Image filtering in PHP 5 with GD extension

PHP includes simple image filtering capabilities provided by the GD extension.

The following filters are available, listed as the PHP constants:

IMG_FILTER_NEGATE
IMG_FILTER_GRAYSCALE
IMG_FILTER_BRIGHTNESS
IMG_FILTER_CONTRAST
IMG_FILTER_COLORIZE
IMG_FILTER_EDGEDETECT
IMG_FILTER_EMBOSS
IMG_FILTER_GAUSSIAN_BLUR
IMG_FILTER_SELECTIVE_BLUR
IMG_FILTER_MEAN_REMOVAL
IMG_FILTER_SMOOTH

Some of them can be adjusted with additional parameters, such as contrast and smooth.

imagefilter($image_resource, FILTER)

Image resource can be created simply from an existing image by using function such as:

$image_resource = imagecreatefromjpeg("laughing-monkey.jpg");

Next step usually is to output the filtered image. This can be done in two ways by using the same function, print to browser or save to a local file. If you do not define the second parameter, the image will be shown to the browser.

imagejpeg($image_resource, "laughing-monkey-filtered.jpg");

A working example of all the filters is available here, applied to my story image thumbnails.