thebookofshaders/07/cross.frag

// Author @patriciogv - 2015
// http://patriciogonzalezvivo.com

#ifdef GL_ES
precision mediump float;
#endif

uniform vec2 u_resolution;
uniform vec2 u_mouse;
uniform float u_time;

float box(in vec2 st, in vec2 size){
    size = vec2(0.5) - size*0.5;
    vec2 uv = smoothstep(size,
                        size+vec2(0.001),
                        st);
    uv *= smoothstep(size,
                    size+vec2(0.001),
                    vec2(1.0)-st);
    return uv.x*uv.y;
}

float cross(in vec2 st, float size){
    return  box(st, vec2(size,size/4.)) + 
            box(st, vec2(size/4.,size));
}

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

    gl_FragColor = vec4( vec3( cross(st,0.4) ) ,1.0);
}