RawTherapee 5.4 Black and White convert results in colour cast

Processing
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I am converting an image to black and white and I get differences in the channels. Should be equal percentage of each but it’s not. :frowning: Can someone explain how to fix it?

why
why1681×1080 534 KB

Here is the PP3 file DSC02098.ARW.pp3 (10.7 KB)

I looked over my settings and the colour cast goes away if I disable the A and B changes in LAB, as well as CIE settings. I like the CIE stuff though …is there any way to keep it and yet make it all grey :stuck_out_tongue: ?

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Back in the day before the Black-and-White and Color Toning tools existed, one would make an image b&w by desaturating it in the L*a*b* tool (not the same thing as desaturating it in the Exposure tool, as there was special code that would make R=G=B when Lab chroma was 0, and I hope I’m not bullshitting you as this is from memory), and the a* and b* curves were used to tone the image. Then Black-and-White and Color Toning came along and we can now do things properly. I suppose the reason a* and b* tone your b&w image is that that part of the code was left alone when the new tools were added to provide backwards-compatibility. And I suppose that as such, it is not possible to make R=G=B when using the a* and b* curve despite enabling the B&W tool.
I think @heckflosse would be able to confirm (or smite).

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Confirmed
If a* or b* curve is used the code to force R=G=B is not called.

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@Morgan_Hardwood @heckflosse Thanks for the swift reply guys!

So I did some more experimenting last night and here’s what I found.

  1. Yes you were right about the A and B channels of LAB. If they are used then the image will not be fully BW no matter what else you do and how many of the many saturation sliders you pull down to 0.
  2. RGB saturation does act differently, but I do remember knowing about that from back in the day. Before the BW conversion tool I used to use the LAB Saturation slider.
  3. Even with the A and B channels untouched, I’d get a slightly different value, usually in the blue (1% or so more than the others) when I’d monitor a spot, and I concluded that it’s due to CIE environment which was on. I did however not want to go without it because I liked the contrast it produced.
  4. I tried to dial down all the saturation sliders I could find, and it finally worked when I turned down the Saturation and the Chroma sliders in CIE - though note, that even though the exported JPG will have equal amounts of RGB in each pixel, the RawTherapee spot measuring tool still gives slightly different measurements between the channels for any given pixel. Weird.

I wonder if a final “desaturate” checkbox could be implemented at the very end of the chain in order to just be sure and not need to measure pixels :stuck_out_tongue:

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Please refer to this thread for a possible solution

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