I’ve finished reading the Color Correction Handbook, and now I’m on to reading the Color Correction Look Book.
I thought it would be good practice with Darktable to try reproducing some of the looks from the Look Book.
The Look Book seems to make lots of use of RGB curves. From what I can tell from the videos I’ve watched, Color Balance RGB seems to be more popular than RGB curves. AFAICT, they basically do the same thing so I’ve been trying to reproduce the examples from the book just via the Color Balance RGB module.
Converting RGB curves to colors in the 4 ways tab seems pretty straightforward (you just add the RGB curves together to calculate the right color for shadows / highlights / mid tones), but then I ran in to this example from Figure 5.4:
I don’t know how to convert this one to Color Balance RGB because it lifts the blackpoint thingy (I don’t know the proper name) but only in the green channel. Is it possible to reproduce this with one instance of Color Balance RGB? Or, is there a different combination of modules I should be using? Should I just be using RGB Curves instead of Color Balance RGB?
I am not familar with the resources you are referring to and I am not sure if others are. Could you share a link?
I would say in my experience that the two modules you ask about are two very different modules and trying to get one to replicate the action of the other would be a totally painful experience and possibly impossible. But I would need to see the resource to make a more valid comment.
Thanks for the important information. So this book is not referring to DT and therefore is not considering how Color Balance RGB works. However, RGB curves is a very universal concept so what the book teaches about curves should be easily adapted to the DT module RGB curves. I feel you should not try and adapt the lessons or advice to Color Balance RGB as it is not similar enough to the RGB curves concept and modules. At least that is my view, but others who are more knowledgeable might have a different answer.
If you want to use a curves-based technique, why don’t you use curves?
Also, color balance rgb is a scene-referred module, can take unbounded input and produce unbounded output. RGB curves map bounded input to bounded output, in the range of 0…1 to 0…1.
Color Balance RGB has nothing to do with curves. In terms of what you’ll typically find in other software, the master tab is a more advanced take on the classic saturation slider, while the 4 ways tab corresponds to lift, gamma, gain, offset (or any of the variations on that concept).
For curves you have a few options. Tone Curve and RGB Curve are exactly the same as curves in other software, with the main difference between the two being the color space they work in (Lab and RGB, respectively), while Tone EQ provides a scene-referred, local contrast-preserving option.
When it comes to working on a single channel, both curve modules will let you do that. Just use a new instance for each channel. Tone EQ and Color Balance RGB don’t have a per-channel option, so will always use all channels as input, but you can choose the output channel by using blend modes.
Not exactly. It does actually work on and output unbounded data, it’s just that the UI doesn’t expose anything outside 0-1, so you have no control over what happens to values above 1. At least for RGB Curve, but I believe Tone Curve is the same.
I will. I’m really just playing around trying to understand how modules relate to each other as well as their advantages / limitations.
Makes sense. The 4 ways tab, specifically changing the hue/chroma for lift, gamma, gain, and offset, could be thought of as placing specific points on the RGB curves and adjusting them to the appropriate color though, right? Clearly you could have more points on RGB Curves than what 4 ways tab provides though. If I’m wrong, what is the difference? (Also just feel free to point me at the docs, I’m just trying to learn about these!)
No. Not at all. As I said, Color Balance RGB has nothing to do with curves. 4 ways is exactly the same as lift/gamma/gain/offset or slope/offset/power, with basically only the specific math being a little different. This should be covered in detail in the first of those two books. It’s also explained in the manual.
This grey ramp can help you to visualize what is happening:
The module has its own masking tab to define the regions impacted so to me its more like the parametric tone curve in RT if anything…I see these modules as quite separate tools even though you could use curves to sort of do some similar things
Okay, I was perhaps being a bit hard in saying Color Balance RGB 4 ways has nothing to do with curves, now that I think about it. You can probably do most or all of what it does with a tone curve, but not the other way around. CC RGB 4 ways sliders act as input to a mathematical function that describes how each pixel value should be shifted. The real answer then is that you don’t place points on the curve, but change the input to the function that describes the curve.
@Donatzsky Allow me a little question that I got from reading the wikipedia article you linked to. It says i.a.:
"The formula for ASC CDL color correction is:
o u t = ( i × s + o ) p
where
i is the input pixel code value (0=black, 1=white)"
How shall I understand this, if CB RGB really builds on this basis, as CB RGB is positioned in the pixelpipe prior to the tone mappers and acts with scene-referred unbounded input?
