@patdavid
First excuse my bad english…
Some new things about Rawpedia and the HDR-SDR and Cam functions in Rawtherapee :
In association with @Wayne_Sutton , we have agreed, to make it clearer, and more educational to :
- remove from “Getting Started” almost everything related to HDR or SDR functions using specific concepts and tools such as “Log encoding”, “Cam16”, “JzCzHz”, Sigmoid"
- these tools are now grouped in paragraph 3 :HDR to SDR : A first Approach (Log Encoding – Cam16 – JzCzHz – Sigmoid)
- Remove the most conceptual parts from this last part and incorporate them into the general module: CIE Color Appearance Model 2002/16 - Cat02/Cat16 - Log Encoding – Jzazbz, especially the whole part concerning Jzazbz :Jzazbz – a new experimental CAM ? (Cam16 & JzCzHz)
- The English version is a bit different from the French one (paragraphs 4, 5 and 6), but the translation should be done this fall (it is a lot of work)
- Rawpedia links :
New link
https://rawpedia.rawtherapee.com/CIECAM02#Local_Adjustments_-Color_Appearance.28Cam16_.26_JzCzHz_Experimental.29
https://rawpedia.rawtherapee.com/Local_Adjustments#HDR_to_SDR:A_First_Approach.28Log_Encoding_-CAM16-JzCzHz-_Sigmoid.29
The whole is clearer (at least from our point of view) and allows a better understanding and an easier reading.
Some explanation on these HDR-SDR type functions, on their implementation in Rawtherapee. Of course, you may or may not take it into account to incorporate it in the loads compared to 5.8 :
« Color Appearance & Lighting (Ciecam02/16) » (main)
- has been enriched with Cam16, much more powerful than Cam02
- possibility to choose a level of complexity " Standard / Advanced »
- some modifications requested by users to make this module more user-friendly
In summary this module which seems complex is located at the end of the process, and allows in particular in " Symmetric " mode to bring a " chromatic adaptation " of high quality when the conditions of shooting and the real illuminant are different.
"HDR to SDR : A first Approach (Log Encoding – Cam16 – JzCzHz - Sigmoid)" (Local Adjustments)
Introduction :
When we look at Darktable, we can see that the software has been entirely rewritten based on 3 principles and tools: Remove the Lab* Mode, Introduce a ‘Filmic’ module (which is a logarithmic encoding of the data), review all the colorimetry in RGB mode, and in a current branch develop a ‘Sigmoid’ module.
I don’t make any judgement, but I can see that these changes have caused a lot of debate and communication. What about Rawtherapee?
What about Rawtherapee?:
Use of the Lab mode: (especially in Local adjustments) :
- Certainly it is true that Lab* passes only 7Ev, but only at the level of the output profile (monitor) whose PCS is in Lab mode and in 8bits. From the various tests that I could make with images with high dynamics at my disposal, when one uses Lab in “float”, the conversion RGB=>Lab, and Lab=>RGB is done without any loss at least on images with a Dynamic Range of 15Ev. Certainly in the long run, if we assume that output profiles can use HDR functions, it may be necessary to use HDR-Lab… But note that today most exchanges are done either on paper (which has a very low Dynamic Range) or on the Web. As for buying a real HDR monitor that passes 4000cd/m2, the prices are around 30000 €.
- It is true that Lab* does not maintain color consistency, especially in red-orange and blue-purple. But in Rawtherapee if in “Local adjustments : Settings” you check the 2 boxes “Avoid Color Shift” and “Munsell correction only”, a series of 200 Lut will correct with a quasi perfection these drifts and will not correct the gamut.
- see in Rawpedia :
https://rawpedia.rawtherapee.com/Toolchain_Pipeline#Colorimetry
The “Log Encoding” module :
- I used the excellent work of Alberto Grigio @agriggio , in ART (with some adaptations), to work in RGB mode. Then to solve the problems of lights and colors (and according to the level of complexity chosen: (Basic, Standard, Advanced), you have at your disposal the tools of Cam16, which allow you to work on the components : J = Lightness and contrast , Q = Brightness and contrast using “Absolute luminance”, and the 3 color components (s : saturation, C : chroma, M : Colorfullness)
The « Cam16 » module :
- it is a very simplified version of the Ciecam & Lighting module present in " main ", therefore much more intuitive, it benefits from the advantages of " Local adjustments " (deltaE, working in " full image ", etc.)
- it has a " Sigmoid Q and Log Encoding Q " module: which of course can use the " Black Ev and White Ev " settings. In both cases, RGB is not used, but the Brighness (Q) which uses “Absolute Luminance” is used as a reference. Of course, depending on the level of complexity (Basic, Standard, Advanced) you have different settings.
- I have equipped it experimentally (by modifying the conversion matrixes) with a slider “HDR PQ (Peak luminance)” which allows a first approach HDR, by allowing to set this function between 100cd/m2 and 10000cd/m2.
The « JzCzHz » module :
- is only accessible in “Advanced” mode
- I encountered many difficulties during the first use (with the original matrix, and PQ set to 10000 cd/m2): lack of saturation, artifacts, etc.
- I tried to work around it with various workarounds: for example the “Remapping” function or the slider to adjust the PQ luminance peak (which specialists or researchers can dispute), but now it works more than correctly.
- JzCzHz (which is a simple transform of Jzazbz), is not a CAM (Zcam we tried does not work) : here again I made some workarounds by giving it some CAM functions for “Scene conditions”: Mean Luminance (Yb%), Absolute Luminance, Surround.
- It has (like Cam16) 2 functions " Log encoding Jz " and “Sigmoid Jz” , there again we do not use the RGB mode, but Jz which takes into account " Absolute luminance "
- the code is not optimized (it is in “double” precision), hence a certain slowness.
These 2 modules (Cam16 and JzCzHz) were developed at the end of 2021 thanks to the precious help of @Wayne_Sutton and @Jade_NL . Thanks to them.
Of course, you can help yourself with the documentation in Rawpedia.
Your opinion is important : what do you think ?
Jacques