thebookofshaders/07/rect.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 rect(in vec2 st, in vec2 size){
	size = 0.25-size*0.25;
    vec2 uv = smoothstep(size,size+size*vec2(0.002),st*(1.0-st));
	return uv.x*uv.y;
}

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

	vec3 color = vec3( rect(st, vec2(0.9) ) );

	gl_FragColor = vec4(color,1.0);
}