Smugmug sharpening

[DanK]

Some time ago, there was a thread on this site about images appearing on Smugmug as if sharpened more.

Well, thanks to the new Smugmug interface, that is now clear, and it is customizable. Choose "organize" from the top menu, then select any gallery, the "settings", then "sharpening." If my site is any indication, the default seems to be amount 0.2, radius 1, threshold 0.05, as well as a parameter I had never heard of, sigma, which they set to 1.0. These can all be changed.

The sigma turns out to be a reference to a standard deviation, which is conventionally shown as sigma. they link to a site that explains:

"It describes the relative weight of pixels as a function of their distance
from the center of the convolution kernel. For small sigma, the outer
pixels have little weight."

[Cantab]

Dan, an interesting post. Do you have any idea what the sigma setting actually does in practice?

[dje]

Dan it's interesting that you have a choice in Smugmug. Many Flickr users have been complaining about the new Flickr which appears to add sharpening which you can't change (as far as I know).

It is my understanding, with Unsharp Mask anyway, that changing the radius setting changes the standard deviation of the distribution curve used to produce blur in the first stage of the Unsharp mask process. Why there would be separate settings for radius and sigma in Smugmug is a puzzle to me but maybe they are using a different sharpening algorithm - your quote refers to a convolution kernel so maybe they are using a de-convolution method and maybe this is more complex than the Unsharp mask.

Dave

[davidedric]

If I have exported from Lightroom, and "sharpened for screen", is there a reason why I would want Smugmug to apply any additional sharpening? Something to do with their display algorithms?

[DanK]

Dave (Ellis),

I had the same question. There are links on the Smugmug page that takes you to an explanation (it takes a few links), but I only skimmed it, and I don't think it explained this in much detail. However, the radius setting is discrete--that is, it counts pixels. The sigma setting is not. It's Gaussian.

Dave (Edric),

That question is why I started hunting. I export to Smugmug from Lightroom, and I always choose "sharpened for screen." Unfortunately, it is hard to get an A/B comparison because the setting is applied to galleries, not individual photos. I think I am going to set up a temporary directory with sharpening turned off and copy some photos into it using the 'collections' option, so I can have the same photo loaded into full-screen view in both and switch back and forth.

I'd be curious to hear if anyone else plays with this and sees the impact.

Dan

[dje]

Dan

I had a look at the info provided by Smugmug on sharpening. It appears that they are using an open source program called ImageMajick for resizing and sharpening (and possibly other things too). As far as I can see, with this program, sigma is the standard deviation of the Gaussian distribution used in the Unsharp Mask process and radius is the number of pixels out from the centre used in the calculations for each pixel.

In most Unsharp mask programs, there is no sigma setting, only radius. My assumption with these programs is that when radius is selected, a value of sigma is set and then the program automatically decides from that value how many pixels out from centre to use in each pixel calculation. eg if you set radius at 0.5 pixels, then sigma is 0.5 and the program might decide to calculate out to say 2 pixels. Anything further out would have no significant effect anyway. Just my theory !

Dave (Edric)

As I read the Smugmug info, the sharpening is not applied to your original posted image. It is only applied to downsized versions of this that can be selected in standard smaller display sizes.

Dave