Hi Dan:
Thank you for your message. I have been noticing color artifacts when
raising the lower point of the tone curve when editing X-Trans RAW images
but not when editing standard Bayer RAW images (e.g., Nikon D800 .NEF). Is
there anything specific to X-Trans RAW images here?
Thanks again. Best, Francisco
On Mon, Jan 5, 2015 at 12:56 AM, Dan Torop <d...@pnym.net> wrote:
> 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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--
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)
------------------------------------------------------------------------------
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
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