Hacking a GIF palette with PHP?

loosus

I have a set of GIF images that are images of letters. Rather than create an almost unlimited amount of images so that I can use whatever color I want for the images, I thought it'd be easier to create a PHP script that replaced the GIF palette with the color that I wanted on-the-fly. This would allow me to pick any color that I wanted while using the same basic "pixel data" of the GIF files.

Is this feasible/possible? Is there anywhere where I can learn enough about the structure of a GIF file to do this?

In case you're curious, I've attached some GIF samples of the images I want to hack.

Thanks in advance!

cahva

Well you could just use imagepalettecopy() and other gd-functions to change palette.

With the imagepalettecopy you could just use one set of images and create "colorthemes"(images with different palettes) and copy palette from them as you like.

loosus

cahva wrote:
Well you could just use imagepalettecopy() and other gd-functions to change palette.

With the imagepalettecopy you could just use one set of images and create "colorthemes"(images with different palettes) and copy palette from them as you like.

I haven't heard of that. I'll read it and reply to this thread again later. Thanks for the reply!

loosus

Okay. I read this, and I don't really think this (imagepalettecopy) is what I need. Why? Because I'd have to create like 16 million pictures to get every color that I need.

I'd like for my query string to look something like this:
pic_output.php?letter=A&color=FFFFF

That way, the script would first pull the base image (A.gif) via the "letter" variable, and then it could be assigned any available color in hex via the "color" variable.

drew010

do you have that font and is it ttf?? why not just generate the image on the fly based on the letter and color passed. its faster to do this than to try to go through the image and replace a certain color with another, especially since the background of the image is white and the inside of the letter is too, because its kind of a hollow letter rather than filled.

cahva

Ok, I had nothing to do so I made you a script. May come in handy for myself too someday 😉

<?php
$rgb=$_GET['color'];
$letter=$_GET['letter'].'.gif';
if (strlen($rgb)==6 && file_exists($letter)) {
	$r=hexdec(substr($rgb,0,2));
	$g=hexdec(substr($rgb,2,2));
	$b=hexdec(substr($rgb,4,2));
} else {
	exit();
}


$im=imagecreatefromgif($letter);
imagecolorset($im,0,$r,$g,$b);

header("Content-type: image/gif");
imagegif($im);
imagedestroy($im);
?>