example of https://thebookofshaders.com/09

09.glsl

void main(void){
    vec2 st = gl_FragCoord.xy/u_resolution.xy;
    vec3 color = vec3(0.0);

    
    // Divide the space in 4
    st = tile(st,4.);
    color += vec3(box(st,vec2(.96),0.01));
    
    st = rotate2D(st,PI*0.25);

    color += vec3(dia(st));    
    color = 1.0 - (box(st,vec2(1.2),0.01) - color);
    
    gl_FragColor = vec4(color,1.0);
}