/**
 * Draw the icons, leaving a semi-transparent rectangle on top of the icon.
 *
 * The top-left corner of the rectangle is rounded and drawned with anti-alias.
 * The anti-alias is done by transitioning from the outer to the inner radius of
 * the rounded corner, and the rectangle sides.
 */

void main()
{
  /* Top-left rounded corner parameters. */
  const float circle_radius_outer = 0.1;
  const float circle_radius_inner = 0.075;

  /**
   * Add a bit transparency to see a bit of the icon, without
   * getting on the way of readability. */
  const float mask_transparency = 0.25;

  vec2 circle_center = vec2(circle_radius_outer - text_width, 0.5);
  fragColor = texture(image, texCoord_interp) * color;

  /* radius in icon space (1 is the icon width).  */
  float radius = length(mask_coord_interp - circle_center);
  float mask = smoothstep(circle_radius_inner, circle_radius_outer, radius);

  bool lower_half = mask_coord_interp.y < circle_center.y;
  bool right_half = mask_coord_interp.x > circle_center.x;

  if (right_half && mask_coord_interp.y < circle_center.y + circle_radius_outer) {
    mask = smoothstep(circle_center.y + circle_radius_inner,
                      circle_center.y + circle_radius_outer,
                      mask_coord_interp.y);
  }
  if (lower_half && mask_coord_interp.x > circle_center.x - circle_radius_outer) {
    mask = smoothstep(circle_center.x - circle_radius_inner,
                      circle_center.x - circle_radius_outer,
                      mask_coord_interp.x);
  }

  fragColor = mix(vec4(0.0), fragColor, max(mask_transparency, mask));
}