LibGDX blending example : substract shapes

BlendingDemo.java

package net.mgsx.blending;

import com.badlogic.gdx.Game;
import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.Input;
import com.badlogic.gdx.ScreenAdapter;
import com.badlogic.gdx.backends.lwjgl.LwjglApplication;
import com.badlogic.gdx.backends.lwjgl.LwjglApplicationConfiguration;
import com.badlogic.gdx.graphics.Color;
import com.badlogic.gdx.graphics.GL20;
import com.badlogic.gdx.graphics.Pixmap.Format;
import com.badlogic.gdx.graphics.Texture;
import com.badlogic.gdx.graphics.g2d.Batch;
import com.badlogic.gdx.graphics.g2d.SpriteBatch;
import com.badlogic.gdx.graphics.glutils.FrameBuffer;
import com.badlogic.gdx.graphics.glutils.ShapeRenderer;
import com.badlogic.gdx.graphics.glutils.ShapeRenderer.ShapeType;
import com.badlogic.gdx.utils.viewport.ScreenViewport;
import com.badlogic.gdx.utils.viewport.Viewport;

public class BlendingDemo extends ScreenAdapter {
	public static void main (String[] arg) {
		LwjglApplicationConfiguration config = new LwjglApplicationConfiguration();
		new LwjglApplication(new Game(){
			@Override
			public void create() {
				setScreen(new BlendingDemo());
			}
		}, config);
	}
	
	private Viewport viewport;
	private ShapeRenderer shapeRenderer;
	private FrameBuffer fbo;
	private Batch batch;
	private boolean drawDirect;
	
	public BlendingDemo() {
		shapeRenderer = new ShapeRenderer();
		viewport = new ScreenViewport();
		int width = Gdx.graphics.getBackBufferWidth();
		int height = Gdx.graphics.getBackBufferHeight();
		fbo = new FrameBuffer(Format.RGBA8888, width, height, false);
		batch = new SpriteBatch();
	}
	
	@Override
	public void render(float delta) {
		
		if(Gdx.input.isKeyJustPressed(Input.Keys.SPACE)) drawDirect = !drawDirect;
		
		if(drawDirect){
			drawShapes();
		}else{
			fbo.bind();
			drawShapes();
			fbo.end();
			
			Texture texture = fbo.getColorBufferTexture();
			int width = Gdx.graphics.getBackBufferWidth();
			int height = Gdx.graphics.getBackBufferHeight();
			
			Gdx.gl.glClearColor(0.5f, 0.5f, 0.5f, 0.0f);
			Gdx.gl.glClear(GL20.GL_DEPTH_BUFFER_BIT | GL20.GL_COLOR_BUFFER_BIT);
			
			viewport.apply(false);
			batch.getProjectionMatrix().setToOrtho2D(0, 0, width, height);
			batch.begin();
			batch.draw(texture, 0, 0, texture.getWidth(), texture.getHeight(), 0, 0, 1, 1);
			batch.end();
		}
	}
	
	private void drawShapes(){
		Gdx.gl.glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
		Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
		
		shapeRenderer.setProjectionMatrix(viewport.getCamera().combined);
		shapeRenderer.setColor(Color.WHITE);
		shapeRenderer.begin(ShapeType.Filled);

		Gdx.gl.glEnable(GL20.GL_BLEND);
		Gdx.gl.glBlendFunc(GL20.GL_ONE, GL20.GL_ZERO);
		shapeRenderer.circle(0, 0, 100);
		shapeRenderer.flush();
		
		Gdx.gl.glBlendFuncSeparate(GL20.GL_ZERO, GL20.GL_ZERO, GL20.GL_ONE_MINUS_SRC_ALPHA, GL20.GL_ONE_MINUS_DST_ALPHA);
		shapeRenderer.circle(100, 0, 64);
		shapeRenderer.flush();
		
		shapeRenderer.end();
		
		// restore state
		Gdx.gl.glDisable(GL20.GL_BLEND);
		Gdx.gl.glBlendFunc(GL20.GL_SRC_ALPHA, GL20.GL_ONE_MINUS_SRC_ALPHA);
	}
	
	@Override
	public void resize(int width, int height) {
		viewport.update(width, height);
	}
	
}