How Can I Resize PNGs with Transparency Using PHP\'s GDlib and Preserve Alpha Channels?

Resizing PNGs with Transparency using PHP's GDlib

Preserving image transparency is essential when dealing with PNG files. While GDlib offers image manipulation capabilities, resizing PNGs with preserved transparency can present a challenge.

One issue encountered when using GD's imagecopyresampled function is that transparent areas in the original PNG are replaced with a solid color. This occurs despite setting imagesavealpha to true.

To resolve this, it's crucial to specify the alpha settings correctly. In PHP, alpha settings apply to the target image, not the source image. By adjusting the target image's alpha properties, we can achieve transparency preservation:

imagealphablending( $targetImage, false );
imagesavealpha( $targetImage, true );

imagealphablending(false) disables any blending and respects the image's alpha channel. imagesavealpha(true) ensures that the PNG's alpha channel is preserved in the output.

Here's a revised PHP code snippet that incorporates these adjustments:

$uploadTempFile = $myField[ 'tmp_name' ]
list( $uploadWidth, $uploadHeight, $uploadType ) 
  = getimagesize( $uploadTempFile );

$srcImage = imagecreatefrompng( $uploadTempFile ); 

$targetImage = imagecreatetruecolor( 128, 128 );   
imagealphablending( $targetImage, false );
imagesavealpha( $targetImage, true );

imagecopyresampled( $targetImage, $srcImage, 
                    0, 0, 
                    0, 0, 
                    128, 128, 
                    $uploadWidth, $uploadHeight );

imagepng(  $targetImage, 'out.png', 9 );

This modified code should effectively resize the PNG image while preserving transparency. Note that it's not guaranteed to be perfect for all cases, but it provides a solid starting point.

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