I am rendering to a texture using a GLSL shader and then sending that texture as input to a second shader. For the first texture I am using RGB channels to send color data to the second GLSL shader, but I want to use the alpha channel to send a floating point number that the second shader will use as part of its program. The problem is that when I read the texture in the second shader the alpha value is always 1.0. I tested this in the following way:
at the end of the first shader I did this:
gl_FragColor(r, g, b, 0.1);
and then in the second texture I read the value of the first texture using something along the lines of
vec4 f = texture2D(previous_tex, pos);
if (f.a != 1.0) {
gl_FragColor = vec4(0.0, 0.0, 0.0, 1.0);
return;
}
No pixels in my output are black, whereas if I change the above code to read
gl_FragColor(r, g, 0.1, 1.0); //Notice I'm now sending 0.1 for blue
and in the second shader
vec4 f = texture2D(previous_tex, pos);
if (f.b != 1.0) {
gl_FragColor = vec4(0.0, 0.0, 0.0, 1.0);
return;
}
All the appropriate pixels are black. This means that for some reason when I set the alpha value to something other than 1.0 in the first shader and render to a texture, it is still seen as being 1.0 by the second shader.
Before I render to texture I glDisable(GL_BLEND);
It seems pretty clear to me that the problem has to do with OpenGL handling alpha values in some way that isn't obvious to me since I can use the blue channel in the way I want, and figured someone out there will instantly see the problem.
