PHP Cookbook

15.7. Getting and Setting a Transparent Color

Problem

You want to set one of an image’s colors as transparent. When the image is overlayed on a background, the background shows through the transparent section of the image.

Solution

Use ImageColorTransparent( ) :

$color = ImageColorAllocate($image, $red, $green, $blue);
ImageColorTransparent($image, $color);

Discussion

Both GIFs and PNGs support transparencies; JPEGs, however, do not. To refer to the transparent color within GD, use the constant IMG_COLOR_TRANSPARENT. For example, here’s how to make a dashed line that alternates between black and transparent:

// make a two-pixel thick black and white dashed line
$style = array($black, $black, IMG_COLOR_TRANSPARENT, IMG_COLOR_TRANSPARENT);
ImageSetStyle($image, $style);

To find the current transparency setting, take the return value of ImageColorTransparent( ) and pass it to ImageColorsForIndex( ) :

$transparent = ImageColorsForIndex($image, ImageColorTransparent($image));
print_r($transparent);
Array
               (
                   [red] => 255
                   [green] => 255
                   [blue] => 255
               )

The ImageColorsForIndex( ) function returns an array with the red, green, and blue values. In this case, the transparent color is white.

See Also

Documentation on ImageColorTransparent( ) at http://www.php.net/imagecolortransparent and on ImageColorsForIndex( ) at http://www.php.net/imagecolorsforindex.