Sharpening Test Image

[tomczak]

Here is a test image that I concocted to better understand how various sharpening settings affect picture elements. It is especially useful to understand the new AS abilities, but also for comparing the halos characteristics of different sharpening methods and settings.

This is obviously not a replacement for testing sharpening settings on a real image. Rather, it helps to understand what a sharpening method and its settings actually do internally (which it turn helps to make wise choices when sharpening real images).

The image background is a simple horizontal gradient (from black to white) with vertical gradient stripes overlay. The vertical stripes are 1, 2 and 5 pixels wide.

The idea behind this image is that it simulates nearly all possible edge contrast (or 'roughness') possibilities, over the entire range of brightness. This allows for separating the sharpening effects, on edges that may have the same contrast (i.e. same difference in brightness), but are in a different brightness range.

The image is in 24 bit colour, and can show the colour artifacts if the sharpening method introduces them.

I find this image particularly useful in exploring and explaining the difference between the new AS upper threshold slider (the right double sliders below the roughness histogram which can exclude the most contrasty edges from sharpening) and the Halo Limit double sliders (which can selectively 'flatten' the peaks of white and/or black halos of the edges that ended up being sharpened).