Overlaying colour on a JPEG with PHP/GD? - php

Question

I have a collection of black and white JPEG's stored on my server. These images are symbol based, where the symbol is a collection of black lines on a white background.
I am trying to use GD to replace the black colour with another colour on the fly based on a variable passed. Currently, I am:
Getting the JPEG as: $image = imagecreatefromjpeg($imgURL),
Converting a HEX code (#FF0000, say) to RGB through PHP,
And then feeding these variables to:
private function colourize_image($image, $colour, $contrast = 0) {
if (!$image) { return false; }
imagealphablending($image, true);
imagesavealpha($image, true);
# Convert hex colour into RGB values
$r = hexdec('0x' . $colour{0} . $colour{1});
$g = hexdec('0x' . $colour{2} . $colour{3});
$b = hexdec('0x' . $colour{4} . $colour{5});
imagefilter($image, IMG_FILTER_COLORIZE, $r, $g, $b);
imagefilter($image, IMG_FILTER_CONTRAST, $contrast);
# Return the GD image object
return $image;
}
For some reason, the function doesn't work at all (it won't overlay a new colour).
Can anyone advise as to where I am going wrong?
Many thanks.

Answer 1

If the color is the only problem, then you could try this:
<php>
//SNIP
$color = preg_replace('/^#/','',$color); //Get rid of "#" if it's there
$r = hexdec("0x{$color[0]}{$color[1]}");
$g = hexdec("0x{$color[2]}{$color[3]}");
$b = hexdec("0x{$color[4]}{$color[5]}");
//SNIP
</php>

Answer 2

You can use the imageistruecolor function to find out whether the JPEG you've just loaded is true colour or palette-based. If it's not true colour, you can create a new true color image of the same width and height, and copy the old image over:
$width = imagesx($jpeg);
$height = imagesy($jpeg);
$image = imagecreatetruecolor($width, $height);
imagecopy($jpeg, $image, 0, 0, 0, 0, $width, $height);
You should then be able to apply the new colours.