As can be seen, the resulting image retains the alpha channel levels of both source and destination images.
The current implementation of imagecopymerge() produces unexpected results when merging two true colour images especially as imagecopy() produces results more inline with what I would expect.
By way of an example, consider the following code....
<?php
$bg = imagecreatefrompng('tux.png');
$over = imagecreatefrompng('ff-logo-sm.png');
imagealphablending($bg, true);
imagesavealpha($bg, true);
imagecopy($bg, $over, 276, 300, 0, 0, 123, 119);
imagepng($bg, 'tux-fox-imagecopy.png');
?>
Using imagecopy the resulting image is....
As can be seen, the resulting image retains the alpha channel levels of
both source and destination images.
Now with imagecopymerge...
<?php
$bg = imagecreatefrompng('tux.png');
$over = imagecreatefrompng('ff-logo-sm.png');
imagealphablending($bg, true);
imagesavealpha($bg, true);
imagecopymerge($bg, $over, 276, 300, 0, 0, 123, 119, 100);
imagepng($bg, 'tux-fox-imagecopymerge-100-without-patch.png');
?>
The resulting image is...
Destination image retains alpha channel information while the alpha channel
of the source image is ignored.
Applying a 50% reduction in opacity....
<?php
$bg = imagecreatefrompng('tux.png');
$over = imagecreatefrompng('ff-logo-sm.png');
imagealphablending($bg, true);
imagesavealpha($bg, true);
imagecopymerge($bg, $over, 276, 300, 0, 0, 123, 119, 50);
imagepng($bg, 'tux-fox-imagecopymerge-50-without-patch.png');
?>
The resulting image is...
With my patch (applied to ext/gd/libgd/gd.c), the results are more inline with what I would expect imagecopymerge to return ( the patch file can be found here.
<?php
$bg = imagecreatefrompng('tux.png');
$over = imagecreatefrompng('ff-logo-sm.png');
imagealphablending($bg, true);
imagesavealpha($bg, true);
imagecopymerge($bg, $over, 276, 300, 0, 0, 123, 119, 100);
imagepng($bg, 'tux-fox-imagecopymerge-100-with-patch.png');
?>
Gives us....
And with 50% opacity reduction...
<?php
$bg = imagecreatefrompng('tux.png');
$over = imagecreatefrompng('ff-logo-sm.png');
imagealphablending($bg, true);
imagesavealpha($bg, true);
imagecopymerge($bg, $over, 276, 300, 0, 0, 123, 119, 50);
imagepng($bg, 'tux-fox-imagecopymerge-50-with-patch.png');
?>
Gives us....
