Can PNG image transparency be preserved when using PHP's GDlib imagecopyresampled?

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

did it for me. Thanks ceejayoz.

note, the target image needs the alpha settings, not the source image.

Edit: full replacement code. See also answers below and their comments. This is not guaranteed to be be perfect in any way, but did achieve my needs at the time.

$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 );

Why do you make things so complicated? the following is what I use and so far it has done the job for me.

$im = ImageCreateFromPNG($source);
$new_im = imagecreatetruecolor($new_size[0],$new_size[1]);
imagecolortransparent($new_im, imagecolorallocate($new_im, 0, 0, 0));
imagecopyresampled($new_im,$im,0,0,0,0,$new_size[0],$new_size[1],$size[0],$size[1]);

I believe this should do the trick:

$srcImage = imagecreatefrompng($uploadTempFile);
imagealphablending($srcImage, false);
imagesavealpha($srcImage, true);

edit: Someone in the PHP docs claims imagealphablending should be true, not false. YMMV.

An addition that might help some people:

It is possible to toggle imagealphablending while building the image. I the specific case that I needed this, I wanted to combine some semi-transparent PNG's on a transparent background.

First you set imagealphablending to false and fill the newly created true color image with a transparent color. If imagealphablending were true, nothing would happen because the transparent fill would merge with the black default background and result in black.

Then you toggle imagealphablending to true and add some PNG images to the canvas, leaving some of the background visible (ie. not filling up the entire image).

The result is an image with a transparent background and several combined PNG images.

Regrading the preserve transparency, then yes like stated in other posts imagesavealpha() have to be set to true, to use the alpha flag imagealphablending() must be set to false else it doesn't work.

Also I spotted two minor things in your code:

1. You don't need to call getimagesize() to get the width/height for imagecopyresmapled()
2. The $uploadWidth and $uploadHeight should be -1 the value, since the cordinates starts at 0 and not 1, so it would copy them into an empty pixel. Replacing it with: imagesx($targetImage) - 1 and imagesy($targetImage) - 1, relativily should do :)

I combined the answers from ceejayoz and Cheekysoft, which gave the best result for me. Without imagealphablending() and imagesavealpha() the image is not clear:

$img3 = imagecreatetruecolor(128, 128);
imagecolortransparent($img3, imagecolorallocate($img3, 0, 0, 0));
imagealphablending( $img3, false );
imagesavealpha( $img3, true );
imagecopyresampled($img3, $srcImage, 0, 0, 0, 0, 128, 128, $uploadWidth, $uploadHeight);
imagepng($img3, 'filename.png', 9);

For anyone having problems with imagecopyresampled or imagerotate with black bars on background, I have found a code example here:

https://qna.habr.com/q/646622#answer_1417035

        // get image sizes (X,Y)
        $wx = imagesx($imageW);
        $wy = imagesy($imageW);

        // create a new image from the sizes on transparent canvas
        $new = imagecreatetruecolor($wx, $wy);

        $transparent = imagecolorallocatealpha($new, 0, 0, 0, 127);
        $rotate = imagerotate($imageW, 280, $transparent);
        imagealphablending($rotate, true);
        imagesavealpha($rotate, true);

        // get the newest image X and Y
        $ix = imagesx($rotate);
        $iy = imagesy($rotate);

        //copy the image to the canvas
        imagecopyresampled($destImg, $rotate, 940, 2050, 0, 0, $ix, $iy, $ix, $iy);