Rotate with empty background color does not yield a transparent background

I defined a rotate effect with an empty background. According to the on-screen help and the GD implementation this should give me a transparent background (in the corner triangles). however the imagick toolkit results in an image with the triangles coloured according to the cornel pixel colors.

I fixed this as below in the given function. Note that in that function there were also references to the not initialized variable $bgcolor whereas $background (the parameter) should have been used.

file: imageapi_imagemagick.module
function: image_imageapi_imagemagick_rotate
new code:

function image_imageapi_imagemagick_rotate(stdClass $image, $degrees, $background = NULL) {
  if (is_null($background)) {
    $bgcolor = 'transparent';
  } elseif (is_int($background)) {
    $bgcolor = '#'. str_pad(dechex($background), 6, 0);
  } else {
    $bgcolor = str_replace('0x', '#', $background);
  }
  $image->ops[] = '-background '. escapeshellarg($bgcolor) .' -rotate '. (float) $degrees;
  return TRUE;
}

Perhaps the literal string transparent should be replaced by something like rgba(0,0,0,0.0) as I'm not sure that transparent will be defined (in colors.xml) in all (supported) installations/versions.