uniform vec4 color;

in vec2 radii;
out vec4 fragColor;

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;
  }
}