Changing the Coordinate System in LibGDX (Java)

LibGDX has a coordinate system where (0,0) is at the bottom-left. (like this image: http://i.stack.imgur.com/jVrJ0.png)

This has me beating my head against a wall, mainly because I'm porting a game I had already made with the usual coordinate system (where 0,0 is in the Top Left Corner).

My question: Is there any simple way of changing this coordinate system?

If you use a Camera (which you should) changing the coordinate system is pretty simple:

camera= new OrthographicCamera(Gdx.graphics.getWidth(), Gdx.graphics.getHeight());
camera.setToOrtho(true, Gdx.graphics.getWidth(), Gdx.graphics.getHeight());

If you use TextureRegions and/or a TextureAtlas, all you need to do in addition to that is call region.flip(false, true).

The reasons we use y-up by default (which you can easily change as illustrated above) are as follows:

• your simulation code will most likely use a standard euclidian coordinate system with y-up
• if you go 3D you have y-up
• The default coordinate system is a right handed one in OpenGL, with y-up. You can of course easily change that with some matrix magic.

The only two places in libgdx where we use y-down are:

• Pixmap coordinates (top upper left origin, y-down)
• Touch event coordinates which are given in window coordinates (top upper left origin, y-down)

Again, you can easily change the used coordinate system to whatever you want using either Camera or a tiny bit of matrix math.

Just to expand a little on what badlogic said above, if you are using a TextureAtlas (with TextureRegions) you need to flip them, as badlogic said, in addition to the camera work. If you are using a TextureAtlas, you can use this code right after loading your atlas:

String textureFile = "data/textures.txt";  
atlas = new TextureAtlas(Gdx.files.internal(textureFile), Gdx.files.internal("data"));  
// Let's flip all the regions.  Required for y=0 is TOP
Array<AtlasRegion> tr = atlas.getRegions();      
for (int i = 0; i < tr.size; i++) {
  TextureRegion t = tr.get(i);
  t.flip(false, true);
}

If you want to hide the transformation and not think about it after setting it up once, you can make a class that inherits all of the functionalities you need, but first transforms the coordinates before passing it to its parent class's function. Unfortunately, this would take a lot of time.

You could alternatively make a method that does the simple y' = height - y transformation on the whole Coordinate object (or whatever it is you're using), and call it once before each operation.

Interesting graphics library, I would say. I found this assessment from the link below:

> Another issue was that different coordinate systems were used in different parts of Libgdx. Sometimes the origin of the axes was in the > bottom left corner with the y-axis pointing upwards and sometimes in > the top left corner of the sprite pointing downwards. When drawing > Meshes the origin was even in the center of the screen. This caused > quite a bit of confusion and extra work to get everything in the > correct place on the screen.

http://www.csc.kth.se/utbildning/kandidatexjobb/datateknik/2011/rapport/ahmed_rakiv_OCH_aule_jonas_K11072.pdf

I just made a class that extends SpriteBatch that overides certain methods adding y = Gdx.graphics.getHeight() - y - height. Simple but effective.

I was able to get textures and fonts rendering correctly using the suggested flipped coordinate system via OrthographicCamera. Here's what I did:

private SpriteBatch batch;
private BitmapFont font;
private OrthographicCamera cam;
private Texture tex;

@Override
public void create () {
	batch = new SpriteBatch();
	
	font = new BitmapFont(true);
	font.setColor(Color.WHITE);
	
	cam = new OrthographicCamera(Gdx.graphics.getWidth(), Gdx.graphics.getHeight());
	cam.setToOrtho(true, Gdx.graphics.getWidth(), Gdx.graphics.getHeight());
	tex = new Texture("badlogic.jpg");
}

@Override
public void dispose() {
    batch.dispose();
    font.dispose();
    tex.dispose();
}

@Override
public void render () {
	cam.update();
	batch.setProjectionMatrix(cam.combined);
	
	Gdx.gl.glClearColor(0, 0, 0, 1);
	Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
	batch.begin();
	
	font.draw(batch, "Test", 50, 50);
	batch.draw(tex, 100, 100, tex.getWidth(), tex.getHeight(), 0, 0, tex.getWidth(), tex.getHeight(), false, true);
	
	batch.end();
}

Important things to notice are:

• The BitmapFont constructor, the boolean flips the font
• For batch.draw() you need to use all those parameters because you need a boolean flipY at the end to flip the texture (I may extend SpriteBatch or make a utility method to avoid passing so many parameters all the time.)
• Notice batch.setProjectionMatrix(cam.combined); in render()

Now we will see if I am back here later tonight doing edits to fix any other issues or discoveries with doing all this.