Unsharp mask tool allows the user to set a radius <= 1.0 and in that case is just overexposes the image. Furthermore, it ignores the decimal part of the number. Any value between 1.01 and 1.99, 2.00-2.99 and so on are the same. It behaves like very old versions of digikam. I checked v4.0.0-beta4 and it does not have this issue.
Git commit 86f5988b7b15b9de8e93750ad5d40db434ab8361 by Gilles Caulier.
Committed on 02/06/2014 at 14:09.
Pushed by cgilles into branch 'master'.
With 4.0.0 release, blur filter algorithm have been changed, and radius argument precision have been dropped due to use an interger value intead a float value.
This commit fix this wrong cast to take a acre about radius precision with UnsharpMask filter
M +3 -1 NEWS
M +3 -3 imageplugins/enhance/sharpentool.cpp
M +1 -1 imageplugins/enhance/sharpentool.h
M +25 -24 libs/dimg/filters/sharp/sharpsettings.cpp
M +1 -1 libs/dimg/filters/sharp/sharpsettings.h
M +4 -4 libs/dimg/filters/sharp/unsharpmaskfilter.cpp
M +2 -2 libs/dimg/filters/sharp/unsharpmaskfilter.h
I'm sorry to say so but the bug doesn't seem to be fixed to me... In digikam 4.2.0 (from Ubuntu's repositories, Ubuntu 14.10) the USM still overexposes images with radius < 1 and "amount" param sets the amount of overexposition... Moreover, radius > 1 results in massive over-sharpening as compared to 3.5.0's behaviour: I used to batch process some pictures in 3.5.0 with radius = 8, amount = 1 and threshold = 0.05 to get a subtly sharpened image, but the very same batch workflow in 4.2.0 yield a completely transformed image (ie. highly oversharpened, much more global contrast, looks like an enormous radius blur on top of an enormous USM contrast amount)
I compiled and installed 4.1.0 and the result was the same.
I get the same on to different machines running Ubuntu 14.10 and digikam 4.1.0 or 4.2.0.
Reproducible: still always