Tuesday, January 19, 2010

Recipe 17.7. Getting and Setting a Transparent Color










Recipe 17.7. Getting and Setting a Transparent Color



17.7.2. Problem


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




17.7.3. Solution


Use
ImageColorTransparent( ):


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





17.7.4. 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);

$transparent = ImageColorsForIndex($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.




17.7.5. See Also


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













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