I do a long description of how the shadhi module works in this video: https://www.youtube.com/watch?v=GVrSePtahJg&list=PLmvlUro_Up1NBX7VK8UUuyWo1B468zEA0&index=8
Just quickly, the sliders in the shadhi module do one of two things: some control how a mask is applied to the image to partition it into three regions: highlights, shadows and midtones. This mask is softened on the edges by either a gaussian or bilateral filter, of which you can control the blur radius to make the transition between the region you're affecting and the region you're leaving alone either sharper or softer. The other sliders then control what is done to the shadows or highlights regions, leaving the midtones alone. Given that you're trying to do a specific adjustment to the shadows under the eyes, it seems to me that you might be better served by making a more targeted adjustment, for example by using the exposure module along with either a drawn or parametric mask to simulate dodge/burn. Regards, Rob On 06/01/15 09:32, KOVÁCS István wrote: > Dear All, > > I've been struggling with darktable's shadows/highlights module. It > seems to do really weird things. The examples shown below are > exagerrations to show the effects more clearly. > The manual says: > > shadows: This slider controls the effect on shadows; positive values > will lighten up shadows while negative values will darken them. > > radius: This slider controls the radius of the involved blurring filter. > So, it seems the image is blurred as part of the processing; I'd > assume that it is the blurred image that is then examined to determine > if a part of the image is dark, midtone or a highlight area, and > adjusted accordingly. > > compress: This slider controls how strong the effect extends to > midtones; high values reduce the effect to the extreme shadows and > highlights; low values cause strong adjustments also to midtones. > So, set to 100% would only affect pure black/pure white (everything > else is considered 'midtones' and not affected); 0 would affect all > tones (anything darker than some mid-point would be considered dark, > anything above it a highlight); 50 means a sizeable chunk of midtones > are left alone. > > This seems straight-forward enough. Unfortunately, I sometimes see > behaviour opposite to the things described above. > > Here's an underexposed image of my son, with 'original' selected in > the history stack (no base curve, sharpening or any other processing): > http://photos.kovacs-telekes.org/Other/Darktable-issues/Shadowshighlights20140105/i-56chChV/0/O/00-base.jpg > > Here it is with with an exposure correction of 1.5EV applied to it: > http://photos.kovacs-telekes.org/Other/Darktable-issues/Shadowshighlights20140105/i-PLR8gMd/0/O/01-exposure.jpg > > And here it is with Shadows/highlights at default settings: > http://photos.kovacs-telekes.org/Other/Darktable-issues/Shadowshighlights20140105/i-CpM2Qjd/0/O/02-sh_defaults.jpg > > Let's leave highlights alone, I only want to lift the shadows around > his eyes. -> highlights = 0 > http://photos.kovacs-telekes.org/Other/Darktable-issues/Shadowshighlights20140105/i-MxLRB8c/0/O/03-sh_shadows_only.jpg > > Let's set radius to 40, that will make the effect stronger on the eyes. > http://photos.kovacs-telekes.org/Other/Darktable-issues/Shadowshighlights20140105/i-638S6BB/0/O/04-eye_closeup_radius40.jpg > > Now his eyes appear brighter, but I'd like to lift the shadows under > the eyes. I'll try to reduce compression, so the not-so-dark parts are > also affected. Let's say compression = 20% > http://photos.kovacs-telekes.org/Other/Darktable-issues/Shadowshighlights20140105/i-MTZ7RR7/0/O/05-eye_closeup_radius40_compress20.jpg > > Some parts actually got darker than without shadows/highlights! Here's > the same crop with S/H turned off: > http://photos.kovacs-telekes.org/Other/Darktable-issues/Shadowshighlights20140105/i-Qgn2vJc/0/O/06-eye_closeup_without_sh.jpg > > Now let's set shadows to 100. This makes the eyes ridiculous, but > shows even more that some shadows are darkened: > http://photos.kovacs-telekes.org/Other/Darktable-issues/Shadowshighlights20140105/i-xFp9pDm/0/O/07-eye_closeup_radius40_compress20_shadows100.jpg > > Let's set shadows to -100, which is supposed to darken them. Some of > them are darkened, but some are lifted up: > http://photos.kovacs-telekes.org/Other/Darktable-issues/Shadowshighlights20140105/i-Nm3bJHh/0/O/08-eye_closeup_radius40_compress20_shadows-100.jpg > > And here we have 2 S/H modules, both set to 'lighten' blend mode, one > with shadows=50, the other with -100: > http://photos.kovacs-telekes.org/Other/Darktable-issues/Shadowshighlights20140105/i-j34DMtb/1/O/09-2_instances_lighten.jpg > > The opposite can be done with hightlights as well: sometimes positive > highlight values will darken highlights instead of burning them out. > > Is this a bug? A feature? If the latter, can the exact behaviour be > documented? > > Thanks, > Kofa > > ------------------------------------------------------------------------------ > Dive into the World of Parallel Programming! The Go Parallel Website, > sponsored by Intel and developed in partnership with Slashdot Media, is your > hub for all things parallel software development, from weekly thought > leadership blogs to news, videos, case studies, tutorials and more. Take a > look and join the conversation now. http://goparallel.sourceforge.net > _______________________________________________ > Darktable-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/darktable-users > ------------------------------------------------------------------------------ Dive into the World of Parallel Programming! The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net _______________________________________________ Darktable-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/darktable-users
