision is not the problem, the problem is when the entities collide with the terrain.

I have a simple .png, upon loading the game I go through all pixels of this image and add all black ones to a list, these are the pixels that is collidable.

Collision-code from Entity-class:

    int ceiling = Integer.MAX_VALUE;
    int floor = 0;
    int rightPoint = 0;
    int leftPoint = Integer.MAX_VALUE;

    for(Point p : main.terrain.points)
        if(getHypoRect().contains(p))
        {
            if(p.x > rightPoint && right)
                rightPoint = p.x;
            if(p.x < leftPoint && left)
                leftPoint = p.x;
            if(p.y < ceiling && up)
                ceiling = p.y;
            if(p.y > floor && down)
                floor = p.y;
        }
    if(rightPoint != 0)
        xCoord = rightPoint - sprite.getImage().getWidth();
    else if(right)
        xCoord+=speed;

    if(leftPoint != Integer.MAX_VALUE)
        xCoord = leftPoint;
    else if(left)
        xCoord-=speed;

    if(ceiling != Integer.MAX_VALUE)
        yCoord = ceiling;
    else if(up)
        yCoord-=speed;

    if(floor != 0)
        yCoord = floor - sprite.getImage().getHeight();
    else if(down)
        yCoord+=speed;

In the for-loop I go through all points (black pixels) in the terrain, then check if the Entity’s collisionbox contains them (speed is added in this method, or rather “hypothetically” added, that’s why it has the weird name).
Now, if the x-value of the point is greater than the “rightpoint”, and if the Entity is walking to the right, it’s overwritten. The same happens with leftPoint (“leftest” point), ceiling and floor.
Then I check if any of these points have been changed, if they have that means we collided with the terrain so we bounce them back. For example, say the Entity is walking to the right and we collided with the terrain, rightPoint have been written with the “rightest” point we collided with so then we set the coordinate so that the Entity is right beside the point that it collided with.

If one would only go at one direction at a time, this would work perfectly, but “weird” things happen when you take multiple inputs into account.

Example: the Entity walks, flat to the floor, towards a ledge on the right, when the Entity is halfway over the ledge, it wants to go down, so both right and down are pressed, what my code will do is check the inputs, put the Entity where it wants to be and then bounce it back if it hit terrain, so it will see that I press right and bounce the Entity back to the end of the ledge.

This is all my own idea of how terraincollision could work, I couldn’t find anything online except how to check if a collision took place, but nothing that covers this.
Any idea what could make my solution work, or a pointer to how terraincollision should be done?

Answer 1

Your bouncing back idea maybe problematic. Try instead to ignore moves that would collide with the terrain:
If the player moves down and right, then first check if down would collide, if yes, then ignore that movement, and check for right, if right is possible do right movement, else block, too.