#ifdef GL_ES
precision mediump float;
#endif
uniform vec2 u_resolution;
uniform vec2 u_mouse;
uniform float u_time;

#define PI 3.14159265359

float rectSDF(vec2 st, vec2 s) {
  st = st*2.-1.;
  return max(abs(st.x/s.x),abs(st.y/s.y));
}

float stroke(in float x_coor, float s, float width){
  float d = step(s,x_coor+width*.5)-step(s,x_coor-width*.5);
  return clamp(d, 0.,1.);
}

float fill(float x, float size) {
  return 1.-step(size, x);
}

void main() {
  vec2 st = gl_FragCoord.xy/u_resolution;
  vec3 color = vec3(0.0,0.0,0.0);
  float alpha = 1.0;
  float sdf = rectSDF(st, vec2(1.));
  color += stroke(sdf,.5,.125);
  color += fill(sdf,.1);

  

  gl_FragColor = vec4(color, alpha);
}