Rawtherapee: Avoid desaturation and colour tint caused by RAW-CA correction

[TooWaBoo]

The RAW-CA correction tool desaturates images. See https://github.com/Beep6581/RawTherapee/issues/4774#issuecomment-418732568
It would be great if the desaturation can be reduced. Ping @heckflosse
CA off

CA on

CA off

CA on

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[TooWaBoo]

@heckflosse
https://filebin.net/grdfzgriwg8tbsqy

[heckflosse]

@TooWaBoo first result:

left is old code, middle is without ca correction, right is new code with avoid desaturation

[TooWaBoo]

This looks awesome, Great job. 👍

[agriggio]

@heckflosse Ingo, do we really need an option for this? Who could possibly want the colour shift introduced by the "old" behaviour?

[heckflosse]

@agriggio Alberto, I'm also fine without an option for this. @Beep6581 @Floessie @TooWaBoo @Hombre57 @Desmis @Thanatomanic @iliasg ?

[Thanatomanic]

Agreed with @agriggio to avoid colour shift by default.

[heckflosse]

@Thanatomanic It is default, but currently only for new files.

[agriggio]

To me the colour shift is a bug, so it seems strage that we have a checkbox that says "do you want to apply the bug fix?" :-)

[TooWaBoo]

Found a bug in Avoid color shift. I get a green circle in this picture .
Iteration = 2 and Avoid color shift = on.

https://filebin.net/kxo1638t5mzfl1qz

[heckflosse]

Let's complete the work on it first. It still has some bugs. Until they are solved it's very nice to have the option in gui to disable avoid colour shift. For example this file, right is with avoid colour shift, ouch

[heckflosse]

@TooWaBoo Thanks for the example. That should be easy to fix

[agriggio]

@heckflosse fair enough, that answers my question :)

[TooWaBoo]

...and another one.
https://filebin.net/rdjs84o0wwjehtc3

[heckflosse]

Fix follows soon

[heckflosse]

Fixed.

[heckflosse]

I just detected a nice side effect from using large number (e.g. 5) of raw ca correction iterations:
It brings out some details in LED blue regions:

Left without raw ca correction, right with raw auto ca correction 5 iterations

[TooWaBoo]

Tested, works fine. Merge it. 😄

[heckflosse]

@TooWaBoo Let's wait a day or two :laughing:

[agriggio]

I'm still :-1: on merging with the parameter, fwiw. I'd rather wait a bit more, make sure that there are no side effects, and get rid of the parameter when it's ready. just my 2c of course

[heckflosse]

@agriggio I agree if there are no side effects. For example we have to test with high ISO files too.

[agriggio]

I'll give it a try when I have the chance

[Desmis]

I agree with Ingo, and test if there are no side effects :)

[heckflosse]

I just pushed new code which executes avoid colour shift per iteration instead of once after all iterations.
This improves the correction of the colour shift a bit when using a large number of iterations though there is still some desaturation when using large number of iterations.

Left is old code with 10 iterations, middle is without raw ca correction, right is new code with 10 iterations.

[agriggio]

@heckflosse I tested a little bit, and in general I like the "avoid colour shift" feature! :+1:
I don't know if this is a known problem, but I got some serious artifacts when pushing the number of iterations too high. This happens regardless of the "avoid colour shift" setting. Here is a raw file and a couple of renderings (neutral profile, just raw CA correction enabled with 10 iterations):
https://filebin.net/8jj8czw7ab1n9x50

[heckflosse]

@agriggio Alberto, thanks for testing. I set the max. number of iterations to 10 only for testing. We should reduce it to 5 or less imho.

[heckflosse]

I tested the new ca correction code with a lot of files. My findings:

1.) I detected no case, where avoid colour shift gave a worse output
2.) for images with moderate ca, 2 iterations almost always gives the best result
3.) for images wit really heavy ca, more than 2 iterations are needed but also can lead to artifacts
4.) for images without or with very low ca even 1 iteration (default before my changes) can lead to artifacts

[Beep6581]

I tested ~50 raw photos mostly from Pentax and Sony but also from a bunch of other cameras.

• A default of 2 iterations worked very well in most cases.
• The more iterations, the more red/blue detail would be altered which should not be altered. Not all photos have such detail.
• Using more than 3 iterations in most cases was a sign that I was trying to fight something which was not normal CA, in which case using a different tool would lead to a better result, e.g. purple fringing caused by extreme contrast is better fixed using the Defringe tool. https://i.imgur.com/Z7cDJjw.jpg
• I agree about hard-coding "avoid color shift" to be always enabled even for old PP3 files.

[heckflosse]

Ok, I will reduce the max. number of iterations to 5 then and hard code 'avoid colour shift'. In first step I will remove 'avoid colour shift' from gui only and keep the procparams/paramsedited stuff for some weeks.
That will allow easier testing of cases, where avoid colour shift could be a culprit.

[Desmis]

With my usual raws, 1 or 2 iterations are enough :)

[heckflosse]

Here's an example (very littel ca) to show the effect of the last fix:

left is 2 iterations without last fix (small greenish border caused by wrong interpolation)
middle is without ca correction (small reddish border)
right is 2 iterations with last fix (perfect)

[heckflosse]

merged with f940490