#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.;
}
float flip(float v, float pct) {
  return mix(v, 1.-v, pct);
}

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

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.);
}

void main() {
  vec2 st = gl_FragCoord.xy/u_resolution;
  vec3 color = vec3(0.0,0.0,0.0);
  float alpha = 1.0;
  vec2 off = vec2(.15,0);
  float right = fill(circleSDF(st-off),.525);  
  float left = stroke(circleSDF(st+off),.5,.05);
  color += flip(right, left);
  

  gl_FragColor = vec4(color, alpha);
}