Looks like the vignetting module is creating "banding". I have first seen this with D800 RAW but it can easily be reproduced with standard jpeg. I have attached a jpeg with plain 40% gray. Import this image and activate the vignetting module. At the inner of the vignette the banding is quite visible in Darkroom and it is also quite visible in the exported jpeg.
I have used extensively Lightroom and never noticed such issue. Do you think this can be fixed somehow?
#1 Updated by Ulrich Pegelow about 7 years ago
I don't see banding in the image you attached. However I think I have an idea where your observation
comes from. It is most likely an unwanted side effect of monitor calibration. I see the same here sometimes
when adding vignetting to a b&w image.
To my understanding the main cause is the limited bit depth (8 bits per channel) between graphics card and
monitor and - consequently - in the calibration curve (vcgt) on your card. For most monitors the curves for
R, G, and B will clearly diverge. These differences combined with a limited bit depth cause aliasing effects
which you see as color banding. That should be especially visible in gradients of dark to mid-tone grays.
Unfortunately there is no easy way out. The only one I know would be an expensive professional
monitor with in-monitor calibration. These often do calibration with 10 or even 12 bit LUTs and
are much less prone to banding. If there is a software option I don't know for sure. Adding some jitter
might reduce banding but also reduce sharpness...
As a quick check would be to temporarily deactivate your monitor calibration and watch your test image
again. This can be done with 'dispwin -c' and restarting darktable.
I added Pascal de Bruijn to the watch list as he has most experience with color management and calibration issues.
#2 Updated by Pascal Obry about 7 years ago
Thanks for your quick answer. Sorry I wasn't clear, the banding is not on the attached image which is plain gray which
can be used to reproduce the issue.
I have this issue on 2 monitors, one which is calibrated and one which is not so I don't think this is due to calibration.
I have attached the export I get from grey-40.jpg with standard vignetting applied. Let me know if there is anything
I can do to help.
#6 Updated by Ulrich Pegelow about 7 years ago
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Well, in both programs vignetting seems to suffer from banding. Maybe to a somewhat different extent, but that
might also be due to differences in the gradients applied.
I'm quite sure that aliasing as described earlier is the cause here. Not only for color but also for tonal
banding. One idea might be to add some jitter to the gradient applied, so that hopefully banding is not as
visible any longer.
#8 Updated by Ulrich Pegelow about 7 years ago
I just added a dithering parameter to module vignetting. Can be found in banch master.
Options are "off", "8-bit output" and "16-bit output". In most cases "8-bit output" should be the right choice
to avoid banding on screen.
Even with the dithering option applied, banding can creep in again quite easily. JPEG's compression has a tendency
to produce banding artifacts. Also image viewers might show banding, especially if you view your image scaled-down.
To check the new option, you best generate an 8-bit output to a lossless format (or JPEG with quality 100%) and inspect
the image with GIMP in 100% view.