void main()
{
  float dist = length(gl_PointCoord - vec2(0.5));

  /* transparent outside of point
   * --- 0 ---
   *  smooth transition
   * --- 1 ---
   * pure point color
   * ...
   * dist = 0 at center of point */

  fragColor.rgb = color.rgb;
  fragColor.a = mix(color.a, 0.0, smoothstep(radii[1], radii[0], dist));

  if (fragColor.a == 0.0) {
    discard;
  }
}