Re: Gaussian blur in GIMP compared to Photoshop

["Jesper de Jong" <jespdj@xxxxxxxxx>]

I understand that the amount of blurring is determined by the standard deviation. So the conclusion is that there is no standard way to compute the standard deviation from the radius parameter that you specify for gaussian blur, and Photoshop and GIMP do this with different formulas - so it's not the case that either Photoshop or GIMP is wrong.

I looked in the sources and found the formula that GIMP uses:

plug-ins/common/gauss.c:932: vert = fabs (vert) + 1.0;
plug-ins/common/gauss.c:933: std_dev = sqrt (-(vert * vert) / (2 * log (1.0 / 255.0)));

(and the same for horizontal about 120 lines below).

Thanks,
Jesper

2007/11/20, Øyvind Kolås <pippin@xxxxxxxx>:

On Nov 20, 2007 12:05 PM, Jesper de Jong <jespdj@xxxxxxxxx> wrote:
> > > Is Gaussian blur not a standard algorithm that has a well-defined
> meaning
> > > for the radius? See for example
> http://en.wikipedia.org/wiki/Gaussian_blur
> > >
> > > If it is, then which one is doing it wrong, CS3 or GIMP?

The amount of blurring achieved by a gaussian blur is specified by the
standard deviation of the blurring kernel not the size of the blurring
kernel. This probably means that GIMP and Photoshop use different ways
of computing the standard deviation used from the user specified
radius.

Using the size of the kernel (which probably is ~2.5x the standard
deviation) might be leading to behavior similar to what GIMP is doing.

/Øyvind K.
--
«The future is already here. It's just not very evenly distributed»
                                                 -- William Gibson
http://pippin.gimp.org/                            http://ffii.org/

-- 
Jesper de Jong
jespdj@xxxxxxxxx
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