Description:
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From manual page: http://www.php.net/book.gmagick
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There are many parallels between Imagick and Gmagick functionality, but one key method seems to have been missed in Gmagick:

Imagick::setImageOpacity — Sets the image opacity level
    http://www.php.net/manual/en/imagick.setimageopacity.php

It appears that the GraphicsMagick PHP port may not support the GraphicsMagick "SetImageOpacity" method, but it's not certain.  I've tried variations on the name without success (eg. SetImageOpacity, setImageOpacity, setimageopacity).  The documentation may not be up to date, but I suspect that the method may have been missed.

    http://www.graphicsmagick.org/api/image.html#setimageopacity 

Test script:
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//Instantiate a new Gmagick object
$image = new Gmagick('p5_abg.png');
$image->setimageopacity( 0.5 );
$image->write('p5_abg_halfopacity.png');


Expected result:
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Same results that I get with Imagick version:

$image = new Imagick('p5_abg.png');
$image->setImageOpacity( 0.5 );
$image->writeImage('p5_abg_halfopacity.png');


Actual result:
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PHP Fatal error:  Call to undefined method Gmagick::setimageopacity() in GMagickTest.php on line ##