Self-made Platform engine: Am I doing it wrong?

Game Mechanics
#1

Hi guys!

I’m currently playing around with java game development a bit.
I usually only write Servers in Java, so this is my first graphical application I am working on.
I learned the basics of how game update and render loops work, and got everything running smoothly so far.

There’s just one problem I came across:
Since I don’t use any framework (I love doing things on my own) I do not have the slightest idea of how I should implement a Platformer movement system.
My current approach looks like this:

// Pseudocode!
if (detectCollision(x, y + verticalVelocity) {
	// Moves this instance to the floor
	setToGround();
} else {
	computeVelocity();
	y += verticalVelocity;
}

if (detectCollision(x + horizontalVelocity, y) {
	// Moves this instance to the wall
	stopMovement();
} else {
	computeMovement();
	x += horizontalVelocity;
}

So what I’m basically doing is to iterate over a List of Blocks which define the collision in the level when detecting collisions.

As long as I don’t use walls which stop the horizontal movement of the player, everything works just fine.
But as soon as I move against a wall, everything goes foobar. I guess that’s because my collision detection is not entirely right:

//Actual code
public int detectCollision(Model model, int offX, int offY)
	{
		int colliding = 0;
		
		int mTop 	= (int) (y + offY);
		int mBot 	= (int) (y + height + offY);
		int mRight 	= (int) (x + width + offX);
		int mLeft 	= (int) (x + offX);
		int oTop	= (int) (model.y);
		int oBot	= (int) (model.y + model.height);
		int oRight	= (int) (model.x + model.width);
		int oLeft	= (int) (model.x);
		int mMidX	= (int) (x + (width / 2) + offX);
		int mMidY	= (int) (y + (height / 2) + offY);
		
		if ((mRight >= oLeft) && (mLeft <= oRight)) {
			if ((mBot >= oTop) && (mBot <= oBot)) {
				colliding += 8;
			}
			if ((mTop <= oBot) && (mTop >= oTop)) {
				colliding += 2;
			}
		}
		if ((mBot >= oTop) && (mTop <= oBot)) {
			if ((mRight >= oLeft) && (mRight <= oRight)) {
				colliding += 1;
			}
			if ((mLeft <= oRight) && (mLeft >= oLeft)) {
				colliding += 4;
			}
		}
				
		return colliding;
	}

What this basically does is returning a number from which I can read WHERE the collision is happening exactly.
Since the feedback is binary, there only is one possible combination for each situation.

This way, I can check if the instance is straight below (e.g. 8, in this case) my entity.
Anyway, this does not seem to work right.

So, my question is: Am I doing this right? Should I just continue optimising this simple JnR engine or am I doing it completely wrong?
If so, can anyone tell me a simple mechanic of how to do that right?

P.S. Sorry if this question was asked already, I used this forum’s search function but could not find any fitting topic.

#2

I don’t think your approach is wrong because there’s a lot of ways to do the same thing in programming. What I would say is that there is a better way, in my opinion. You can implement collision points - each side of character has collision points that stop it from moving and a little further are collision points that are here just to detect on which side it is colliding with a wall or something. To illustrate my point, here’s an image:

Red squares are collision points that stop the movement.
Blue squares are collision points that are used as sensor (to detect which side is colliding).
You could also, instead of three sensor squares on each side, just have one rectangle, though.

This is how I apply collision detection to the game I’m developing except I don’t need sensor squares since the game doesn’t care what side is colliding, but what I illustrated in the picture is exactly how I would go about developing a platformer game. I’m not sure if it is the most efficient way, but I find it to be the best I know for now.

Kevin Glass explains all in an article Tile Maps. Just add sensor squares and gravity to his implementation.

EDIT: I guess you might not need the sensor squares, after all. You can check which side collided solely with collision points that stop the movement, but sometimes it seems as if it is better to have a sensor square a little further (inside other blocks) just in case.

#3

hello man, i have some days doing the same thing ( my own java game engine ), maybe i can help you, if i can help you, just let me know,

i have implemented, movement in x and y cordinates, colision, draw background ( images, tiles, hud, etc), i have a class sprite, i can rotate sprites or images, i can blend the backgrounds, detect colisions with tiles, add some control with keyboards, set animations for sprites, transitions, etc…