Can PNG image transparency be preserved when using PHP's GDlib imagecopyresampled?
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09-06-2019 - |
Question
The following PHP code snippet uses GD to resize a browser-uploaded PNG to 128x128. It works great, except that the transparent areas in the original image are being replaced with a solid color- black in my case.
Even though imagesavealpha
is set, something isn't quite right.
What's the best way to preserve the transparency in the resampled image?
$uploadTempFile = $myField[ 'tmp_name' ]
list( $uploadWidth, $uploadHeight, $uploadType )
= getimagesize( $uploadTempFile );
$srcImage = imagecreatefrompng( $uploadTempFile );
imagesavealpha( $targetImage, true );
$targetImage = imagecreatetruecolor( 128, 128 );
imagecopyresampled( $targetImage, $srcImage,
0, 0,
0, 0,
128, 128,
$uploadWidth, $uploadHeight );
imagepng( $targetImage, 'out.png', 9 );
Solution
imagealphablending( $targetImage, false );
imagesavealpha( $targetImage, true );
did it for me. Thanks ceejayoz.
note, the target image needs the alpha settings, not the source image.
Edit: full replacement code. See also answers below and their comments. This is not guaranteed to be be perfect in any way, but did achieve my needs at the time.
$uploadTempFile = $myField[ 'tmp_name' ]
list( $uploadWidth, $uploadHeight, $uploadType )
= getimagesize( $uploadTempFile );
$srcImage = imagecreatefrompng( $uploadTempFile );
$targetImage = imagecreatetruecolor( 128, 128 );
imagealphablending( $targetImage, false );
imagesavealpha( $targetImage, true );
imagecopyresampled( $targetImage, $srcImage,
0, 0,
0, 0,
128, 128,
$uploadWidth, $uploadHeight );
imagepng( $targetImage, 'out.png', 9 );
OTHER TIPS
Why do you make things so complicated? the following is what I use and so far it has done the job for me.
$im = ImageCreateFromPNG($source);
$new_im = imagecreatetruecolor($new_size[0],$new_size[1]);
imagecolortransparent($new_im, imagecolorallocate($new_im, 0, 0, 0));
imagecopyresampled($new_im,$im,0,0,0,0,$new_size[0],$new_size[1],$size[0],$size[1]);
I believe this should do the trick:
$srcImage = imagecreatefrompng($uploadTempFile);
imagealphablending($srcImage, false);
imagesavealpha($srcImage, true);
edit: Someone in the PHP docs claims imagealphablending
should be true, not false. YMMV.
An addition that might help some people:
It is possible to toggle imagealphablending while building the image. I the specific case that I needed this, I wanted to combine some semi-transparent PNG's on a transparent background.
First you set imagealphablending to false and fill the newly created true color image with a transparent color. If imagealphablending were true, nothing would happen because the transparent fill would merge with the black default background and result in black.
Then you toggle imagealphablending to true and add some PNG images to the canvas, leaving some of the background visible (ie. not filling up the entire image).
The result is an image with a transparent background and several combined PNG images.
I have made a function for resizing image like JPEG/GIF/PNG with copyimageresample
and PNG images still keep there transparency:
$myfile=$_FILES["youimage"];
function ismyimage($myfile) {
if((($myfile["type"] == "image/gif") || ($myfile["type"] == "image/jpg") || ($myfile["type"] == "image/jpeg") || ($myfile["type"] == "image/png")) && ($myfile["size"] <= 2097152 /*2mb*/) ) return true;
else return false;
}
function upload_file($myfile) {
if(ismyimage($myfile)) {
$information=getimagesize($myfile["tmp_name"]);
$mywidth=$information[0];
$myheight=$information[1];
$newwidth=$mywidth;
$newheight=$myheight;
while(($newwidth > 600) || ($newheight > 400 )) {
$newwidth = $newwidth-ceil($newwidth/100);
$newheight = $newheight-ceil($newheight/100);
}
$files=$myfile["name"];
if($myfile["type"] == "image/gif") {
$tmp=imagecreatetruecolor($newwidth,$newheight);
$src=imagecreatefromgif($myfile["tmp_name"]);
imagecopyresampled($tmp, $src, 0, 0, 0, 0, $newwidth, $newheight, $mywidth, $myheight);
$con=imagegif($tmp, $files);
imagedestroy($tmp);
imagedestroy($src);
if($con){
return true;
} else {
return false;
}
} else if(($myfile["type"] == "image/jpg") || ($myfile["type"] == "image/jpeg") ) {
$tmp=imagecreatetruecolor($newwidth,$newheight);
$src=imagecreatefromjpeg($myfile["tmp_name"]);
imagecopyresampled($tmp, $src, 0, 0, 0, 0, $newwidth, $newheight, $mywidth, $myheight);
$con=imagejpeg($tmp, $files);
imagedestroy($tmp);
imagedestroy($src);
if($con) {
return true;
} else {
return false;
}
} else if($myfile["type"] == "image/png") {
$tmp=imagecreatetruecolor($newwidth,$newheight);
$src=imagecreatefrompng($myfile["tmp_name"]);
imagealphablending($tmp, false);
imagesavealpha($tmp,true);
$transparent = imagecolorallocatealpha($tmp, 255, 255, 255, 127);
imagefilledrectangle($tmp, 0, 0, $newwidth, $newheight, $transparent);
imagecopyresampled($tmp, $src, 0, 0, 0, 0, $newwidth, $newheight, $mywidth, $myheight);
$con=imagepng($tmp, $files);
imagedestroy($tmp);
imagedestroy($src);
if($con) {
return true;
} else {
return false;
}
}
} else
return false;
}
I suppose that this might do the trick:
$uploadTempFile = $myField[ 'tmp_name' ]
list( $uploadWidth, $uploadHeight, $uploadType )
= getimagesize( $uploadTempFile );
$srcImage = imagecreatefrompng( $uploadTempFile );
$targetImage = imagecreatetruecolor( 128, 128 );
$transparent = imagecolorallocate($targetImage,0,255,0);
imagecolortransparent($targetImage,$transparent);
imagefilledrectangle($targetImage,0,0,127,127,$transparent);
imagecopyresampled( $targetImage, $srcImage,
0, 0,
0, 0,
128, 128,
$uploadWidth, $uploadHeight );
imagepng( $targetImage, 'out.png', 9 );
The downside is that the image will be stripped of every 100% green pixels. Anyhow, hope it helps :)
Regrading the preserve transparency, then yes like stated in other posts imagesavealpha() have to be set to true, to use the alpha flag imagealphablending() must be set to false else it doesn't work.
Also I spotted two minor things in your code:
- You don't need to call
getimagesize()
to get the width/height forimagecopyresmapled()
- The
$uploadWidth
and$uploadHeight
should be-1
the value, since the cordinates starts at0
and not1
, so it would copy them into an empty pixel. Replacing it with:imagesx($targetImage) - 1
andimagesy($targetImage) - 1
, relativily should do :)
Here is my total test code. It works for me
$imageFileType = pathinfo($_FILES["image"]["name"], PATHINFO_EXTENSION);
$filename = 'test.' . $imageFileType;
move_uploaded_file($_FILES["image"]["tmp_name"], $filename);
$source_image = imagecreatefromjpeg($filename);
$source_imagex = imagesx($source_image);
$source_imagey = imagesy($source_image);
$dest_imagex = 400;
$dest_imagey = 600;
$dest_image = imagecreatetruecolor($dest_imagex, $dest_imagey);
imagecopyresampled($dest_image, $source_image, 0, 0, 0, 0, $dest_imagex, $dest_imagey, $source_imagex, $source_imagey);
imagesavealpha($dest_image, true);
$trans_colour = imagecolorallocatealpha($dest_image, 0, 0, 0, 127);
imagefill($dest_image, 0, 0, $trans_colour);
imagepng($dest_image,"test1.png",1);
Pay attention to the source image's width
and height
values which are passed to imagecopyresampled
function. If they are bigger than actual source image size, the rest of image area will be filled with black color.
I combined the answers from ceejayoz and Cheekysoft, which gave the best result for me. Without imagealphablending() and imagesavealpha() the image is not clear:
$img3 = imagecreatetruecolor(128, 128);
imagecolortransparent($img3, imagecolorallocate($img3, 0, 0, 0));
imagealphablending( $img3, false );
imagesavealpha( $img3, true );
imagecopyresampled($img3, $srcImage, 0, 0, 0, 0, 128, 128, $uploadWidth, $uploadHeight);
imagepng($img3, 'filename.png', 9);