Dear Aurélien
Am 18.04.19 um 21:59 schrieb Aurélien Pierre:
please don't do a deconvolution in the Lab color model. Natural blur is
a phenomenon happening to photons and described by a convolution
product. If you want to revert it, you need to use a physically
meaningful color space, e.g. a linear one as close as possible to the
spectral space.
That would be camera RGB, linearly encoded (be sure the gamma is
reverted if the input file is a JPEG/TIFF/PNG file), before it is messed
up with non-linear transfer functions in the pipe (from Parseval's
theorem of energy conservation in convolutions).
The sharpening module happens too late in the pipe (after tone curves)
and works in Lab, which is nonsensical (Lab == perceptual model built
upon human vision : that means nothing in optics). I would squeeze it
right after the denoising modules, but I'm not sure yet if it's better
to have it before or after the lens correction.
my main motivation to include the deconvolution into the existing
sharpening module was to keep the number of modules small. Your
concerns regarding the Lab space, however, are absolutely valid.
Currently I am not sure if deconvolution should happen at a very early
stage of the pipeline or at the end. Just considering the physics, I
would certainly agree with your arguments to put the module at the
beginning, possibly even just after demosaic. This is the right place
when you want to reverse the effect of the point spread function of your
optical system.
I think, however, deconvolution might also be used as a---let's
say---artistic tool to counteract all the interpolation and averaging
effects that happen along the pixel pipe (demosaic, lens correction, etc.).
Since quite some time I use a custom module, which allows me to use all
sorts of GMIC filters in darktable. It works in RGB space at the end of
the pixel pipe just before gamma. I use it mainly for the RL
deconvolution. Employing a Gaussian kernel with a width of about one
pixel (or slightly less) it allows me to turn reasonably sharp images
into very sharp images and I am rather satisfied with the results.
Imho, the results are better than the ones one gets by a traditional
unsharp mask filter.
Heiko
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