Have you tried this Teodor?

a) Activate an instance of exposure IOP (or gradient) with a drawn mask 
selecting the sky area. Set your exposure to -1 or whatever it needs for the 
sky.  Set the parametric mask to work on input values and with it exclude the 
darker ranges so that mountain areas present in the drawn mask get affected 
less.
b) Create a second instance of exposure IOP with the same mask and invert so a 
to select the non-sky area only. Increase exposure as required (I am assuming 
you have originally underexposed the shot so as to retain detail in the sky).  
Use the parametric mask in input value mode to reduce its effect on any 
brighter sky areas that have crept into the mask.

Working that way has so far always been satisfactory for me. I don't find exact 
masks are necessary for this sort of operation, especially when feathered.  
There is usually somewhere on the sky/land border that doesn't look quite right 
but never anything that can't be fixed by adjusting a couple of the mask 
control points.  Actually I find this approach much better than an 'exact' mask 
as it also works for things like trees on the horizon where a mask around each 
branch would be impractical.

Rgds,
Rob.

-----Original Message-----
From: Teodor Petrov [mailto:[email protected]]
Sent: 04 June 2014 19:56
To: [email protected]
Subject: Re: [Darktable-users] Automatic curve fitting for exposure masking?

Hi,

I see one major limitation with the drawn+parametric mask work-flow.
The problem is:
1. I have a sky, clouds and mountains
2. I want to underexpose the sky/clouds by 0.5-1-2ev 3. to do so I'm using 
either a gradient or a path drawn mask 4. then to prevent darkening of the 
summits of the mountains, I'm trying to use the parametric mask 5. I'm using 
the grey channel by removing the darkest colours 6. but sometimes I have as 
light colour as some of the colours in the clouds 7. when this happens I cannot 
exclude some of the summits without excluding some of the clouds

I'm thinking if it will be possible to add another mask combine mode, where the 
parametric masks are in effect only where the drawn masks are in the range 
(1e6; 1-1e6) .
This way I can use the feathering
of the drawn mask as a tool to constrain the parametric mask in 2d.

What do you think? Is this possible?
If it is, where one have to look in the code?

Best regards,
Teodor

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