WALLPAPER GROUPS → 22× (pgg)

vec2 pgg(vec2 uv){
  vec2 id = floor(uv);
  uv = fract(uv);
  if(mod(floor(id.x) + floor(id.y), 2.) == 0.) uv = 1. - uv.yx;
  if(uv.y > uv.x) uv = 1. - uv;
  return uv;
}