opengl - Using unProject correctly in Java Libgdx

Question

I want to make a button clickable, but it isn't working - it seems like I need to use unproject() but I can't figure out how. The code in question is:

Texture playButtonImage;
SpriteBatch batch;
ClickListener clickListener;
Rectangle playButtonRectangle;
Vector2 touchPos;
OrthographicCamera camera;

@Override
public void show() {
    playButtonImage = new Texture(Gdx.files.internal("PlayButton.png"));

    camera = new OrthographicCamera();
    camera.setToOrtho(false, 800, 480);
    batch = new SpriteBatch();

    playButtonRectangle = new Rectangle();
    playButtonRectangle.x = 400;
    playButtonRectangle.y = 250;
    playButtonRectangle.width = 128;
    playButtonRectangle.height = 64;
}

@Override
public void render(float delta) {
    Gdx.gl.glClearColor(0, 0, 0.2f, 1);
    Gdx.gl.glClear(GL10.GL_COLOR_BUFFER_BIT);

    camera.update();
    batch.setProjectionMatrix(camera.combined);

    batch.begin();
    batch.draw(playButtonImage, playButtonRectangle.x, playButtonRectangle.y);
    batch.end();

    if (Gdx.input.isTouched()) {
        Vector2 touchPos = new Vector2();
        touchPos.set(Gdx.input.getX(), Gdx.input.getY());


        if (playButtonRectangle.contains(touchPos)) {
            batch.begin();
            batch.draw(playButtonImage, 1, 1);
            batch.end();
        }
    }
}

Answer 1

In general you use camera.unproject(Vector) to transform your screen coordinates from a click or touch to your gameworld. This is needed because the origin is not necessarily the same and using the camera you can also zoom in and out, move around, rotate and so on. Unprojecting will take care of all of that and give you your game world coordinate matching the pointers position.

In your example it would go like this:

Vector3 touchPos = new Vector3(Gdx.input.getX(), Gdx.input.getY(), 0);
camera.unproject(touchPos);

Having this said you should actually not do this UI task manually. Libgdx has also some shipped UI functionality which is called a Stage (see this). There are lots of widgets already available (see this). They use skins (you can get a basic one from here, you need all the uiskin.* files). They automatically forward inputevents to the socalled Actors, e.g. a button, and you just need to implement handling of those events.