AlphaBlending.md

Alpha Masking Without Shaders

Shaders are often more efficient, but for simple purposes this may do the trick. It also means it supports GLES10.

The three textures can be downloaded here.

BlendTest.java

import com.badlogic.gdx.ApplicationListener;
import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.backends.lwjgl.LwjglApplication;
import com.badlogic.gdx.graphics.GL10;
import com.badlogic.gdx.graphics.OrthographicCamera;
import com.badlogic.gdx.graphics.Texture;
import com.badlogic.gdx.graphics.Texture.TextureFilter;
import com.badlogic.gdx.graphics.g2d.SpriteBatch;

public class BlendTest implements ApplicationListener {

	public static void main(String[] args) {
		LwjglApplication app = new LwjglApplication(new BlendTest(), "Test", 480, 320, true);
	}
	
	OrthographicCamera cam;
	SpriteBatch batch;
	Texture bg, sprite, alphaMask;

	@Override
	public void create () {
		cam = new OrthographicCamera();
		batch = new SpriteBatch();
		
		bg = new Texture("data/badlogic.jpg");
		sprite = new Texture("data/grass.png");
		alphaMask = new Texture("data/mask.png");
		alphaMask.setFilter(TextureFilter.Linear, TextureFilter.Linear);
	}

	@Override
	public void resize (int width, int height) {
		cam.setToOrtho(false, width, height);
		batch.setProjectionMatrix(cam.combined);
	}

	@Override
	public void render () {
		Gdx.gl.glClear(GL10.GL_COLOR_BUFFER_BIT);
		
		
		int w = Gdx.graphics.getWidth();
		int h = Gdx.graphics.getHeight();
		
		batch.begin();
		
		//tell SpriteBatch not to apply any blend modes...
		
		//regular blending mode
		batch.setBlendFunction(GL10.GL_SRC_ALPHA, GL10.GL_ONE_MINUS_SRC_ALPHA);
		
		//draw background with regular blending
		batch.draw(bg, 0, 0, w, h); 
		batch.flush();

		//the sprite we want the circle mask applied to
		int x = 25;
		int y = 50;
		int spriteWidth = 200;
		int spriteHeight = 200;
		
		//now draw the alpha into the frame buffer
		Gdx.gl.glColorMask(false, false, false, true);
		batch.setBlendFunction(GL10.GL_ONE, GL10.GL_ZERO);
		
		batch.draw(alphaMask, x, y, spriteWidth, spriteHeight);
		batch.flush();
		
		//now that the buffer has our alpha, we simply draw the sprite with the mask applied
		Gdx.gl.glColorMask(true, true, true, true);
		batch.setBlendFunction(GL10.GL_DST_ALPHA, GL10.GL_ONE_MINUS_DST_ALPHA);
		
		batch.draw(sprite, x, y, spriteWidth, spriteHeight);
		batch.flush();
		
		batch.end();
	}

	@Override
	public void pause () {
		
	}

	@Override
	public void resume () {
		
	}

	@Override
	public void dispose () {
		batch.dispose();
		alphaMask.dispose();
		sprite.dispose();
		bg.dispose();
	}
	
	
}