MaskTest2.java

package mdesl.line2dx.test;

import com.badlogic.gdx.ApplicationListener;
import com.badlogic.gdx.Gdx;
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 MaskTest2 implements ApplicationListener {
 
  OrthographicCamera cam;
	SpriteBatch batch;
	Texture bg, sprite, alphaMask;
 
	@Override
	public void create () {
		cam = new OrthographicCamera();
		batch = new SpriteBatch();
		
		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);
	}
	
	private void drawBackground(SpriteBatch batch) {
		//regular blending mode
		batch.setBlendFunction(GL10.GL_SRC_ALPHA, GL10.GL_ONE_MINUS_SRC_ALPHA);
		
		
		//... draw background entities/tiles here ...
		
		
		//flush the batch to the GPU
		batch.flush();
	}
	
	private void drawAlphaMask(SpriteBatch batch, float x, float y, float width, float height) {
		//disable RGB color, only enable ALPHA to the frame buffer
		Gdx.gl.glColorMask(false, false, false, true);
		
		//change the blending function for our alpha map
		batch.setBlendFunction(GL10.GL_ONE, GL10.GL_ZERO);
		
		//draw alpha mask sprite(s)
		batch.draw(alphaMask, x, y, width, height);
		
		//flush the batch to the GPU
		batch.flush();
	}
	
	private void drawForeground(SpriteBatch batch, int clipX, int clipY, int clipWidth, int clipHeight) {
		//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);
		
		//The scissor test is optional, but it depends 
		Gdx.gl.glEnable(GL10.GL_SCISSOR_TEST);
		Gdx.gl.glScissor(clipX, clipY, clipWidth, clipHeight);
		
		//draw our sprite to be masked
		batch.draw(sprite, 0, 0, 250, 250);
		
		//remember to flush before changing GL states again
		batch.flush();
		
		//disable scissor before continuing
		Gdx.gl.glDisable(GL10.GL_SCISSOR_TEST);
	}
 
	@Override
	public void render () {
		Gdx.gl.glClearColor(0.5f, 0.5f, 0.5f, 1.0f);
		Gdx.gl.glClear(GL10.GL_COLOR_BUFFER_BIT);
		
		
		int w = Gdx.graphics.getWidth();
		int h = Gdx.graphics.getHeight();
		
		//start the batch
		batch.begin();
		
		//draw background
		drawBackground(batch);
		
 
		//the sprite we want the circle mask applied to
		int x = 25;
		int y = 50;
		int spriteWidth = 200;
		int spriteHeight = 200;
		
		//draw the alpha mask
		drawAlphaMask(batch, x, y, spriteWidth, spriteHeight);
		
		//draw our foreground elements
		drawForeground(batch, x, y, spriteWidth, spriteHeight);
		
		batch.end();
	}
 
	@Override
	public void pause () {
		
	}
 
	@Override
	public void resume () {
		
	}
 
	@Override
	public void dispose () {
		batch.dispose();
		alphaMask.dispose();
		sprite.dispose();
	}
	
	
}

You hava to write some code in render method.

batch.setBlendFunction(GL20.GL_SRC_ALPHA, GL20.GL_ONE_MINUS_SRC_ALPHA); batch.flush();