Problems resizing transparent PNG image

PHP
1

Hello,

I am creating an application that will help me resize images quickly.

Some of the images I wanted to resize in in PNG format, with alpha channel, but when I resize using imagecopyresampled, the output image is not ahh, transparent anymore.

Any ideas please?

This is the way this application work: This application opens the source image, and calculate the destination size, create a new true color image, and then imagecopyresampled to the destination image resource, then write to file.

2

I’m not sure why none of it’s transparent, but it really doesn’t surprise me. You don’t have much control over image resizing and you need to do it in a decent graphics package if you want to keep the quality.

3

imagealphablending()
http://www.php.net/manual/en/function.imagealphablending.php
imagesavealpha()
http://www.php.net/manual/en/function.imagesavealpha.php

If you want to resize a png-24 image and preserve the alpha channel you need to set imagealphablending($im_dest, false) on the destination image just after creating it with imagecreatetruecolor() and do a imagesavealpha($im_dest, true) on it before saving it:


<?php

$im = ImageCreateFromPNG('redfade.png');

$im_dest = imagecreatetruecolor (500, 300);
imagealphablending($im_dest, false);

imagecopyresampled($im_dest, $im, 0, 0, 0, 0, 300, 300, 500, 300);

imagesavealpha($im_dest, true);
imagepng($im_re, 'small_redfade.png');

?>
4

Oh, thank you very much!

Looks like imagesavealpha is the key. I’ll try it.

5

For all of you looking for a way to transform images on the fly, you might want to check the open-source project Asido: there are a lot of useful features ([URL=“http://www.asido.info/about/features/”]http://www.asido.info/about/features/), including [URL=“http://www.asido.info/about/features/#resize”]various types of image resize - like fit resize (resize if bigger) and frame resize (force-fit), [URL=“http://www.asido.info/about/features/#watermark”]watermarking, [URL=“http://www.asido.info/about/features/#copy”]image copy, [URL=“http://www.asido.info/about/features/#crop”]cropping, etc. There is no problem working w/ transparent images at all - check the examples on the [URL=“http://asido.info”]Asido website. Also, it can use various platforms: not only GD but ImageMagick too (either via shell or via any of the php extensions like php_magickwand.dll or php_imagick.dll). Here’s the list of drivers: [URL=“http://www.asido.info/about/drivers/”]http://www.asido.info/about/drivers/