Draw metaball-like circles.

touchMetaball.frag

uniform float time;
uniform vec2 resolution;
uniform vec2 touches[10];

float circle(vec2 p, vec2 center, float r) {
  float d = distance(p, center);
  return 1.0 * r / d;
}

void main( void ) {
  vec2 position = ( gl_FragCoord.xy / resolution.xy );
  vec3 color = vec3(0.0, 0.0, 0.0);
  for(int i=0; i<10; i++){
    float cr = circle(position, vec2(touches[i].x, touches[i].y), 0.02);
    float cg = circle(position, vec2(touches[i].x, touches[i].y), 0.3);
    float cb = circle(position, vec2(touches[i].x, touches[i].y), 0.4);
    color.r += cr;
    color.g += cg * 0.2;
    color.b += cb * 0.5;
  }

  gl_FragColor = vec4( color.r, color.g * (sin(time*1.2) * 0.1 + 0.9), color.b * (cos(time*1.5) * 0.1 + 0.9), 0.5 );
}