#version 330 core

uniform float time;
uniform vec2 resolution;
uniform vec2 mouse;

out vec4 color;

float
quadratic_bezier (float x, vec2 a)
{
  float epsilon = 0.00001;
  a.x = clamp (a.x, 0, 1);
  a.y = clamp (a.y, 0, 1);
  if (a.x == .5)
    a += epsilon;

  // solve t from x (an inverse operation)
  float om2a = 1 - 2*a.x;
  float t = (sqrt (a.x*a.x + om2a*x) - a.x)/om2a;
  float y = (1 - 2*a.y)*(t*t) + 2*a.y*t;
  return y;
}

float
line_segment (vec2 p, vec2 a, vec2 b)
{
  vec2 pa = p - a;
  vec2 ba = b - a;
  float h = clamp (dot (pa, ba)/dot (ba, ba), 0, 1);
  return smoothstep (0, 1/resolution.x, length (pa - ba*h));
}

void
main ()
{
  vec2 st = gl_FragCoord.xy/resolution;
  float px = 1/resolution.y;

  // control point
  // vec2 cp = mouse/resolution;
  vec2 cp = vec2 (cos (time), sin (time))*0.45 + 0.5;
  float l = quadratic_bezier(st.x, cp);
  vec3 c = vec3 (smoothstep (l, l + px, st.y));

  // draw control point
  c = mix (vec3 (.5), c, line_segment (st, vec2 (0), cp));
  c = mix (vec3 (.5), c, line_segment (st, vec2 (1), cp));
  float d = distance (cp, st);
  c = mix (vec3 (1, 0, 0), c, smoothstep (.01, .01 + px, d));

  color = vec4 (c, 1);
}