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

#define PI 3.14159265359

const vec3 uAColor = vec3(.5,.5,.5);
const vec3 uBColor = vec3(.5,.5,.5);
const vec3 uCColor = vec3(1.,1.,1.);
const vec3 uDColor = vec3(.0,.33,.67);

//get colors from http://dev.thi.ng/gradients/
vec3 cosPalette(float t) { return uAColor + uBColor*cos(6.28318*(uCColor*t+uDColor)); }

float circleSDF(vec2 st) { return length(st-.5)*2.; }

void main() {
  vec2 st = gl_FragCoord.xy/u_resolution;
  vec3 color = vec3(.0,.0,.0);
  float alpha = 1.0;
  float circle = length(st-.5)*2.;
  float s = u_time/8.;
  float f_circle = fract((circle-s)*5.);
  
  //color += f_circle;
  color += step(.5,f_circle);
  gl_FragColor = vec4(color, alpha);
}