Create a transparent png file using PHP - 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:
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
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");
?>

Answer 3

1) You can create a new png file without any existing one.
2) You get a black color image because you use imagecreatetruecolor();. It creates a highest quality image with a black background. As you need a lowest quality image use imagecreate();
<?php
$tt_image = imagecreate( 100, 50 ); /* width, height */
$background = imagecolorallocatealpha( $tt_image, 0, 0, 255, 127 ); /* In RGB colors- (Red, Green, Blue, Transparency ) */
header( "Content-type: image/png" );
imagepng( $tt_image );
imagecolordeallocate( $background );
imagedestroy( $tt_image );
?>
You can read more in this article: How to Create an Image Using PHP