Resize and write PNG with transparent background

Question

I save an image PNG with transparent background normally, but when I use this code to resize the image it is with black and non-transparent background as it was before being resized:

include("resize-class.php");

$resizeObj = new resize("/home/roteirodoimovelc/public_html/cp/clientes/".$cliente."/marcadagua/".$nome_imagem."");
$resizeObj -> resizeImage("".$width."", "".$height."", 'exact');
$resizeObj -> saveImage("/home/roteirodoimovelc/public_html/cp/clientes/".$cliente."/marcadagua/".$nome_imagem."", 100);

What can it be?

Follows the class used:

<?php

   # ========================================================================#
   #
   #  Author:    Jarrod Oberto
   #  Version:   1.0
   #  Date:      17-Jan-10
   #  Purpose:   Resizes and saves image
   #  Requires : Requires PHP5, GD library.
   #  Usage Example:
   #                     include("classes/resize_class.php");
   #                     $resizeObj = new resize('images/cars/large/input.jpg');
   #                     $resizeObj -> resizeImage(150, 100, 0);
   #                     $resizeObj -> saveImage('images/cars/large/output.jpg', 100);
   #
   #
   # ========================================================================#


        Class resize
        {
            // *** Class variables
            private $image;
            private $width;
            private $height;
            private $imageResized;

            function __construct($fileName)
            {
                // *** Open up the file
                $this->image = $this->openImage($fileName);

                // *** Get width and height
                $this->width  = imagesx($this->image);
                $this->height = imagesy($this->image);
            }

            ## --------------------------------------------------------

            private function openImage($file)
            {
                // *** Get extension
                $extension = strtolower(strrchr($file, '.'));

                switch($extension)
                {
                    case '.jpg':
                    case '.jpeg':
                        $img = @imagecreatefromjpeg($file);
                        break;
                    case '.gif':
                        $img = @imagecreatefromgif($file);
                        break;
                    case '.png':
                        $img = @imagecreatefrompng($file);
                        break;
                    default:
                        $img = false;
                        break;
                }
                return $img;
            }

            ## --------------------------------------------------------

            public function resizeImage($newWidth, $newHeight, $option="auto")
            {
                // *** Get optimal width and height - based on $option
                $optionArray = $this->getDimensions($newWidth, $newHeight, $option);

                $optimalWidth  = $optionArray['optimalWidth'];
                $optimalHeight = $optionArray['optimalHeight'];


                // *** Resample - create image canvas of x, y size
                $this->imageResized = imagecreatetruecolor($optimalWidth, $optimalHeight);
                imagecopyresampled($this->imageResized, $this->image, 0, 0, 0, 0, $optimalWidth, $optimalHeight, $this->width, $this->height);


                // *** if option is 'crop', then crop too
                if ($option == 'crop') {
                    $this->crop($optimalWidth, $optimalHeight, $newWidth, $newHeight);
                }
            }

            ## --------------------------------------------------------

            private function getDimensions($newWidth, $newHeight, $option)
            {

               switch ($option)
                {
                    case 'exact':
                        $optimalWidth = $newWidth;
                        $optimalHeight= $newHeight;
                        break;
                    case 'portrait':
                        $optimalWidth = $this->getSizeByFixedHeight($newHeight);
                        $optimalHeight= $newHeight;
                        break;
                    case 'landscape':
                        $optimalWidth = $newWidth;
                        $optimalHeight= $this->getSizeByFixedWidth($newWidth);
                        break;
                    case 'auto':
                        $optionArray = $this->getSizeByAuto($newWidth, $newHeight);
                        $optimalWidth = $optionArray['optimalWidth'];
                        $optimalHeight = $optionArray['optimalHeight'];
                        break;
                    case 'crop':
                        $optionArray = $this->getOptimalCrop($newWidth, $newHeight);
                        $optimalWidth = $optionArray['optimalWidth'];
                        $optimalHeight = $optionArray['optimalHeight'];
                        break;
                }
                return array('optimalWidth' => $optimalWidth, 'optimalHeight' => $optimalHeight);
            }

            ## --------------------------------------------------------

            private function getSizeByFixedHeight($newHeight)
            {
                $ratio = $this->width / $this->height;
                $newWidth = $newHeight * $ratio;
                return $newWidth;
            }

            private function getSizeByFixedWidth($newWidth)
            {
                $ratio = $this->height / $this->width;
                $newHeight = $newWidth * $ratio;
                return $newHeight;
            }

            private function getSizeByAuto($newWidth, $newHeight)
            {
                if ($this->height < $this->width)
                // *** Image to be resized is wider (landscape)
                {
                    $optimalWidth = $newWidth;
                    $optimalHeight= $this->getSizeByFixedWidth($newWidth);
                }
                elseif ($this->height > $this->width)
                // *** Image to be resized is taller (portrait)
                {
                    $optimalWidth = $this->getSizeByFixedHeight($newHeight);
                    $optimalHeight= $newHeight;
                }
                else
                // *** Image to be resizerd is a square
                {
                    if ($newHeight < $newWidth) {
                        $optimalWidth = $newWidth;
                        $optimalHeight= $this->getSizeByFixedWidth($newWidth);
                    } else if ($newHeight > $newWidth) {
                        $optimalWidth = $this->getSizeByFixedHeight($newHeight);
                        $optimalHeight= $newHeight;
                    } else {
                        // *** Sqaure being resized to a square
                        $optimalWidth = $newWidth;
                        $optimalHeight= $newHeight;
                    }
                }

                return array('optimalWidth' => $optimalWidth, 'optimalHeight' => $optimalHeight);
            }

            ## --------------------------------------------------------

            private function getOptimalCrop($newWidth, $newHeight)
            {

                $heightRatio = $this->height / $newHeight;
                $widthRatio  = $this->width /  $newWidth;

                if ($heightRatio < $widthRatio) {
                    $optimalRatio = $heightRatio;
                } else {
                    $optimalRatio = $widthRatio;
                }

                $optimalHeight = $this->height / $optimalRatio;
                $optimalWidth  = $this->width  / $optimalRatio;

                return array('optimalWidth' => $optimalWidth, 'optimalHeight' => $optimalHeight);
            }

            ## --------------------------------------------------------

            private function crop($optimalWidth, $optimalHeight, $newWidth, $newHeight)
            {
                // *** Find center - this will be used for the crop
                $cropStartX = ( $optimalWidth / 2) - ( $newWidth /2 );
                $cropStartY = ( $optimalHeight/ 2) - ( $newHeight/2 );

                $crop = $this->imageResized;
                //imagedestroy($this->imageResized);

                // *** Now crop from center to exact requested size
                $this->imageResized = imagecreatetruecolor($newWidth , $newHeight);
                imagecopyresampled($this->imageResized, $crop , 0, 0, $cropStartX, $cropStartY, $newWidth, $newHeight , $newWidth, $newHeight);
            }

            ## --------------------------------------------------------

            public function saveImage($savePath, $imageQuality="100")
            {
                // *** Get extension
                $extension = strrchr($savePath, '.');
                $extension = strtolower($extension);

                switch($extension)
                {
                    case '.jpg':
                    case '.jpeg':
                        if (imagetypes() & IMG_JPG) {
                            imagejpeg($this->imageResized, $savePath, $imageQuality);
                        }
                        break;

                    case '.gif':
                        if (imagetypes() & IMG_GIF) {
                            imagegif($this->imageResized, $savePath);
                        }
                        break;

                    case '.png':
                        // *** Scale quality from 0-100 to 0-9
                        $scaleQuality = round(($imageQuality/100) * 9);

                        // *** Invert quality setting as 0 is best, not 9
                        $invertScaleQuality = 9 - $scaleQuality;

                        if (imagetypes() & IMG_PNG) {
                             imagepng($this->imageResized, $savePath, $invertScaleQuality);
                        }
                        break;

                    // ... etc

                    default:
                        // *** No extension - No save.
                        break;
                }

                imagedestroy($this->imageResized);
            }


            ## --------------------------------------------------------

        }
?>

Answer 1

The problem is with the alpha, to solve this you can use the functions imagealphablending, imagesavealpha and imagecolorallocatealpha.

As described here imagepng() and Transparency in GD library with PHP for example:

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

An example of use How do I resize pngs with Transparency in PHP?:

<?php
$newImg = imagecreatetruecolor($nWidth, $nHeight);
imagealphablending($newImg, false);
imagesavealpha($newImg, true);
$transparent = imagecolorallocatealpha($newImg, 255, 255, 255, 127);
imagefilledrectangle($newImg, 0, 0, $nWidth, $nHeight, $transparent);
imagecopyresampled($newImg, $im, 0, 0, 0, 0, $nWidth, $nHeight, $imgInfo[0], $imgInfo[1]);

However if you want to add a watermark (copymerge) you may need to use this example (http://php.net/manual/en/function.imagecopymerge.php#88456), I’m not sure if the alpha problem has been fixed or improved, but the following code solves the problem if you’re using older versions of php like 5.2 or 5.3:

<?php 
/** 
* PNG ALPHA CHANNEL SUPPORT for imagecopymerge(); 
* This is a function like imagecopymerge but it handle alpha channel well!!! 
**/ 

// A fix to get a function like imagecopymerge WITH ALPHA SUPPORT 
// Main script by aiden dot mail at freemail dot hu 
// Transformed to imagecopymerge_alpha() by rodrigo dot polo at gmail dot com 
function imagecopymerge_alpha($dst_im, $src_im, $dst_x, $dst_y, $src_x, $src_y, $src_w, $src_h, $pct){ 
    if(!isset($pct)){ 
        return false; 
    } 
    $pct /= 100; 
    // Get image width and height 
    $w = imagesx( $src_im ); 
    $h = imagesy( $src_im ); 
    // Turn alpha blending off 
    imagealphablending( $src_im, false ); 
    // Find the most opaque pixel in the image (the one with the smallest alpha value) 
    $minalpha = 127; 
    for( $x = 0; $x < $w; $x++ ) 
    for( $y = 0; $y < $h; $y++ ){ 
        $alpha = ( imagecolorat( $src_im, $x, $y ) >> 24 ) & 0xFF; 
        if( $alpha < $minalpha ){ 
            $minalpha = $alpha; 
        } 
    } 
    //loop through image pixels and modify alpha for each 
    for( $x = 0; $x < $w; $x++ ){ 
        for( $y = 0; $y < $h; $y++ ){ 
            //get current alpha value (represents the TANSPARENCY!) 
            $colorxy = imagecolorat( $src_im, $x, $y ); 
            $alpha = ( $colorxy >> 24 ) & 0xFF; 
            //calculate new alpha 
            if( $minalpha !== 127 ){ 
                $alpha = 127 + 127 * $pct * ( $alpha - 127 ) / ( 127 - $minalpha ); 
            } else { 
                $alpha += 127 * $pct; 
            } 
            //get the color index with new alpha 
            $alphacolorxy = imagecolorallocatealpha( $src_im, ( $colorxy >> 16 ) & 0xFF, ( $colorxy >> 8 ) & 0xFF, $colorxy & 0xFF, $alpha ); 
            //set pixel with the new color + opacity 
            if( !imagesetpixel( $src_im, $x, $y, $alphacolorxy ) ){ 
                return false; 
            } 
        } 
    } 
    // The image copy 
    imagecopy($dst_im, $src_im, $dst_x, $dst_y, $src_x, $src_y, $src_w, $src_h); 
} 

// USAGE EXAMPLE: 
$img_a = imagecreatefrompng('image1.png'); 
$img_b = imagecreatefrompng('wm2.png'); 

// SAME COMMANDS: 
imagecopymerge_alpha($img_a, $img_b, 10, 10, 0, 0, imagesx($img_b), imagesy($img_b),50); 

// OUTPUT IMAGE: 
header("Content-Type: image/png"); 
imagesavealpha($img_a, true); 
imagepng($img_a, NULL);