I recently had occasion to shoot a bunch of night-time images of skiers wearing headlamps with 50mm f/1.4 and a 5DSR – a recipe for purple-fringing galore.
I don’t mind the fringing itself, but Darktable handles it much less-well than does the Canon SOOC preview.
Sigmoid’s new primary attenuation sliders will probably resolve this problem really well. They are brilliant at tackling certain lighting problems like the one you have shown here. If it was a playraw I would have attempted the fix to show you.
It seems that you first need to learn how to work with darktable. Simply opening a raw file, expecting it to look reasonable, is not how it works. Start with this tutorial:
It will ensure you have a good understanding of the basic workflow. This is needed to get the most out of the other advice here.
Those purple areas are very highly saturated. filmic, turned on by default if you use that workflow, will try to preserve colours in highlights, reducing brightness.
You can use sigmoid, which rolls off to white; you can tell filmic to roll off to white (set look / highlights saturation mix to 0); you can use filmic v6 or v5 (options / color science) with options / preserve chrominance set to no or luminance Y.
When filmic did not try so hard to preserve colour, people were complaining that it was producing flat highlights.
I think what’s happening is it first applies the tone curve to the brightness, and then multiplies the RGB channels with a value derived from the original colour (RGB ratios) of the pixel.
Look at what happened to a ‘purple’ (clipped) sun here, and the explanation given by filmic’s developer, Aurélien: Magenta highlights vs raw clipping indicator vs filmic white level - #60 by anon41087856.
I’m experienced developing images in Darktable. I work with a few thousand selects each year in DT – essentially everything you find here has been processed with DT.
I’m new to playraws – that looks super fun. I’ve uploaded one here.
My question wasn’t “how do I mitigate these clipped/saturated halos”?, but rather, “Why is it happening at all?” i.e., quantitatively, what’s making it happen?
Even if disable filmic and highlight reconstruction, then pull back exposure, the halos remain highly saturated in blue – I don’t think they should, even quantitatively, given that the SOOC preview handles it so cleanly and that reality doesn’t show anything of the sort, even through the viewfinder.
For those who asked me to give sigmoid a spin, here’s the screenshot (using the playraw file)
color calibration is part of white balance. You either use white balance set to camera reference, and apply color calibration with one of the the adaptation methods, or you use white balance with some other value, disabling color calibration. Since the module changes white balance, it changes colours, so some effect on the highly saturated areas is expected.
If the fringes come from overexposure, you should suppress them in highlight reconstruction, if possible.
Speaking of color calibration, it also has a gamut compression slider that affects highly saturated colours the most, and often helps with e.g. blue LEDs.
The new primaries module can also be used. Search the forum for instructions. The new version of sigmoid has a built-in version. Again, search the forum, detailed instructions have been provided.
This can be partially corrected with the chromatic aberration module by switching on “very large chromatic aberration”, increasing strength and using “darken only” in this case:
The whole thing is much easier with the Sigmoid module. You just take the “smooth” preset and increase a little blue attenuation slider to desaturate blue in highlights:
In addition, you can also use the rgb primaries module to move the blue channel to cyan and the red channel to yellow, which completely eliminates purple fringing:
Hello Boris,
Thank you for your detailed explanation and for sharing your knowledge. Your professional experience is a great help to me and certainly to the whole community.
Greetings from Brussels,
Christian
@4kbt here is my fix for the problem using the blue attenuation slider in the primaries section of the Sigmoid module. I feel this is a very simple and straight forward fix to problems like this. These sliders are also good for controlling strong colored stage lighting. The screen shot shows image as opened on the left and attenuation on the right.
