Correct — but irrespective of dt or RT,
color management in a Mac does not
behave like in non-Macs, which just might
be the culprit here…
For instance, what is your display profile set to?
@Claes I think the problem is not CM. Look at the first two screen shots.
There is clearly some sharpening (USM or other) that is not getting exported.
I was hoping for a style set in export, but alas, no luck.
Currently no further idea what is going on.
@kulkarniankur is this happening to all your images or just this one?
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This is happening in other images too. I have a bird whose detail I am trying to bring out, but it looks washed out and lacking in brightness or sharpness. I suspect something is going wrong with sharpening or brightening or local contrast applied to the masked area.
This is a difficult story involving a lot of image-processing maths.
- You simply have algorithms that rely on pixel-per-pixel data like blurring or sharpening. You must expect output data to be depending on scale. In modules like D&S we try to minimize that effect but it can’t be removed completely. That’s life (or that’s maths)
- If you scale at some point, you always must take some scaling maths into account. (Read about nyquist for example.) If you look into the feathers structure you will run into problems very fast. Also read about “moire” here.
- Some scaling algo are more prone to “overshoot” (lanc variants are one example)
- If you talk about preview, i guess you mean the thumbs in lighttable view or the top-left pre/overview? If you want to see what you will have while exporting (leaving out jpeg compression artefacts again) you have to zoom-in to 100% to avoid any scaling differences.
Just some additional points to think about leaving color maths out of discussion here.
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Just for clarity –
a) I did scale by 1, and the problem continues to remain.
b) By preview, I don’t refer to the top-left in darkroom, but the main central one.
As @grubernd said, there is some sharpening that is just not getting exported.
Just a thought, should I be looking at gamut clipping and enabling/disabling something?
If it is not a hassle to you could you provide an original file with its xmp file where you can clearly reproduce the problem? I am really curious as to what is going on here.
The xmp is in my first post here, as is the exported file: Masked portions different in preview and export
My original file is attached. Note: I am shooting in jpg.
Thanks for trying!
Thank you very much!
But I would really need the original xmp that goes with the file, because otherwise we are trying to fix the cause before the reason has happened. 
This file here:
does not contain any edits.
kofa@eagle:~$ grep operation /tmp/P1030322_edited.jpg.xmp
darktable:operation="colorin"
darktable:operation="colorout"
darktable:operation="gamma"
darktable:operation="flip"
When loaded into darktable:
Interestingly, the settings of the input color profile are:
But if I switch between 0 original and 1 input color profile, the image changes drastically:
embedded is not an option in the drop-down, though:
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Sorry, my mistake. I attached the wrong xmp. I believe this is the right one @grubernd @kofa
P1030321.JPG.xmp (36.3 KB)
Well, output is exactly as shown in darktable … if checked at 100%.
The rest is preview scaling.
Given the nature of this file this is not really a surprise - just as @hannoschwalm has said, Math is brutal. It does exactly what it is supposed to do. And with noise and JPG artefacts all over the place the outcome at different zoom levels can be quite different.
Sorry, there is nothing we can do. 
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PS: The preview downscaler of darktable is a little heavy on the sharpening, which enhances the effect seen here. I just downscaled the full size output to screen size in other software and it is not too different from what darktable shows.
As far as I know, the fit-to-screen preview in darktable is done from a smaller intermediate size, so what you see is ( downscale - apply all settings) instead of (apply all settings - downscale).
Since your edit is rather brutal (sorry for the blunt statement here), some differences are to be expected.
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Ok then I need a practical suggestion: if there is going to be a slight mismatch between the preview and the export (as seen by the naked eye), what can I do to make sure my exported file “looks” like the one I want to look like. One option is simply to sharpen the original more till the export looks ok, but that is time consuming.
You will have to evaluate sharpness at 100%.
The original photo has strong noise suppression, but still much patterned noise:
I think that brings out the worst in all the scaling algos.
Maybe if you could start with a raw file, you’d have better chances. Also, if you are editing JPG (non-raw) files, it’s better to switch the module order to JPG (at least for me, loading your sidecar resulted in using the order recommended for raw files):
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The most important “practical suggestion” as i see it would be:
If you want the best quality and need some sort of processing, you must shoot raw 
Whatever camera you are using! If you have noise plus jpeg patterns - every sharpening algo will fail. It might also help a bit to use bilinear as the scaling operators in preferences.
That’s a very interesting and very specific point depending a lot on the camera firmware as they try to keep the impression of sharpness (often introducing high frequency content close to nyquist) or create some blotches with hard edges (also high frequency). And this is very tricky to control in a good way.
I agree RAW would be a better starting point for post processing. Unfortunately, I shoot birds, often on hikes and that often give me a very brief moment to capture them. Hence I carry around a light bridge camera with big zoom, and shoot in burst mode. RAW would slow down the shooting a lot, and a better camera/lens would be difficult to lug around.
This eventually gets me to some noisy jpg pics with a bit of haze due to long distance or shady trees or early morning light.
Now that I am wiser I will try a faster sd card for raw shooting. Thanks again to everyone for your efforts 
Regards,
Ankur
This is so much nicer without much additional noise! And the match between the export and preview is much better.
Can you guide me on what your thought process was and why these modules (and why the custom order?)