imagerotate transparency

[The Little Guy]

I would like to select a color that isn't in an image, any thoughts on the best way? Maybe this isn't even the best way to do this.

 

I am using imagerotate, and for the 3rd parameter, I would like the uncovered area to  be transparent. I have done this, but I had to use a color then make every color in the image of that color transparent. so, either what is the best way:

 

A. Is there a good way to do this?

B. save every pixel color in an array then loop though the array testing colors that are not in the array and setting that as the transparent value.

C. Other. What?

[kicken]

Just use a color for the background that has it's opacity set to 127.

 

$img = ImageCreateFromPNG('blah');
$transparent = imagecolorallocatealpha($img, 0, 0, 0, 127);
imagerotate($img, 90, $transparent);

ImageSaveAlpha($img, true);
header('Content-type: image/png');
ImagePNG($img);

[The Little Guy]

ahhh!!! good call!

[The Little Guy]

That didn't work...

 

It uses the color, but doesn't keep the alpha.