Alpha blending not working on Android libgdx but works on Desktop

Question

I'm having a weird problem with alpha blending in libgdx. What I'm doing is drawing a background picture, drawing a mask (done using ShapeRenderer) and then drawing the foreground. The mask works fine on my Desktop but not on my Android devices (Galaxy S2, Lenovo ideaTab A2109). I really have no idea what's going on. Could someone help me out? Or at least give me a hint on what's going on. Thanks

Result on my Desktop:

Result on my phone:

My Render function:

Gdx.gl.glClearColor(0f, 0f, 0f, 0);
Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
batch.setProjectionMatrix(camera.combined);
camera.zoom = 1f;
camera.update();
batch.begin();
drawBackground(batch);
batch.end();

frameBuffer.begin();
line.drawMask(false);
frameBuffer.end();
fboRegion.setTexture(frameBuffer.getColorBufferTexture());

batch.begin();
drawMask(batch);
drawForeground(batch)
batch.end();

draw functions:

 private void drawBackground(SpriteBatch batch){
    batch.setBlendFunction(GL20.GL_SRC_ALPHA, GL20.GL_ONE_MINUS_SRC_ALPHA);
    page1.draw(batch);
    page2.draw(batch);
    batch.flush();
}

private void drawMask(SpriteBatch batch){
    Gdx.gl.glColorMask(false, false, false, true);

    batch.setBlendFunction(GL20.GL_ONE, GL20.GL_ZERO);
    batch.draw(fboRegion, 0, 0);
    batch.flush();
}

private void drawForeground(SpriteBatch batch){
    Gdx.gl.glColorMask(true, true, true, true);

    batch.setBlendFunction(GL20.GL_DST_ALPHA, GL20.GL_ONE_MINUS_DST_ALPHA);
    page3.draw(batch);
    batch.flush();
}

Edit: How about using shaders instead of an Alpha Mask? eg replace all white pixels from ShapeRenderer with foreground image pixels. Is it a good idea?

Answer 1

OpenGL ES 2.0 doesn't guarantee 8888 for a frame buffer's color buffer. it will only work on some devices. You have to use RGBA4444, RGB565, or RGB5_A1.

I do this so I can at least use 8888 on devices with OpenGL ES 3.0 and on the desktop:

Format format = (Gdx.graphics.getGL30()==null && 
    Gdx.app.getType()!=ApplicationType.Desktop) ?
        Format.RGB565 : Format.RGBA8888;