I’ve just made a comments on php.net’s manual page under imagecopyresized() about resizing transparent PNGs with GD. Here is the post again, hopefully it can help someone. Comments are welcome.
Belows is the code snipet that allows you to resize a transparent PNG and composite it into another image. The code is tested to work with PHP5.1.2, GD2, but I think it can also work with other versions of PHP and GD.
The code has been commented to help you read through it. The idea of resizing the transparent PNG image is to create a new destination image which is completely transparent then turn off the imageAlphaBlending of this new image so that when the PNG source file is copied, its alpha channel is still retained.
===Code:===
/**
* Compose a PNG file over a src file.
* If new width/ height are defined, then resize the PNG (and keep all the transparency info)
* Author: Alex Le - http://www.alexle.net
*/
function imageComposeAlpha( &$src, &$ovr, $ovr_x, $ovr_y, $ovr_w = false, $ovr_h = false)
{
if( $ovr_w && $ovr_h )
$ovr = imageResizeAlpha( $ovr, $ovr_w, $ovr_h );
/* Noew compose the 2 images */
imagecopy($src, $ovr, $ovr_x, $ovr_y, 0, 0, imagesx($ovr), imagesy($ovr) );
}
/**
* Resize a PNG file with transparency to given dimensions
* and still retain the alpha channel information
* Author: Alex Le - http://www.alexle.net
*/
function imageResizeAlpha(&$src, $w, $h)
{
/* create a new image with the new width and height */
$temp = imagecreatetruecolor($w, $h);
/* making the new image transparent */
$background = imagecolorallocate($temp, 0, 0, 0);
ImageColorTransparent($temp, $background); // make the new temp image all transparent
imagealphablending($temp, false); // turn off the alpha blending to keep the alpha channel
/* Resize the PNG file */
/* use imagecopyresized to gain some performance but loose some quality */
imagecopyresized($temp, $src, 0, 0, 0, 0, $w, $h, imagesx($src), imagesy($src));
/* use imagecopyresampled if you concern more about the quality */
//imagecopyresampled($temp, $src, 0, 0, 0, 0, $w, $h, imagesx($src), imagesy($src));
return $temp;
}
?>
===Example usage:===
header('Content-type: image/png');
/* Open the photo and the overlay image */
$photoImage = ImageCreateFromJPEG('images/MiuMiu.jpg');
$overlay = ImageCreateFromPNG('images/hair-trans.png');
$percent = 0.8;
$newW = ceil(imagesx($overlay) * $percent);
$newH = ceil(imagesy($overlay) * $percent);
/* Compose the overlay photo over the target image */
imageComposeAlpha( $photoImage, $overlay, 86, 15, $newW, $newH );
/* Open another PNG file, then resize and compose it */
$watermark = imagecreatefrompng('images/watermark.png');
imageComposeAlpha( $photoImage, $watermark, 10, 20, imagesx($watermark)/2, imagesy($watermark)/2 );
/**
* Open the same PNG file then compose without resizing
* As the original $watermark is passed by reference, it was resized already.
* So we have to reopen it.
*/
$watermark = imagecreatefrompng('images/watermark.png');
imageComposeAlpha( $photoImage, $watermark, 80, 350);
Imagepng($photoImage); // output to browser
ImageDestroy($photoImage);
ImageDestroy($overlay);
ImageDestroy($watermark);
?>
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