Sharpen an image using PHP and GD

PHP and GD can sharpen images by providing a matrix to imageconvolution:

// create the image resource from a file
$i = imagecreatefromjpeg('otter.jpg');

// define the sharpen matrix
$sharpen = array(
	array(0.0, -1.0, 0.0),
	array(-1.0, 5.0, -1.0),
	array(0.0, -1.0, 0.0)
);

// calculate the sharpen divisor
$divisor = array_sum(array_map('array_sum', $sharpen));

// apply the matrix
imageconvolution($i, $sharpen, $divisor, 0);

// output the image
header('Content-Type: image/jpeg');
imagejpeg($i);

Otter, before and after sharpening

Image courtesy Kevin Law