[mffowler]

What you need to apply is an unsharp mask, like the Unsharp mask algorithm by Torstein Hц╦nsi. This is what vBulletin uses and the code basically looks like:

////////////////////////////////////////////////////////////////////////////////////////////////
////
////                  p h p U n s h a r p M a s k
////
////		Unsharp mask algorithm by Torstein Hц╦nsi 2003.
////		thoensi@netcom.no
////		Please leave this notice.
////
///////////////////////////////////////////////////////////////////////////////////////////////

function UnsharpMask(&$img, $amount = 100, $radius = .5, $threshold = 3)
{

	// $img is an image that is already created within php using
	// imgcreatetruecolor. No url! $img must be a truecolor image.

	// Attempt to calibrate the parameters to Photoshop:
	if ($amount > 500)
	{
		$amount = 500;
	}
	$amount = $amount * 0.016;
	if ($radius > 50)
	{
		$radius = 50;
	}
	$radius = $radius * 2;
	if ($threshold > 255)
	{
		$threshold = 255;
	}

	$radius = abs(round($radius)); 	// Only integers make sense.
	if ($radius == 0)
	{
		return true;
	}

	$w = imagesx($img);
	$h = imagesy($img);
	$imgCanvas = imagecreatetruecolor($w, $h);
	$imgCanvas2 = imagecreatetruecolor($w, $h);
	$imgBlur = imagecreatetruecolor($w, $h);
	$imgBlur2 = imagecreatetruecolor($w, $h);
	imagecopy ($imgCanvas, $img, 0, 0, 0, 0, $w, $h);
	imagecopy ($imgCanvas2, $img, 0, 0, 0, 0, $w, $h);


	// Gaussian blur matrix:
	//
	//	1	2	1
	//	2	4	2
	//	1	2	1
	//
	//////////////////////////////////////////////////

	// Move copies of the image around one pixel at the time and merge them with weight
	// according to the matrix. The same matrix is simply repeated for higher radii.
	for ($i = 0; $i < $radius; $i++)
	{
		imagecopy ($imgBlur, $imgCanvas, 0, 0, 1, 1, $w - 1, $h - 1); // up left
		imagecopymerge ($imgBlur, $imgCanvas, 1, 1, 0, 0, $w, $h, 50); // down right
		imagecopymerge ($imgBlur, $imgCanvas, 0, 1, 1, 0, $w - 1, $h, 33.33333); // down left
		imagecopymerge ($imgBlur, $imgCanvas, 1, 0, 0, 1, $w, $h - 1, 25); // up right
		imagecopymerge ($imgBlur, $imgCanvas, 0, 0, 1, 0, $w - 1, $h, 33.33333); // left
		imagecopymerge ($imgBlur, $imgCanvas, 1, 0, 0, 0, $w, $h, 25); // right
		imagecopymerge ($imgBlur, $imgCanvas, 0, 0, 0, 1, $w, $h - 1, 20 ); // up
		imagecopymerge ($imgBlur, $imgCanvas, 0, 1, 0, 0, $w, $h, 16.666667); // down
		imagecopymerge ($imgBlur, $imgCanvas, 0, 0, 0, 0, $w, $h, 50); // center
		imagecopy ($imgCanvas, $imgBlur, 0, 0, 0, 0, $w, $h);

		// During the loop above the blurred copy darkens, possibly due to a roundoff
		// error. Therefore the sharp picture has to go through the same loop to
		// produce a similar image for comparison. This is not a good thing, as processing
		// time increases heavily.
		imagecopy ($imgBlur2, $imgCanvas2, 0, 0, 0, 0, $w, $h);
		imagecopymerge ($imgBlur2, $imgCanvas2, 0, 0, 0, 0, $w, $h, 50);
		imagecopymerge ($imgBlur2, $imgCanvas2, 0, 0, 0, 0, $w, $h, 33.33333);
		imagecopymerge ($imgBlur2, $imgCanvas2, 0, 0, 0, 0, $w, $h, 25);
		imagecopymerge ($imgBlur2, $imgCanvas2, 0, 0, 0, 0, $w, $h, 33.33333);
		imagecopymerge ($imgBlur2, $imgCanvas2, 0, 0, 0, 0, $w, $h, 25);
		imagecopymerge ($imgBlur2, $imgCanvas2, 0, 0, 0, 0, $w, $h, 20 );
		imagecopymerge ($imgBlur2, $imgCanvas2, 0, 0, 0, 0, $w, $h, 16.666667);
		imagecopymerge ($imgBlur2, $imgCanvas2, 0, 0, 0, 0, $w, $h, 50);
		imagecopy ($imgCanvas2, $imgBlur2, 0, 0, 0, 0, $w, $h);
	}
	imagedestroy($imgBlur);
	imagedestroy($imgBlur2);

	// Calculate the difference between the blurred pixels and the original
	// and set the pixels
	for ($x = 0; $x < $w; $x++)
	{ // each row
		for ($y = 0; $y < $h; $y++)
		{ // each pixel

			$rgbOrig = ImageColorAt($imgCanvas2, $x, $y);
			$rOrig = (($rgbOrig >> 16) & 0xFF);
			$gOrig = (($rgbOrig >> 8) & 0xFF);
			$bOrig = ($rgbOrig & 0xFF);

			$rgbBlur = ImageColorAt($imgCanvas, $x, $y);

			$rBlur = (($rgbBlur >> 16) & 0xFF);
			$gBlur = (($rgbBlur >> 8) & 0xFF);
			$bBlur = ($rgbBlur & 0xFF);

			// When the masked pixels differ less from the original
			// than the threshold specifies, they are set to their original value.
			$rNew = (abs($rOrig - $rBlur) >= $threshold) ? max(0, min(255, ($amount * ($rOrig - $rBlur)) + $rOrig))	: $rOrig;
			$gNew = (abs($gOrig - $gBlur) >= $threshold) ? max(0, min(255, ($amount * ($gOrig - $gBlur)) + $gOrig))	: $gOrig;
			$bNew = (abs($bOrig - $bBlur) >= $threshold) ? max(0, min(255, ($amount * ($bOrig - $bBlur)) + $bOrig))	: $bOrig;

			$pixCol = imagecolorallocate ($img, $rNew, $gNew, $bNew);
			imagesetpixel ($img, $x, $y, $pixCol);
		}
	}
	imagedestroy($imgCanvas);
	imagedestroy($imgCanvas2);

	return true;
}

If you have a copy of vBulletin- see how it works in functions_image.php as that is basically what Boomer is doing.

Boomer- ever used this?

- Mike