Hi Francisco,
This is interesting... The trouble seems to come via the tone curve
module's automatic scale chroma setting. (The noise doesn't seem to
depend upon the demosaicing algorithm -- it's there with VNG or
Markesteijn.)
When "scale chroma" is set, each pixel processed has its saturation ("a"
and "b") adjusted proportionally to how its luminance ("L") is changed.
That is, if the luminance for that pixel is increased by 5%, its
saturation is increased by 5%. For the tone curve you drew, which pushes
up the black point, this breaks down. The very dim pixels in this curve
have their luminance increased by approx. 10x, and hence "scale chroma"
then correspondingly saturates them by about 10x. As the shirt is so
red, this makes the saturation for the shirt especially garish in its
darker areas (indeed, the "a" value becomes greater than 100).
Furthermore, the saturation changes are jumpy because slight luminance
changes in the shadows produce radically amplified saturation changes --
and this makes visible the noise inherent to the shadows. All this adds
up to the horrible artifacts which you are seeing.
There probably would be a way to rewrite the tone curve's "scale chroma"
to handle this case, but it wouldn't be backwards compatible, so
doubtful dt devs would go for it? (There already seems to be some code
there to better handle very low luminance values.)
What if instead you use "scale chroma" set to "manual"? There's no
artifacts then, and the results appear pleasing. Alternately, in the
tutorial you link to, it looks as though the tone curve change is
happening in RGB colorspace, not Lab. You could simulate this in dt
via enabling the color balance module and increasing the "factor"
for its"lift" parameter. Or even by lowering the "black" in the
exposure module?
Best, Dan
On Sun, Jan 4, 2015, at 06:22 PM, Francisco Cribari wrote:
> My second camera is a Fuji X100S, which has an X-Trans sensor. I am
> getting color artifacts in several images when I raise the lower end
> of the tone curve. The problem does NOT happen with my Nikon D800 RAW
> images, only with X-Trans RAW images. I would also like to point out
> that it is common to raise the lower end of tone curve and to lower
> its upper end. This is done to give the image a "vintage/classic"
> look. See, e.g.,
>
> https://photographylife.com/that-classic-vintage-look
>
> For some reason, that tends to cause color artifacts in X-Trans RAW
> images processed using Darktable.
>
> Here is an example:
>
> RAW (.RAF) file:
> https://dl.dropboxusercontent.com/u/2171814/_FFX7093.RAF
>
> XMP file:
> https://dl.dropboxusercontent.com/u/2171814/_FFX7093_01.RAF.xmp
>
> Zoom in the image and take a look at the man's shirt (near his
> right arm). (I have already reported another case in this mailing
> mailing list.)
>
> I run DT version 1.6.1+30~g6b508d0 on an Ubuntu notebook. Any
> suggestions? Thank you. Francisco
>
> --
> Francisco Cribari - http://about.me/cribariFrancisco
> CribaraaahhhFrancisco Cribari - http://about.me/crihhhrancisco Cribari
> - http://about.me/cribariffaaaaFrancisco Cribari -
> http://www.cribari.com.br - "All theory, my friend, is grey, but green
> is life's glad golden tree." --Goethe (Faust)
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