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Cut translucent square in texture

How to remove (cut) a transparent rectangle in a texture so that the hole is translucent.

On Android, I would use the Xfermodes approach:

How to use masks in android

But in libgdx I have to use opengl. So far, I almost got what I was looking for using glBlendFunc. From this nice and very useful page, I tell you that

glBlendFunc(GL_ZERO, GL_ONE_MINUS_SRC_ALPHA);

should solve my problem, but I tried this and it didn’t work as expected:

batch.end();
batch.begin();
//Draw the background
super.draw(batch, x, y, width, height);
batch.setBlendFunction(GL20.GL_ZERO,
        GL20.GL_ONE_MINUS_SRC_ALPHA);

//draw the mask
mask.draw(batch, x + innerButtonTable.getX(), y
        + innerButtonTable.getY(), innerButtonTable.getWidth(),
        innerButtonTable.getHeight());

batch.end();
batch.setBlendFunction(GL20.GL_SRC_ALPHA,
        GL20.GL_ONE_MINUS_SRC_ALPHA);
batch.begin();

It just makes the mask area plain black, while I expected transparency, any ideas.

This is what I get:

Mask will be drawn black

This is what I expected:

Mask area should be transparent

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1 answer

I solved the problem using a stencil buffer:

Gdx.gl.glClear(GL_STENCIL_BUFFER_BIT);
batch.end();
//disable color mask
Gdx.gl.glColorMask(false, false, false, false);
Gdx.gl.glDepthMask(false);
//enable the stencil
Gdx.gl.glEnable(GL20.GL_STENCIL_TEST);
Gdx.gl.glStencilFunc(GL20.GL_ALWAYS, 0x1, 0xffffffff);
Gdx.gl.glStencilOp(GL_REPLACE, GL_REPLACE, GL_REPLACE);

batch.begin();
//draw the mask
mask.draw(batch, x + innerButtonTable.getX(), y
        + innerButtonTable.getY(), innerButtonTable.getWidth(),
        innerButtonTable.getHeight());

batch.end();
batch.begin();

//enable color mask 
Gdx.gl.glColorMask(true, true, true, true);
Gdx.gl.glDepthMask(true);
//just draw where outside of the mask
Gdx.gl.glStencilFunc(GL_NOTEQUAL, 0x1, 0xffffffff);
Gdx.gl.glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);
//draw the destination texture
super.draw(batch, x, y, width, height);
batch.end();
//disable the stencil
Gdx.gl.glDisable(GL20.GL_STENCIL_TEST);
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