Why is the background painted black when resizing a png shock with a transparent background using php?

Question

PHP

When resizing images (png with a transparent background) using a php script, the background is painted black.
Why does this happen and how to avoid it?
Here is the whole script:

<?php
//Maximize script execution time
ini_set('max_execution_time', 0);

//Initial settings, Just specify Source and Destination Image folder.
$ImagesDirectory = 'C:\Users\Sun\Desktop\origin-picture-logo/';//Source Image Directory End with Slash
$DestImagesDirectory = 'D:\Color/'; //Destination Image Directory End with Slash
$NewImageWidth ; //New Width of Image
$NewImageHeight; // New Height of Image
$Quality ; //Image Quality
 


//Function that resizes image.
function resizeImage($SrcImage,$DestImage, $MaxWidth=20,$MaxHeight= 29,$Quality)
{
//echo $SrcImage;
global  $iWidth;
global  $iHeight;
global  $img;
$img = $SrcImage;

    list($iWidth,$iHeight,$type)    = getimagesize($SrcImage);
    $ImageScale             = min($MaxWidth/$iWidth, $MaxHeight/$iHeight);
    $NewWidth               = ceil($ImageScale*$iWidth);
    $NewHeight              = ceil($ImageScale*$iHeight);
    $NewCanves              = imagecreatetruecolor($NewWidth, $NewHeight);
    imagealphablending($NewCanves, false);
    imagesavealpha($NewCanves, true);
    switch(strtolower(image_type_to_mime_type($type)))
{
case 'image/png':
$NewImage = imagecreatefrompng($SrcImage);
break;
case 'image/png':
 $NewImage = imagecreatefrompng($SrcImage);

break;
case 'image/gif':
$NewImage = imagecreatefromgif($SrcImage);
break;
default:
return false;
}




// Resize Image
    if(imagecopyresampled($NewCanves, $NewImage,0, 0, 0, 0, $NewWidth, $NewHeight, $iWidth, $iHeight))
    {
//	echo	$iWidth;
      $stndartwidth  = $iWidth;
// copy file
        if(imagejpeg($NewCanves,$DestImage,$Quality))
        {
            imagedestroy($NewCanves);
            return true;
        }
    }
}


//Open Source Image directory, loop through each Image and resize it.
if($dir = opendir($ImagesDirectory)){
    while(($file = readdir($dir))!== false){

        $imagePath = $ImagesDirectory.$file;
        $destPath = $DestImagesDirectory.$file;
        $checkValidImage = @getimagesize($imagePath);
         
        if(file_exists($imagePath) && $checkValidImage) //Continue only if 2 given parameters are true
        {
            //Image looks valid, resize.
resizeImage($imagePath,$destPath,$NewImageWidth = 230,$NewImageHeight = 138,$Quality = 100);
//if ($iWidth > $iHeight ){echo 'dsfd' ;resizeImage($imagePath,$destPath,$NewImageWidth = 280,$NewImageHeight = 280,$Quality = 100); ;}      
//if ($iWidth < $iHeight ){echo 'sadas' ;resizeImage($imagePath,$destPath,$NewImageWidth = 260,$NewImageHeight = 260,$Quality = 100) ;      ;}
          echo	  $iWidth;
            if(resizeImage($imagePath,$destPath,$NewImageWidth,$NewImageHeight,$Quality))
            {
              
                echo $file.' resize Success!<br />';
                /*
                Now Image is resized, may be save information in database?
                */

            }else{
                echo $file.' resize Failed!<br />';
            }
        }
    }
  //	echo  $imagePath;
    closedir($dir);
}

?>

Answer 1

The point is that imagecreatetruecolor creates an image with a black background. When the “color pairing mode” is on, a transparent pixel will not replace the background, but a new color will be calculated in accordance with the new color and the background color, taking into account the alpha channel of both colors - as a result, for a completely transparent pixel, we get the background color - i.e. black.
The solution is the following
After the line:
Add this:
// Turn off the color pairing mode
// Turn on the preservation of the alpha channel
Replace the line
with
And in the line below, specify the quality 0-9, if the maximum, then set $Quality=9

resizeImage($imagePath,$destPath,$NewImageWidth=230,$NewImageHeight=138,$Quality=100);

Answer 2

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