Use PHP imagecreatefromgif() to modify the right 150 pixels of a 760x1 pixel gif image

Question

I have a 1 pixel tall by 760 pixel wide image that I use as a repeating vertical background image. The right side of this image is filled with a spot color (the remaining left side of the image is white).

The purpose of this background image, in my css based layout, is that it provides the illusion that the sidebar background color runs all the way down the page (easy to do with tables, but no so much with CSS positioning).

What I need to do is to figure a way to feed a php script (background-image.php) which contains the imagecreatefromgif function, a hex number and have it use that to repaint the spot color of the image to match the spot color that's passed in and save the resulting image onto the server, overwriting the default one.

Ideally, I'd not like to have to call this function everytime the template loads, and onlydo it when the user elects to change the template colors. So once they do that, I'd just like to modify the existing image I've got on the server which will always be called "sidebar_bg.gif"

Any ideas on how to do this are much appreciated.

Answer 1

Something like this could do it:

$token = md5(serialize(array($red, $green, $blue)));

if (!file_exists('cachedir/'.$token.'.gif')) 
{
    $img = imagecreatefromgif('origfilename.gif');

    $color = imagecolorallocate($img, $red, $green, $blue);

    for ($i = $startPixel-1; $i < $endPixel; $i++)
    {
        imagesetpixel($img, $i, 0, $color);
    }

    imagegif($img, 'cachedir/'.$token.'.gif');
}

serveFile($token);

EDIT: Added caching to example code

Answer 2

Just an addition to this post. You can convert HEX color into RGB notation with the folowing function:

function hexToRGB ($hexColor)
{
    $output = array();
    $output['red']   = hexdec($hexColor[0].$hexColor[1]);
    $output['green'] = hexdec($hexColor[2].$hexColor[3]);
    $output['blue']  = hexdec($hexColor[4].$hexColor[5]);

    return $output;
}

e.g. try:

var_dump(hexToRGB("FFFFFF"));