Speeding Precise Gaussian Blur

[Robert Schleif]

I like the backgrounds of my flower images to be highly blurred. Precise Gaussian is fine for this except that it runs very slowly. The following seems to be an effective blurring procedure that, for my images, is about five times faster than Precise Gaussian. Feather a mask of the flower with a radius of about +100. The difference between the enlarged mask and the original mask is a band around the flower. Use this difference mask with Precise Gaussian and a radius of about 70 to blur the background in the band around the flower. Then use Gaussian blur with a radius of about 70 to blur the rest of the background using the enlarged mask. Some fiddling with the radius used with Gaussian blur can be needed to eliminate a halo. So far it has not been necessary to use the Brightness transform on the difference mask to adjust the fall-off of the mask darkness in the border region, but this could be a way to improve the match between blurring in the band and in the rest of the background.

[jsachs]

That should work. Unfortunately the blurring math is a little different between Gaussian and Precise Gaussian so the same radius only produces approximately the same result.