Description:
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Two PNG files handled the same way, and one works with transparency and the other does not.  The included code cuts a transparent "hole" in each file, but only one has the red back ground show through.

I'll be happy to look into patching it, but I need to confirm it's a problem.  It show this behavior on all of my machines, but I cannot determine if it's a PHP or a libgd issue.



Test script:
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<?php
sometimesworks("https://maps.googleapis.com/maps/api/staticmap?center=40.714728,-73.998672&zoom=12&size=400x400&maptype=hybrid", "google");
sometimesworks("https://lh4.googleusercontent.com/UQvV_dpBa3_rVf25pvLXKD3OwzF4FtPnHBHkzdWqjCQ5mlFqcFfId9echIgDMv_xYRRYzLaKEXphw7g=w2447-h1106", "myhouse");

function sometimesworks($p_image, $p_prefix)
{
$image_top =imagecreatefrompng($p_image);
$brush = imagecolorallocate($image_top, 0, 0, 0);  //create a black brush
imagefilledpolygon($image_top, explode(",", "10,10, 120,22, 80,280, 200, 191"), 4, $brush);  //fill a polygon
imagecolortransparent($image_top, $brush);  //turn the black to be transparent
imagepng($image_top, './'.$p_prefix.'.transparent.png');  //save the file to confirm that it is working.

//create a big red square
$image_bottom = imagecreatetruecolor(imagesx($image_top), imagesy($image_top));
$red = imagecolorallocate($image_bottom, 255, 0, 0);
imagefill($image_bottom, 0, 0, $red);
imagepng($image_bottom, './'.$p_prefix.'.red.png');

//merge the top onto the bottom.
$out = imagecreatetruecolor(imagesx($image_top), imagesy($image_top));
imagecopy($out, $image_bottom, 0, 0, 0, 0, imagesx($image_top), imagesy($image_top));
imagecopy($out, $image_top, 0, 0, 0, 0, imagesx($image_top), imagesy($image_top));
imagepng($out, './'.$p_prefix.'.out.png');
}

Expected result:
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Both files should have red seen through the alpha mask holes in the PNG images.

Actual result:
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One file shows transparency, and the other does not; however, the code is identical as function.  Only the files are different.