GLSL - how to force rendering a fragment?

Question

is there any way to force rendering a particuar fragment. As far as I know, fragment shaders are called only for pixels within rasterized triangles. What I need to do is to draw a mark(say a single red pixel) in a constant position on the viewport. I mean something like this:

void main(void) {
if(gl_FragCoord.x == vec2(300.5, 300.5)){
    gl_FragColor = vec4(1.0,0.0,0.0,1.0);
}
else {
gl_FragColor = getColorFromSampler();
}

,while there's no quad nor nothing behind fragment (300.5, 300.5). I don't want to affect performance (no fake background and stuff). How to proceed in such situation?

Answer 1

Is there anything speaking against just rendering a single point on top of the other stuff?

So just render a single GL_POINTS primitive at the given pixel. Either use an appropriate orthographic projection to specify the vertex position directly in window space, or just compute the clip space position of the pixel and use an identity vertex transformation. And then all that you need is a simple passthrough fragment shader writing your color of choice.

While you say you want "nothing behind the fragment", I still think that single fragment rendered under the red mark doesn't hurt anyone, at least not more (rather less) than your branch inside the fragment shader just for a single pixel (or any other more elaborate technique using the stencil buffer). If you have any other more strict reason why you cannot render anything else at that position except for the mark, you might want to clarify your problem a bit more.