thebookofshaders/07/triangle-making.frag

// Author @patriciogv - 2015
// http://patriciogonzalezvivo.com

#ifdef GL_ES
precision mediump float;
#endif

uniform vec2 u_resolution;
uniform vec2 u_mouse;
uniform float u_time;

// Based on https://www.shadertoy.com/view/4sSSzG
float triangleDF(vec2 st, 
                vec2 p0, vec2 p1, vec2 p2){
  vec3 e0, e1, e2;

  e0.xy = normalize(p1 - p0).yx * vec2(+1.0, -1.0);
  e1.xy = normalize(p2 - p1).yx * vec2(+1.0, -1.0);
  e2.xy = normalize(p0 - p2).yx * vec2(+1.0, -1.0);

  e0.z = dot(e0.xy, p0);
  e1.z = dot(e1.xy, p1);
  e2.z = dot(e2.xy, p2);

  float a = max(0.0, dot(e0.xy, st) - e0.z);
  float b = max(0.0, dot(e1.xy, st) - e1.z);
  float c = max(0.0, dot(e2.xy, st) - e2.z);

  return length(vec3(a, b, c)*2.0);
}

void main(){
  vec2 st = gl_FragCoord.xy/u_resolution.xy;
  vec3 color = vec3(0.0);

  // Distance Field in 3 channels
  float df = triangleDF(st, 
                       vec2(0.40,0.45), 
                       vec2(0.60,0.45), 
                       vec2(0.5,0.60));
  color = vec3(df);

  // Make a shape of it
  float size = fract(u_time*0.2);
  float border = 0.025;
  color.rb += smoothstep(size+border,size+1e-7,df)-
              smoothstep(size+0.001,size+1e-7,df);

  gl_FragColor = vec4(color,1.0);
}