recommended Gaussian blur sigma for halving image dimensions

[NicolasRobidoux]

What is the "standard" recommendation (if there is any) for the gaussian blur sigma to be used to reduce an image from, say, 512x512 down to 256x256?

(I'll check Turkowski but figured there may be someone on this list with a better recommendation.)

[NicolasRobidoux]

I think I answered my own question by reading Ken Turkowski's Filters for Common Resampling Tasks:

For halving, if you don't filter further, it depens on the phase.

sigma = 1/sqrt(2) if the phase is 0.

Possibly sigma = 1/2 if the phase is .5.

(Of course, I'm stll willing to read opinions.)

[anthony]

You could also simply resize the image with a -filter Gaussian

Of course if you still want to define the sigma you can do that (with a recent version of IM)
http://www.imagemagick.org/Usage/resize/#filter_sigma

But even if your IM version is not recent enough, you can still modify the sigma by using the blur expert setting
http://www.imagemagick.org/Usage/resize/#filter_blur