thebookofshaders/08/cross.frag

// Author @patriciogv ( patriciogonzalezvivo.com ) - 2015

#ifdef GL_ES
precision mediump float;
#endif

uniform vec2 u_resolution;
uniform float u_time;

float box(vec2 _st, vec2 _size){
    _size = vec2(0.5) - _size*0.5;
    vec2 uv = smoothstep(_size,
                        _size+vec2(0.0001),
                        _st);
    uv *= smoothstep(_size,
                    _size+vec2(0.0001),
                    vec2(1.0)-_st);
    return uv.x*uv.y;
}

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

	vec3 color = vec3( box(st, vec2(0.4,0.1)) + box(st, vec2(0.1,0.4)) );

	gl_FragColor = vec4(color,1.0);
}