Recipe 15.8 Getting and Setting a Transparent Color
15.8.1 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.
15.8.2 Solution
Use ImageColorTransparent(
):
$color = ImageColorAllocate($image, $red, $green, $blue);
ImageColorTransparent($image, $color);
15.8.3 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.
15.8.4 See Also
Documentation on ImageColorTransparent( ) at
http://www.php.net/imagecolortransparent and on
ImageColorsForIndex( ) at
http://www.php.net/imagecolorsforindex.
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