Some background about Capture Sharpening.
Why did I code it?
RT has RL deconvolution sharpening since many years but it was known to produce (mainly bright) halos.
Based on a discussion somewhere here on pixls.us I tried to apply RL earlier in the pipeline on linear data.
That already worked better but now gave dark halos instead of bright ones.
I could partly fix the dark halos by using a more correct gaussian blur, but not complete.
As it already worked better than before, I cared about usability.
The old RL (still) has this settings:
Lots of settings to find the right ones. Let’s exclude Blur radius (which is for creative unsharpening), amount and damping as I never used them after Contrast threshold was implemented. Still 3 settings remained marked with red rectangles in this screenshot:
It turned out to be quite easy to calculate the Radius value from the green channel of the raw data (at least for bayer, since yesterday also for xtrans), means now the user can mostly stick with the auto radius and has only 2 settings to adjust.
Furthermore, as Capture sharpening works on the full size image, also the Contrast threshold could be auto-calculated => only one setting to adjust, which is the iterations setting.
I tried to find a good default value for iterations (which is 20), but I still got dark halos (we will come back later to that)
While coding all that stuff, I had the idea, to allow a radius increasing from center of the image to the corners to fight the decreasing sharpness of a lens towards the outer regions.
So I added another adjuster (Corner radius boost), which defaults to 0, but can be increased to increase sharpening towards the outer regions of an image (it can also be decreased)
So now, we again had 2 settings, though corner radius at 0 still works fine especially when using full frame (FX) lenses on APS (Dx) sensors.
The screenshot shows the two setting which were not auto calculated with red rectangles
Still, with all the improvements, there were this damn dark halos.
Fortunately I found a way to reduce them (not fix them completely) by adding an option to limit the iterations of the RL deconvolution. To do that, I had to reduce the tile size from about 194 to 32 (so that the iteration limitation is done locally, not for the whole image or large tiles)
Means we got ony more ui control (the new checkbox) which mostly allows us to keep another setting untouched. That also turned out to work quite well, so now we have this gui
And the only thing wich deserves a change (if at all) is the one marked with the red rectangle if you want to increase sharpening towards the outer regions of the image:
Just stick with default values and maybe set Corner radius boost to e.g. 0.15 and for most cases this will work just fine.