Description:
------------
---
From manual page: http://www.php.net/imagick.extentimage
---
Undocumented flip-flop in argument interpretation on Imagick::extentImage(int $width, int $height, int $x, int $y):

Prior to ImageMagick 6.5.7-8 (1623), $x was positive when shifting to the left and negative when shifting to the right, and $y was positive when shifting an image up and negative when shifting an image down.

Somewhere betwen ImageMagick 6.3.7 (1591) and ImageMagick 6.5.7-8 (1623), the axes of $x and $y were flipped, so that $x was negative when shifting to the left and positive when shifting to the right, and $y was negative when shifting an image up and positive when shifting an image down.

Somewhere between ImageMagick 6.5.7-8 (1623) and ImageMagick 6.6.9-7 (1641), the axes of $x and $y were flipped back to pre-ImageMagick 6.5.7-8 (1623) functionality.

Test script:
---------------
<?php
$imagick = new Imagick();
$imagick_version = $imagick->getVersion();
if($imagick_version['versionNumber'] == 1623)
  $pos_factor = 1;
else
  $pos_factor = -1;

$imagick->readImage($path_to_some_image_to_center);
$imagick->resizeImage($new_width, $new_height, imagick::FILTER_LANCZOS, 1);
$imagick->setImageBackgroundColor(new ImagickPixel('#000000'));
$imagick->extentImage($max_width, $max_height, $pos_factor * ($max_width - $new_width) / 2, $pos_factor * ($max_height - $new_height) / 2);
?>

Expected result:
----------------
Using the above code to scale and fit image, centered, into an incongruent container should result in a properly centered image.

Actual result:
--------------
If $x and $y do not use the signs (positive or negative) expected by the installed version of Imagick, the image is shifted in the wrong direction!  This should be noted in the documentation as a warning.