What measure does "threshold" represent in the NR widget?

[mjdl]

I'm curious and want to better understand the NR widget: increasing the threshold percentage seems to blur increasingly large differences in adjacent or close-by pixels, but I can't figure out how its behaviour differs from the "level" control... and the manual is not giving more information.

[ksinkel]

The level control is a akin to a radius control and determines the size of the pixel neighborhood noise reduction considers. The correct setting depends alot on the exact nature of the noise. In some cases different setting can make a big difference, in other cases there is little difference between settings. Level should be set empirically to give the best results.

Threshold works together with the sharpen control. It allows you to exclude areas that have small differences from being sharpened. For example by increasing the threshold setting you can prevent smooth areas like sky or skin from being sharpened. Sharpening such areas introduces undesirable exaggeration of texture in areas which should be smooth.

Kiril

[mjdl]

Threshold works together with the sharpen control.

Ah-ha, thank you for the explanations: I had assumed from the manual's indication "Set the Threshold Control. This control has the greatest affect [sic] over how much noise reduction is applied" (p. 203) that the threshold control were somehow directly worked with the level control in noise reduction, but it is actually the threshold for the sharpening effect--no wonder I was having trouble figuring out the best adjustments.

[tomczak]

The way I understand the 'Threshold' in NR transform is that it divides the image into 'Low Roughness' areas and 'High Roughness' areas (how this roughness is defined, depends on the Level). Then the 'smoothing' is performed on the 'Low Roughness' parts, and the sharpening (if any) on the 'High Roughness' parts. If Threshold=0%, all the pixels are considered to have 'High Roughness' and thus no smoothing is done. Sharpening, if any, is done only on the High Roughness pixels - those that are smoothed are excluded. The Hard/Soft Threshold settings determines how the transition between Rough and Smooth pixels is treated.

I think that in a way, despite the NR method being different, the idea is similar to what AS is doing (but with AS having potentially far grater control over the results), with NR using a single threshold value for both NR and Sharpening.

This is also not that far off from Bilateral Sharpen - the difference being that BS leaves the 'High Roughness' pixels alone, and allows to either smooth or sharpen only 'Low roughness' areas.

http://www.dl-c.com/board/viewtopic.php?f=4&t=524

[mjdl]

Thank you for taking the time for this detailed explanation of how the various NR controls work, as well as the link to the earlier discussion of Adavenced Sharpen's Speck Removal tool and the Mask texture tool.