GLSL check if fragment is on geometry

I am currently writing the positions of my geometry to the RGB channels of gl_FragColor and I would like to write 1.0 to the alpha channel if the fragment is part of geometry, and 0.0 if its e开发者_JS百科mpty.

Is there a simple way to tell if a fragment is geometry or not? Maybe through gl_FragCoord.z?

thanks

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Every processed fragment is generated because the geometry is rendered. Fragments not belonging to the geometry rasterization result are not processed by the fragment shader.

So, the solution is very simple:

gl_FragColor.a = 1.0;

However, you need an RGBA texture.