how to darken blue sky


(Kasyap Sankara) #1


I am new to rawtherapee and i would like to know how i can darken blue skies. Also, there appears to be a way to apply mask to a section of the image as evident from color tonig section of rawpedia but it does not say how to apply the mask. Can anyone shed some light on these two topics. Thanks.

(Ingo Weyrich) #2

To darken a blue sky you can use the LH curve in L*a*b Adjustments


there is also a graduated ND filter

(Kasyap Sankara) #4

Thanks for the reply. It worked. I tried similar method to darken sky in black and white mode also using luminance equalizer method.


graduated filter, not rawtherapee though

(Paul Goelz) #6

I too have been trying to darken a bright sky and enhance low contrast clouds in a raw image from a Sony a6000. The desired end result is a black and white image. I adjust the exposure and white / black points, enable black and white, then use the equalizer to selectively darken the light blue sky as shown above. Using Capture One Sony, this is fairly easy, with excellent (striking, actually) results. But in RawTherapee, while the sky does darken and the contrast between the sky and clouds increases, the resulting image is very grainy in low contrast areas of the sky. It looks like maybe the internal bit depth is insufficient? I have tried absolutely every setting and configuration and the grainy appearance remains. The same image and roughly the same approach yields a smooth image in Capture One Sony.

Note that I am enhancing very low contrast areas in the sky. But again, Capture One produces smooth results and Raw Therapee does not.



@pgoelz Although I haven’t used it, RT has an experimental version called Local Lab build, which allows you to select and modify local regions. It gets updated every so often.


You provide some textual comparison between CO and RT.
If you want either some help to use RT or help dev to improve RT it is not sufficient.
It would be helpful if you could provide: what version of RT you use, a raw file you don’t mind to share, the jpg from CO and RT and the PP3.

(Paul Goelz) #9

Sorry… I wasn’t clear as I could have been… my actual question is what is the internal processing bit depth in RawTherapee? What I am seeing looks like somewhere internally, my RAW file is downgraded to 8 bits/pixel prior to the B/W and equalize function.

Here are the files in question:

Original RAW file (Sony a6000)

Output from CaptureOne Sony

Output from RawTherapee

RawTherapee original image pp3 file


I highly doubt that is the case. RT makes every effort to process everything in the highest fidelity and has really sophisticated and optimized code to do that. First thing I would suggest is to go to 100% zoom and see if you have any issues with that. I will let those who are more familiar with the internals follow up with you. (Could be a bug… though I haven’t looked into your specific issue.)

(Ingo Weyrich) #11

32 bit float

(Paul Goelz) #12

OK, well 32 bit float would seem to be sufficient if that is per channel. So it must be a setting… but where? I have spent literally hours on this image, trying to duplicate what I achieved relatively easily in CaptureOne Sony.

I apply a basic set of generic corrections, tweek the exposure and white / black points and contrast, then enable B/W and use the equalizer to pull out the blue (sky) and boost the yellow and red (prairie grass) to enhance the clouds and the prairie grass. With some careful tweeking this works fine in CaptureOne. It has the same effect in RawTherapee, but the contrast enhanced areas of sky and clouds are missing a lot of subtle detail. It looks a LOT like my old Dell monitor that claimed 8 bits/pixel but in fact could only display 6 bits/pixel. Blacks especially looked muddy and lacked subtle detail.

I really want to like RawTherapee… in my testing with images that do not require anywhere near as much contrast enhancement, RawTherapee actually produced subtly more detailed images. Not hugely, but noticeable. So my issues with this image are puzzling. Yes, I could continue to use CaptureOne Sony but I dislike the way you are locked into the CaptureOne database and can break things if you move an image outside of CaptureOne. I much prefer the way RawTherapee stores all settings and adjustments in a file per image. Move both and you can re-organize just using Windows.


(Mica) #13

Getting used to a new tool takes time and effort, especially a tool with so many powerful features as RT.

A great way to learn is to post a raw to our play raw, state your problem, then study the sidecar files you get back.


@pgoelz The first order of business to get to know RT is to become involved in the PlayRaws. You could examine others’ pp3s and also try your hand at developing the images shared. It is great fun!

There are a lot of PlayRaws with sky. :wink:

(Ingo Weyrich) #15

It is per channel

(Paul Goelz) #16

Thanks, I guess I can rule that out.


(Paul Goelz) #17

OK, I’m zeroing in on the issue. With a lot of experimentation, I have managed to get close to the results I get from Capture One. However, there seems to be some sort of odd and bafflingly intermittent issue with the Noise Reduction function where it at times stops responding to luminance slider input. There have been many times when the luminanace slider did absolutely nothing at all. Neither did enabling or disabling the noise reduction function (with the slider at either end of its range). The end result was that while I THOUGHT I was applying noise reduction, the onscreen image was still noisy as if it was not de-noised at minimum zoom, 1:1 zoom or max zoom. Since I am attempting to enhance the subtle contrast details between sky and light cloud, the added noise from the stuck noise processing made the enhanced image very noisy / blotchy.

When the noise reduction function is working correctly, the cloud detail is similar to what I get in Capture One.

Note that I have also found that the relatively low JPG quality default in the queue simply compounds the issue. Yes, I know I can change it, but it took a while to separate out the noise reduction issue from the similar effects of a low quality JPG render.

This is RawTherapee v5.5 running under Windows 10 with 8GB RAM and an i7 CPU. Display adapter is Nvidia GT 710 and the display is Dell S2716DG.


(Stefan Chirila) #18

This is strange indeed. As I was reading your post I was going to suggest to make sure to either review the image at 100%, or to at least have a small preview square set to 100%, since any other zoom usually doesn’t display the more hard-to-compute things. But since you mentioned you already were at 100% this is irrelevant.

Honestly at this point, as many people seem interested in the issue and are showing themselves willing to help (myself included), I think it would be a good idea to upload the raw file, the pp3 file your rawtherapee edit produces, a screenshot of what it looks like (the problem) in RawTherapee, and a screenshot of what you want it to look like (the Capture One version).

I’m actually quite interested in this myself because of an issue I encountered. I like to do relatively light edits, and I find that when I brighten a lot, contrast ratios drop and finer details are less obvious (similar to your cloud situation with light coloured objects near each other).

Okay I see you uploaded the Raw file further up …I’mma download that and see what I can do with it :slight_smile:

(Paul Goelz) #19

More on this. When the noise reduction appears to get stuck, the only way I have found to get it un-stuck is to apply a default profile to the image. Not sure if that clears a bug or simply un-does a setting that is in conflict with something.

I can’t swear to it but the noise reduction seems to get stuck after I move the “detail” slider to the right. That produces the expected reaction, but then when I move it back to the left nothing happens. This is a bit intermittent and I am not sure of what I am seeing yet.



@pgoelz It might be clearer if you showed us what you meant. It is hard to understand when it is just words. Take screenshots, crop and drag and drop. :wink: