A Question on OpenGL ES 2.0 and Alpha / Stencil Tests

Question

I have a quad covering the area between -0.5, 0.5 and 0.5, -0.5 on a cleared viewport with a stencil and alpha buffer. In the fragment shader I apply a texture which happens to have a shape -- in this case a circle -- outside of which it is fully transparent.

I am trying to figure out how I can essentially "cut" that non-alpha textured shape out of the next draw of the shape, such that I draw the first quad, offset to some degree (say between -0.3, 0.5 and 0.8, -0.5) and draw again, and only the non-overlap of the non-alpha texture is drawn of the second quad's texture.

It is easy enough doing this with a stencil buffer, such that it applies to the quad and is blind to the texture, however I would like to apply it to the texture.

So as an example of the function what I want actually rendered of the conceptual circle texture would be a crescent in that case. I am not sure what tests I should be using for this.

Tommy Tommy · Accepted Answer · 2011-03-24T22:06:07

I think you want to stick with the stencil buffer, but the alpha test isn't available in ES 2.0 per the philosophy that anything that can be done in a shader isn't supplied as fixed functionality.

Instead, you can insert one of your own choosing inside the fragment shader, thanks to the discard keyword. Supposing you had the most trivial textured fragment shader:

varying mediump vec2 texCoordVarying;
uniform sampler2D tex2D;

void main()
{
    gl_FragColor = texture2D(tex2D, texCoordVarying);
}

You could throw in an alpha test so that pixels with an alpha of less than 0.1 don't proceed down the pipeline, and hence don't affect the stencil buffer with:

varying mediump vec2 texCoordVarying;
uniform sampler2D tex2D;

void main()
{
    vec4 colour = texture2D(tex2D, texCoordVarying);

    if(colour.a > 0.1)
        gl_FragColor = colour;
    else
        discard;
}

A Question on OpenGL ES 2.0 and Alpha / Stencil Tests

1 Answers