#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));
}

vec2 rotate(vec2 st, float a) {
  st = mat2(cos(a),-sin(a),sin(a),cos(a))*(st-.5);
  return st+.5;
}

float triSDF(vec2 st) {
  st = (st*2.-1.)*2.;
  return max(abs(st.x)*.866025+st.y*.5,-st.y*.5);
}

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;

  st = rotate(st, radians(-25.));
  float sdf = triSDF(st);
  //why does dividing the triangle with a larger triangle create a smaller black triangle area at the bottom?
  sdf /= triSDF(st+vec2(.0,.2));
  color += sdf; //fill(abs(sdf),.56);

  gl_FragColor = vec4(color, alpha);
}