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 );

After couple of hours of testing and kicking my head against the wall, I think I've found solution. Problem was about allocating transparent color using imagecolorallocate(). I did not get it at first sight. It was totally wrong approach. However, imagecolorallocatealpha() has helped me alot.

Also, alpha blending must be off before saving alpha channel on working layer. However, it must be done right after creating blank true-color image, like

  $im = imagecreatetruecolor($w, $h);
  $alphacolor = imagecolorallocatealpha($img, $r, $g, $b, 127);
  imagealphablending($im, false);
  imagesavealpha($im, true);

This code is a key for getting smooth corners in transparent area after resize-down.

After all, I've wrote this function

  function imageCreateCorners($sourceImageFile, $radius) {
  # function body
  }

I've tested it with couple of images and it returned image with smooth corners for every bg color.

  imagepng(imageCreateCorners('jan_vesely_and_james_gist.jpg', 9), 'test.png');

Output

Original image

IN BROWSER (Same png file 'test.png')

It finally returns fully transparent alpha channel so you can use that image on every background you want.

I almost forgot to post function code :)

function imageCreateCorners($sourceImageFile, $radius)

  function imageCreateCorners($sourceImageFile, $radius) {
    # test source image
    if (file_exists($sourceImageFile)) {
      $res = is_array($info = getimagesize($sourceImageFile));
      } 
    else $res = false;

    # open image
    if ($res) {
      $w = $info[0];
      $h = $info[1];
      switch ($info['mime']) {
        case 'image/jpeg': $src = imagecreatefromjpeg($sourceImageFile);
          break;
        case 'image/gif': $src = imagecreatefromgif($sourceImageFile);
          break;
        case 'image/png': $src = imagecreatefrompng($sourceImageFile);
          break;
        default: 
          $res = false;
        }
      }

    # create corners
    if ($res) {

      $q = 10; # change this if you want
      $radius *= $q;

      # find unique color
      do {
        $r = rand(0, 255);
        $g = rand(0, 255);
        $b = rand(0, 255);
        }
      while (imagecolorexact($src, $r, $g, $b) < 0);

      $nw = $w*$q;
      $nh = $h*$q;

      $img = imagecreatetruecolor($nw, $nh);
      $alphacolor = imagecolorallocatealpha($img, $r, $g, $b, 127);
      imagealphablending($img, false);
      imagesavealpha($img, true);
      imagefilledrectangle($img, 0, 0, $nw, $nh, $alphacolor);

      imagefill($img, 0, 0, $alphacolor);
      imagecopyresampled($img, $src, 0, 0, 0, 0, $nw, $nh, $w, $h);

      imagearc($img, $radius-1, $radius-1, $radius*2, $radius*2, 180, 270, $alphacolor);
      imagefilltoborder($img, 0, 0, $alphacolor, $alphacolor);
      imagearc($img, $nw-$radius, $radius-1, $radius*2, $radius*2, 270, 0, $alphacolor);
      imagefilltoborder($img, $nw-1, 0, $alphacolor, $alphacolor);
      imagearc($img, $radius-1, $nh-$radius, $radius*2, $radius*2, 90, 180, $alphacolor);
      imagefilltoborder($img, 0, $nh-1, $alphacolor, $alphacolor);
      imagearc($img, $nw-$radius, $nh-$radius, $radius*2, $radius*2, 0, 90, $alphacolor);
      imagefilltoborder($img, $nw-1, $nh-1, $alphacolor, $alphacolor);
      imagealphablending($img, true);
      imagecolortransparent($img, $alphacolor);

      # resize image down
      $dest = imagecreatetruecolor($w, $h);
      imagealphablending($dest, false);
      imagesavealpha($dest, true);
      imagefilledrectangle($dest, 0, 0, $w, $h, $alphacolor);
      imagecopyresampled($dest, $img, 0, 0, 0, 0, $w, $h, $nw, $nh);

      # output image
      $res = $dest;
      imagedestroy($src);
      imagedestroy($img);
      }

    return $res;
    }

Function returns GD object or false.

Function works with solid JPEG, GIF and PNG images. Also, it works great with transparent PNGs and GIFs.

imagecopy does not support using two images with alpha channels. take a look at imagecopymerge.

http://php.net/manual/en/function.imagecopymerge.php

There are plenty examples in the user comments sections, and a finished implementation for what you want:

http://www.php.net/manual/en/function.imagecopymerge.php#92787