CSS optimizers url update does not include base path

I've also removed the condition if the directory is just '.', because this is never true when base path (or even '/') is prepended before.

Sorry, but my last patch does not fix this issue. The path is still incorrect.

This is my unminified css before preprocessing with advagg:

.icon-foo {
  background-image: url("../img/icon-foo.svg");
}

(the img folder is next to the css folder)

This is my unminified css after preprocessing with advagg (without patch #2):

.icon-foo {
  background-image: url("/sites/default/themes/custom/my_theme/img/icon-foo.svg");
}

And this is my unminified css after preprocessing with advagg with patch #2:

.icon-foo {
  background-image: url("/subdirectory/drupal/sites/default/themes/custom/my_theme/dist/css../img/icon-foo.svg");
}

The double dot is still part of the path. I'm not a regex expert, so I don't know how to fix this:

return preg_replace('/url\(\s*([\'"]?)(?![a-z]+:|\/+)([^\'")]+)([\'"]?)\s*\)/i', 'url(\1' . $directory . '\2\3)', $contents);

OK, I managed to fix the double dot issue and it was not because of the regex pattern. The following part is still needed:

    // If the file is in the current directory, make sure '.' doesn't appear in
    // the url() path.
    $directory = $directory == '.' ? '' : $directory . '/';

Could somebody explain why this line is necessary? And why are the urls updated anyway?