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alan hogan dot com slash contact ¶
17 years ago
As creamdog noted before, the alpha channel IS available from this function! (The manual should probably be updated to include this!)

$rgba = imagecolorat($im,$x,$y);
$alpha = ($rgba & 0x7F000000) >> 24;

$alpha will then contain the TRANSPARENCY (NOT OPACITY) level. So 127, the max, would be completely transparent, and 0 would be completely opaque.

Using this information, it is possible to write a dithering png-to-gif function like the completely working simple one below:

<?php
$im = imagecreatefrompng($pngRel);
        $transparentColor = imagecolorallocate($im, 0xfe, 0x3, 0xf4 );
        $height = imagesy($im);
        $width = imagesx($im);
        for($x = 0; $x < $width; $x++){
            for($y = 0; $y < $height; $y++) {
                $alpha = (imagecolorat($im,$x,$y) & 0x7F000000) >> 24;
                //DITHER!
                if ($alpha > 3 && (
                        $alpha >=127-3 || 
                        (rand(0,127))>=(127-$alpha)
                    )){
                    imagesetpixel($im,$x,$y,$transparentColor);
                }

            }
        }
        imagecolortransparent($im, $transparentColor);
        imagegif($im, $gifRel);//save
        header("Content-type: image/gif");
        readfile($gifRel); //pass thru to browser

?>
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