当使用 PHP 的 GDlib 图像采样时，PNG 图像透明性能否得到保护？

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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.

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最佳答案

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 );

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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 for imagecopyresmapled()
The $uploadWidth and $uploadHeight should be -1 the value, since the cordinates starts at 0 and not 1, so it would copy them into an empty pixel. Replacing it with: imagesx($targetImage) - 1 and imagesy($targetImage) - 1, relativily should do :)

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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]);

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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 :)

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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.

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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;
}

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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);

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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.

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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);

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For anyone having problems with imagecopyresampled or imagerotate with black bars on background, I have found a code example here:

https://qna.habr.com/q/646622#answer_1417035

        // get image sizes (X,Y)
$wx = imagesx($imageW);
$wy = imagesy($imageW);


// create a new image from the sizes on transparent canvas
$new = imagecreatetruecolor($wx, $wy);


$transparent = imagecolorallocatealpha($new, 0, 0, 0, 127);
$rotate = imagerotate($imageW, 280, $transparent);
imagealphablending($rotate, true);
imagesavealpha($rotate, true);


// get the newest image X and Y
$ix = imagesx($rotate);
$iy = imagesy($rotate);


//copy the image to the canvas
imagecopyresampled($destImg, $rotate, 940, 2050, 0, 0, $ix, $iy, $ix, $iy);