The long question about tone curves in RawTherapee

I found this interesting and also a bit conflicting while reviewing the RT documentation…
There are two tags used by Adobe that might not be used by RT and this could impact what you are seeing when you say the dcamprof ones just seem better… The conflict in reading this section is I can’t be sure if RT does or does not use the exposure bump if there is one in the DCP profile… At first to me it seems to say it doesn’t but later it says it does and I believe also says it ignores the black setting… not sure if any of this matters for you but it might for anyone trying to use the Adobe DCP profiles…they might want to confirm these points mentioned below…

" DCP Baseline Exposure

The DCP may indicate an exposure offset that corresponds to an offset of the exposure slider. The purpose of this is typically to make the brightness of the image match the brightness of the camera’s own JPEGs, which can be useful if you’re shooting with auto-exposure. Currently this offset is applied “under the surface” so you don’t see it on the exposure slider.

Note that if you are using Adobe’s proprietary profiles those are expecting that the DNG’s “baseline exposure” tag is applied too (the profile’s offset is added on top). Currently there is no support for the DNG tag so you need to find that out on your own (using exiftool for example) and then set that offset using the exposure slider if you want to get the exact same brightness as in Adobe Camera Raw.

Implementation Notes

Third-Party DCP Support

The DNG Camera Profile, DCP, is the preferred camera input profile format for RawTherapee. All elements of the 1.4 DNG specification are supported, with the exception of the black render tag (see below). A DCP can be a pure matrix profile, it can have a LUT (typically 2.5D) to improve the colorimetric accuracy, and then it can have an embedded curve and a separate “look table” on top. It may also add an exposure offset. All these elements can be toggled on/off via checkboxes. However, you should be aware that a profile will produce the most accurate colors when all the elements it was designed to include are enabled. For example, using a tone curve changes the color appearance, so if you disable an embedded tone curve to get a linear profile you can’t count on the colors being absolutely accurate. Nonetheless, most photographers rely on an aesthetically-pleasing appearance, and not on absolute accuracy, so this should not be a concern unless absolute accuracy is crucial. Typically, third-party profiles would come from Adobe Camera Raw/Lightroom/DNG Converter, and RawTherapee supports them. Many of Adobe’s profiles lack a tone curve, but in Adobe’s world that does not mean that no tone curve should be applied, but that Adobe’s default curve should be applied. RawTherapee will therefore identify Adobe profiles (from the copyright string) and add the default curve to those (which you can toggle with the tone curve checkbox).

Adobe DNG Converter may add a “baseline exposure” to the DNG file. Some of Adobe’s DCPs are designed to work with that baseline exposure and then produce a default output which is about the same brightness and contrast as the out-of-camera JPEG. RawTherapee can honor this baseline exposure if the DCP contains one.

The DCP format also has a black render tag. This indicates if the raw converter should do “automatic” black subtraction or not. RawTherapee ignores this tag - you can perform manual black subtraction with the Raw Black Points tool or with the black slider in the Exposure tool. As many of Adobe’s profiles indicate auto black subtraction and Adobe Camera Raw/Lightroom does it, in those cases RawTherapee will render slightly lower contrast and brighter shadows."

@Thanatomanic is the resident expert on RT parametric curves. I believe they are piecewise catmull-rom splines at x^5 smoothness. Additionally besides the base curve there are four segments for the range, but Roel will be able to describe them accurately.

Thank you! It was very interesting video! I’ve checked Adobe resources for camera raw DNG and they only have DNG Profile Editor for macOS and Windows, they don’t have options for me as the only OS that I have is Linux.

As I told, in general I am petty happy about the DCP that DCamProf produce, couple of years ago, I compared the profiles that produce DCamProf, Color Checker software and the Pentax profiles from Adobe and DCamProf provided best results, so I choose it.

With DCP from DCamProf I don’t have any problems with skin-tones. That problem was on the first steps, when I tried to use Pentax profiles.

Currently the only thing, I am not happy is Highlights rendering. As I understand it’s rather not about profile generation, but more about the S curve. Because when we apply S curve it affects colors, and I am not happy about how it affect them.

To show what I am talking about let me show several examples:
Here 4 images of street:

  1. With curve in DCP profile from DCamProf

  2. Without curve in DCP profile, but with same curve in RawTherapee (film-like curve)

  3. Without curve in DCP profile, but with same curve in RawTherapee (perceptual)

  4. Without curve in DCP profile, but with curve splitted to two curves in RawTherapee

It is still not result of instrument I want to have, but it illustrate the effect (I’d like to remove yellow from lights, because for me it looks more natural, you can compare the first and last images)

Thank you, Richard!

NP its a single exe file in windows…no install so I think it might work very easily in a VM. I just suggested it as it has a nice visual way to tweak color profiles from images which I find can be useful and or with a color checker. So I think it would be easy to run on your Linux box but you seem happy and all set so good…maybe some others reading might have an interest…

Its funny the middle two images look better not washed out and blown out a bit… I think a simple hue rotation would likely handle the issue without a bunch of curve manipulation… but that is just how I see the images you provided… Using DT a recent post had issues with a Canon camera and red… a simple 5 degree hue rotation in general… was all that was needed to correct the red nicely and when done globally didn’t really mess with the other colors… but everyone has to find their own way… I am not sure as I don’t use RT nearly as much as DT how you would do that, ie apply a global hue rotation but for sure also in that Adobe example you would simply click the foliage adjust to taste and save the DCP …

Thank you Todd!

I think, you right! I’ll try to use it in VirtualBox.

Regarding you comment about the colors:

Its funny the middle two images look better not washed out and blown out a bit…

The picture was taken 1:54 pm so it was pretty strong noon light. This is a reason why I think that more washed out colors look properly for this situation.

All images have the same exposure compensation +1 (in RawTherapee) in real live I would adjust it, but to compare the effects I decided it should be the same.

As I didn’t have the instrument to split curve properly, I just tried to mimic it by two curves, so the contrast for the last image a little bit different.

The last images probably also look too neutral, but I think that this is good, because now I can add any effects and it shouldn’t be any problems with color balance (it’s the same for all images)

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An interesting exercise if you don’t have a colorchecker is to download a test shot from imaging resources for your camera. Then load in your desired profile. Take the HSL tool and work with the H vs H equalizer… just use the 6 primary spots and line them up with the primary nodes on the vector scope. Hover over the patches to get a marker on the vectorscope and then make some small tweaks… I did the no NR 100 iso shot I think… the biggest adjustment was on magenta if I recall and yellow is by far the most saturated patch … it clearly impact red… to me the small tweaks make the red a “more” red hue and a bit less pinkish… anyway its a good way to monitor some of you musings as you play with things… There are a range taken with different iso and with and without NR…

Thank you, Todd! It’s very interesting resource!