CGA monohromatic shader

precision highp float;

varying vec3 n;
varying vec2 uv;

uniform sampler2D tex;
uniform vec2 resolution;

const vec3 BLACK = vec3(0.0);
const vec3 WHITE = vec3(1.0);

const float pix = 1.0;


vec3 middle(float size) {
    vec4 pos = floor(gl_FragCoord / pix);
    if(mod(pos.x, size) < 1.0 && mod(pos.y, size) < 1.0) {
        return BLACK;
    }
    if(mod(pos.x+size/2.0,size) < 1.0 && mod(pos.y+size/2.0,size) < 1.0) {
        return BLACK;
    }
    return WHITE;
}

void main(void)
{
    // constants & options
    vec2 pos = floor(gl_FragCoord.xy/pix)/300.0*pix;
    const float grads = 5.0; // color levels

    vec4 img = texture2D(tex, pos);
    float gray = (img.x + img.y + img.z) / 1.5;
    
    gray = floor(gray*grads) / grads;
    
    float st = 1.0/grads;
    vec3 color = vec3(gray);
    for(int i=0; i<int(grads); i++) {
        if(gray < st*1.02*float(i+1) && gray >= st*0.98*float(i)) {
            if(i==0) { color = BLACK; break; }
            if(i==int(grads)) { color = WHITE; break; }

            color = middle(2.0+float(i-2)*2.0);
            //break;
        }
    }
    
    gl_FragColor = vec4( color, 1.0);
}