How is Threshold parameter calculated and expressed in Unsharp Mask?

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Nut
Nut 2017-3-22
编辑： Nut 2017-3-22
Hi,
I'm trying to understand the imsharpen function:
https://it.mathworks.com/help/images/ref/imsharpen.html
I would like to understand how the Threshold parameter is calculated. It is about the minimum contrast, but how the contrast is calculated? The Threshold must be a scalar in the range [0 1], so I think it is a ratio: but of what?
As example, if I want to exclude the edges characterized by a difference of intensity equal or less than 20, which is the value to set as Threshold?
I also tried to investigate by using the imgradient function, but the resultant matrix can have values greater than 1, so I think it is not what I'm looking for.
Finally, in my case it could be very very useful to return a matrix (of the same size of the image) containing the value of contrast pixel by pixel, as calculated in order to apply the Threshold.
Thanks for your answers, kind regards.

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