Bad quality of GIF image in PHP GD

Question

I have a watermark script I am working on, the results are good on PNG and on JPG images however a gif image, not so good. I am using PHP and GD

Below you can see the difference in quality of the watermark.

Anyone know how to improve this?

For the gif version I am using

$image = imagecreatefromgif($source_file);
imagecopymerge($image, $watermark, $x, $y, 0, 0, $water_width, $water_height, 65);
imagegif($image, $source_file);

gif image = bad quality gif image http://img2.pict.com/fd/46/00/1471179/0/gif.gif

jpg image = good jpg image http://img2.pict.com/82/a1/5a/1471181/0/jpg.jpg

Answer 1

GIF images have a fixed palette that can contain a maximum of 256 colors. The issue here is probably that the image your inserting uses colors that isn't available in the target image.

I have never tried this, but it might be worth a shot. You could try converting the gif image to a true color image first, then do the watermarking and after that converting it back to gif.

$image = imagecreatefromgif($source_file);
// create a true color image of the same size
$image2 = imagecreatetruecolor(imagesx($image), imagesy($image));
// copy the original gif image on to the true color image
imagecopy($image2, $image, 0, 0, 0, 0, imagesx($image), imagesy($image));
// copy the watermark onto the new true color image
imagecopymerge($image2, $watermark, $x, $y, 0, 0, $water_width, $water_height, 65);
// write the new image to disk
imagegif($image2, $source_file);

Try it and see if it makes a difference.

There's also a couple of palette manipulation functions available that might help:

1. imagecolormatch()
2. imagetruecolortopalette()
3. imagepalettecopy()

I'm not sure how you would apply them, but I'm guessing that there's a few things you could do to improve the results.

Answer 2

GIF only supports a color palette of 256 colors. Therefore, the colors of your watermark image have to be mapped to this palette. This caused them to be rendered and saved with different colors than before. Due to this small palette, GIF is not recommended for photos in general, anyways.

Answer 3

GIF images will never look great, as the colour palette is 256 colours. As MrMage says, colour mapping causes a severe approximation of the true colours of the image. You are better off with PNGs, they do support transparency.