The particularities of Ciecam in Rawtherapee (here in Local Adjustments).
Process 1 - Scene conditions (Scene-reffered)
Let’s take a look at the main components displayed in the Cam16 module:
- Mean luminance (Yb%) and Absolute luminance are calculated very early in the process, to avoid pollution by other processing. This data is essential not only for Log encoding (Mean luminance) calculations, but also for taking into account image characteristics at the time of shooting.
- Absolute luminance - expressed in candelas per m²:
** is calculated on the one hand from exif data: exposure time, f-stop, exposure compensation on the camera body, and on the other, by user-defined corrections (exposure and raw white point). This is a fairly good approximation. In the absence of data, or if these are out of range, the system chooses 2000 cd/m².
** this value (and to a lesser degree Mean luminance (gray point)) determines how well Cam16 works, for example in differentiating between lightness and brightness, or chroma (C) and Colorfulness (M). The value of Absolute luminance conditions part of the correspondence between what our eye/brain pair feels and the software system; its usual value is between 1 and 10,000 cd/m².
An example taken from Rawpedia:
https://rawpedia.rawtherapee.com/CIECAM02#the_Hunt’s_effect
I’ll pass quickly over the other components that have already been described (white distribution, black distribution, Black Ev , White Ev), which are the characteristics of the Dynamic Range estimated before processing and are covered in the 2 tutorials already mentioned.
White balance
Admittedly, it doesn’t appear directly in “Color Appearance”, but from my point of view (there are other alternatives depending on the software), it must be located as far upstream as possible in the process so that the corrections made by Cam16, which can be summed up as “Trying to take into account by software, the physiological aspects due to the perception of the eye and the brain”.
If this consideration is based on erroneous data, the whole thing falls apart.
I recommend the use of “WBauto - temperature correlation” which, by working on both Temperature and Tint (green), will approximate shooting conditions.
Don’t hesitate to activate the “Show White Balance Auto temperature correlation settings” checkbox in “Preferences”.
The need to take other physiological parameters into account
The Hunt effect described above is not the only one that Ciecam tries to take into account.
For further explanation - see the tutorial.
https://rawpedia.rawtherapee.com/CIECAM02
However, I’d like to mention one that I feel is important: the Stevens effect. Take a look at these two images: the two black and white bars are exactly the same. Yet they don’t give the same impression.
This effect is notably taken into account in Process 1, by “Surround” (default - Average), try changing to “Dim” and also in Process 3, by “Surround”.
Process 2 - Cam16 Image Adjustments.
Rather than give you a long lecture, I suggest you try it out for yourself, using a well-known image on Rawtherapee (The blue horse). But of course you can choose another image
Raw file (Creative Common Attribution-share Alike 4.0): [2010_MONTR_033.NEF - Google Drive]
- Select - in Settings - Add spot - Full Image.
- Select the “Color Appearance (Cam16 & JzCzHz)” tool, of course, with Cam16.
- Select “Advanced” for complexity, to be able to use all tools.
- Set Scope=100, for full reproduction of Ciecam settings.
- Set several Lockable Color Pickers in Lab mode - as I’ve done.
Now try each of the following sliders in turn:
-
Lightness (J) = 50, then Brightness (Q) = 50. You can see that not only the luminance changes, but also the “a” and “b” values. Overall, Brightness is less aggressive.
-
Chroma (C) = 50, then Saturation (s) = 50, then Colorfulness (M) = 50. Obviously, we’re acting on the “a” and “b” components, but also on luminance “L”. Note the action of Saturation, less aggressive in the shadows.
Then, with “Local adjustments” deactivated and Lab* Adjustments" - Chromaticity = 50 activated in “Exposure - tab”, compare the almost linear response of the system, which has virtually no effect on Luminance.
Process 3 - Viewing conditions (Display-reffered)
This process must not deviate from its intended purpose. Adapting the system to the environment, reflects the media on which the final image will be viewed (monitor, TV, projector, …), as well as its environment. This process will take the data from the process 2 and “bring them” to the support so that the viewing conditions and environment are taken into account.
- Mean Luminance (Yb=18%) - corresponds to the average luminance of the output device. Typically 18, corresponding to a correctly calibrated monitor.
- Absolute luminance, is the relative luminance of the background, otherwise known as the ambient lighting level in the room you’re in (default = 16).
- Surround , by default “average” you can choose “Dim”, if for example the TV is set against a dark wall.
The rest in part 3.
Jacques