Change image background

Question

Now I got this image with a white background, but want to change it to something else...

I've tried using imagecolorset, with a index of 16777215 (which is white), but it won't work D:

Anyone else who had this problem? And even better, knows how to fix it?

Code:

$img = imagecreatefrompng("http://chart.apis.google.com/chart?cht=qr&chs=300x300&chl=hellow"); 
$index = imagecolorat($img, 0, 0);
imagecolorset($img, $index, 0, 0, 255);
header("content-type:image/png");
imagepng($img);
imagedestroy($img);

Answer 1

It seems that the Google chart API is creating Truecolor images. Note:

[ghoti@pc ~]$ cat imgtest1.php 
<?php

$url = "http://chart.apis.google.com/chart?cht=qr&chs=300x300&chl=hello";
$img = imagecreatefrompng($url);

print "Colours before: " . imagecolorstotal($img) . "\n";
imagetruecolortopalette($img, FALSE, 2);
print "Colours after:  " . imagecolorstotal($img) . "\n";

[ghoti@pc ~]$ php imgtest1.php 
Colours before: 0
Colours after:  2
[ghoti@pc ~]$ 

So converting to a palette image with imagetruecolortopalette() seems to fix your problem.

<?php

$url = "http://chart.apis.google.com/chart?cht=qr&chs=100x100&chl=hello";
$img = imagecreatefrompng($url);         # fetch the image
imagetruecolortopalette($img, FALSE, 2); # convert image from TC to palette
$bg = imagecolorat($img, 0, 0);          # get the bg colour's index in palette
imagecolorset($img, $bg, 0, 0, 255);     # make it blue

header("content-type:image/png");
imagepng($img);
imagedestroy($img);

And the result: