Unsharp Mask...

This filter uses the common "Unsharp Mask" (also known as "USM") technique to sharpen the alpha channel.

The unsharp mask technique applies a Gaussian blur to the image, then determines the difference between the blurred and original pixels and offsets this difference to increase sharpness.
It is similar to the same named technique used in analog photo laboratories.

Settings

The settings are closely related in such a way that there are multiple ways to achieve the same result.

Amount

The "Amount" determines how much pixels are sharpened.
Range is 0% to 500%, default 100% is considered the maximum for photorealistic sharpening.

Radius

Sets the radius of the Gaussian Blur used, in pixels.
Range is 0 to 250 pixels.
A large radius will pick up more details but may also see small gradient changes as details.
A small radius will not blur the pixel enough to detect any edges.
It is generally best to keep radius roughly between 2 and 10.
A large radius may also cause the sharpened edges to become "fat".

Threshold

This sets the minimum threshold in difference between the original pixel and the blurred pixel.
If the difference is greater than the threshold, the pixel is sharpened.
Range is from 0 to 255.
A low threshold will pick up more details but may also detect and sharpen noise, making the image more grainy.
It is advised to set threshold as low as the noise level in the image allows.
Setting a high threshold can be used for special effects.

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