// fragment shader to demo position to texture and texture to position mapping 

uniform sampler2D grid;

void main() {
    // base color, with alpha channel for grid masking
    gl_FragColor = vec4(gl_Color.xyz,-1);

    // overlay red grid for texture coordinates
    gl_FragColor = mix(gl_FragColor, vec4(1,0,0,1),
		       texture2D(grid, 2.*gl_TexCoord[0].xy).r);

    // overlay green grid for position
    gl_FragColor = mix(gl_FragColor, vec4(0,1,0,1),
		       texture2D(grid, gl_TexCoord[1].xy/gl_TexCoord[1].w).r);

    // toss fragments not on grid
    if (gl_FragColor.a < 0.) discard;
}