Create a transparent png file using PHP

Question

Currently I would like to create a transparent png with the lowest quality .

The code:

<?php
function createImg ($src, $dst, $width, $height, $quality) {
    $newImage = imagecreatetruecolor($width,$height);
    $source = imagecreatefrompng($src); //imagecreatefrompng() returns an image identifier representing the image obtained from the given filename.
    imagecopyresampled($newImage,$source,0,0,0,0,$width,$height,$width,$height);
    imagepng($newImage,$dst,$quality);      //imagepng() creates a PNG file from the given image. 
    return $dst;
}

createImg ('test.png','test.png','1920','1080','1');
?>

However, there are some problems:

1. Do I need to specific a png file before creating any new file? Or can I create without any existing png file?

   Warning: imagecreatefrompng(test.png): failed to open stream: No such file or directory in

   C:\DSPadmin\DEV\ajax_optipng1.5\create.php on line 4

2. Although there are error message , it still generate a png file , however, what I found that is the file is a black color image , do I need to specific any parameter to make it transparent?

Thanks.

Answer 1

To 1) imagecreatefrompng('test.png') tries to open the file test.png which then can be edited with GD functions.

To 2) To enable saving of the alpha channel imagesavealpha($img, true); is used. The following code creates a 200x200px sized transparent image by enabling alpha saving and filling it with transparency.

<?php
$img = imagecreatetruecolor(200, 200);
imagesavealpha($img, true);
$color = imagecolorallocatealpha($img, 0, 0, 0, 127);
imagefill($img, 0, 0, $color);
imagepng($img, 'test.png');

Answer 2

Take a look at:

• imagecolorallocatealpha
• imagefill

An example function copies transparent PNG files:

    <?php
    function copyTransparent($src, $output)
    {
        $dimensions = getimagesize($src);
        $x = $dimensions[0];
        $y = $dimensions[1];
        $im = imagecreatetruecolor($x,$y); 
        $src_ = imagecreatefrompng($src); 
        // Prepare alpha channel for transparent background
        $alpha_channel = imagecolorallocatealpha($im, 0, 0, 0, 127); 
        imagecolortransparent($im, $alpha_channel); 
        // Fill image
        imagefill($im, 0, 0, $alpha_channel); 
        // Copy from other
        imagecopy($im,$src_, 0, 0, 0, 0, $x, $y); 
        // Save transparency
        imagesavealpha($im,true); 
        // Save PNG
        imagepng($im,$output,9); 
        imagedestroy($im); 
    }
    $png = 'test.png';

    copyTransparent($png,"png.png");
    ?>