uniform vec4 color;
uniform vec4 outlineColor;

in vec4 radii;
out vec4 fragColor;

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

  // transparent outside of point
  // --- 0 ---
  // smooth transition
  // --- 1 ---
  // pure outline color
  // --- 2 ---
  // smooth transition
  // --- 3 ---
  // pure point color
  // ...
  // dist = 0 at center of point

  float midStroke = 0.5 * (radii[1] + radii[2]);

  if (dist > midStroke) {
    fragColor.rgb = outlineColor.rgb;
    fragColor.a = mix(outlineColor.a, 0.0, smoothstep(radii[1], radii[0], dist));
  }
  else {
    fragColor = mix(color, outlineColor, smoothstep(radii[3], radii[2], dist));
  }

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