imagepng() and transparency in GD library with PHP

Question

在PHP中使用函数imagepng()时，如何确保保存的图像以透明背景保存？

Answer 1

Simply do this:

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

Before outputting. Make sure that all source files (if you used any) are set to PNG 32-bit with transparency - if not the output may differ with black background or transparency does not comply.

Answer 2

Here is the example

     $newimage = imagecreatetruecolor($dst_w, $dst_h);
     imagealphablending($newimage, false);
     imagesavealpha($newimage, true);
     $transparentindex = imagecolorallocatealpha($newimage, 255, 255, 255, 127);
     imagefill($newimage, 0, 0, $transparentindex);

Answer 3

Here is an example of the imagecolortransparent function (if it helps):

<?php
// Create a 55x30 image
$im = imagecreatetruecolor(55, 30);
$red = imagecolorallocate($im, 255, 0, 0);
$black = imagecolorallocate($im, 0, 0, 0);

// Make the background transparent
imagecolortransparent($im, $black);

// Draw a red rectangle
imagefilledrectangle($im, 4, 4, 50, 25, $red);

// Save the image
imagepng($im, './imagecolortransparent.png');
imagedestroy($im);
?>

Answer 4

There's a function called imagecolortransparent that allows you to set which color is made transparent. I don't know if this answers your question.