PHPでPNG画像をimagepngでブラウザへ返すと透過情報が消えてしまうので

http://www.petitec.com/2011/06/php-enable-alpha-color-in-png/
によるとimagesavealphaをセットすると良いようだ。

<?php
            $img = imagecreatefrompng($path);
            imagesavealpha($img, TRUE);
            imagepng($img);
            return;

同じような問題で拡大縮小した際の話だと
http://phpspot.org/blog/archives/2007/09/phppng.html
の情報を元に
http://mediumexposure.com/smart-image-resizing-while-preserving-transparency-php-and-gd-library/
を見れば良いらしい。将来の自身への情報保存の為に転載しておく。

<?php
function smart_resize_image( $file, $width = 0, $height = 0, $proportional = false, $output = 'file', $delete_original = true, $use_linux_commands = false )
  {
    if ( $height <= 0 && $width <= 0 ) {
      return false;
    }

    $info = getimagesize($file);
    $image = '';

    $final_width = 0;
    $final_height = 0;
    list($width_old, $height_old) = $info;

    if ($proportional) {
      if ($width == 0) $factor = $height/$height_old;
      elseif ($height == 0) $factor = $width/$width_old;
      else $factor = min ( $width / $width_old, $height / $height_old);   

      $final_width = round ($width_old * $factor);
      $final_height = round ($height_old * $factor);

    }
    else {
      $final_width = ( $width <= 0 ) ? $width_old : $width;
      $final_height = ( $height <= 0 ) ? $height_old : $height;
    }

    switch ( $info[2] ) {
      case IMAGETYPE_GIF:
        $image = imagecreatefromgif($file);
      break;
      case IMAGETYPE_JPEG:
        $image = imagecreatefromjpeg($file);
      break;
      case IMAGETYPE_PNG:
        $image = imagecreatefrompng($file);
      break;
      default:
        return false;
    }

    $image_resized = imagecreatetruecolor( $final_width, $final_height );

    if ( ($info[2] == IMAGETYPE_GIF) || ($info[2] == IMAGETYPE_PNG) ) {
      $trnprt_indx = imagecolortransparent($image);

      // If we have a specific transparent color
      if ($trnprt_indx >= 0) {

        // Get the original image's transparent color's RGB values
        $trnprt_color    = imagecolorsforindex($image, $trnprt_indx);

        // Allocate the same color in the new image resource
        $trnprt_indx    = imagecolorallocate($image_resized, $trnprt_color['red'], $trnprt_color['green'], $trnprt_color['blue']);

        // Completely fill the background of the new image with allocated color.
        imagefill($image_resized, 0, 0, $trnprt_indx);

        // Set the background color for new image to transparent
        imagecolortransparent($image_resized, $trnprt_indx);


      } 
      // Always make a transparent background color for PNGs that don't have one allocated already
      elseif ($info[2] == IMAGETYPE_PNG) {

        // Turn off transparency blending (temporarily)
        imagealphablending($image_resized, false);

        // Create a new transparent color for image
        $color = imagecolorallocatealpha($image_resized, 0, 0, 0, 127);

        // Completely fill the background of the new image with allocated color.
        imagefill($image_resized, 0, 0, $color);

        // Restore transparency blending
        imagesavealpha($image_resized, true);
      }
    }

    imagecopyresampled($image_resized, $image, 0, 0, 0, 0, $final_width, $final_height, $width_old, $height_old);

    if ( $delete_original ) {
      if ( $use_linux_commands )
        exec('rm '.$file);
      else
        @unlink($file);
    }

    switch ( strtolower($output) ) {
      case 'browser':
        $mime = image_type_to_mime_type($info[2]);
        header("Content-type: $mime");
        $output = NULL;
      break;
      case 'file':
        $output = $file;
      break;
      case 'return':
        return $image_resized;
      break;
      default:
      break;
    }

    switch ( $info[2] ) {
      case IMAGETYPE_GIF:
        imagegif($image_resized, $output);
      break;
      case IMAGETYPE_JPEG:
        imagejpeg($image_resized, $output);
      break;
      case IMAGETYPE_PNG:
        imagepng($image_resized, $output);
      break;
      default:
        return false;
    }

    return true;
  }