GLSL practice midterm

Question

I have this problem on a practice midterm that I don't understand.

void main(void){
    int i;
    for(i=0; i< gl_VerticesIn; i++){
        gl_Position = gl_PositionIn[i];
        EmitVertex();   
}
    EndPrimitive();
    for(i=0; i< gl_VerticesIn; i++){
        gl_Position = gl_PositionIn[i];
        gl_Position.xy = gl_Position.yx;
        EmitVertex();
    }
    EndPrimitive(); 
}

I have been reading documentation, and I think that this is part of a geometry shader, and I think it is inverting the x and y coordinates of each point, but I don't have any way to verify this. I tried checking it in a program and it made slight differences in the coloring of the scene, but it didn't seem to change the geometry at all, so if someone could help explain this that would be awesome. Thanks!

Answer 1

This is indeed a part of a geometry shader.

First part of the shader (ending with first EndPrimitive()) is simplest possible pass-through geometry shader that does absolutely nothing to the geometry.

Second part is almost the same, except for the swizzling with the xy. It duplicates geometry, but changing the x and y coordinates, so it effectively mirrors the image across the line that connects down-left and up-right corner of the screen.

So, geometry is being duplicated and mirrored across the diagonal of the screen.