Two ways of using Filmic RGB

Yes, “gain” is good. Perhaps “brightness” or “lightness” for less technical users.

“Exposure compensation” or (worse) “exposure correction” can mislead users into thinking the camera exposure was wrong and needs correcting in post. In fact, the camera exposure might be perfect ETTR, causing neither clipping nor noise, and the only problem is that neutral processing results in an image that is subjectively too dark or too light.

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Brightness and lightness are super confusing too, since multiple models use those therms.

True. Maybe “brighten” or “lighten”?

Those are the terms that I use for my own commands. In any case, as long as the tool is clear on what it does and honours the tradition it has been adapted from, it should be fine.

Brightness and lightness are already taken and have a different meaning.

Besides the actual process of keeping a shutter open, exposure means the relative luminance of a light emission in log2 encoding. It has in post-processing just as much meaning as on camera. To add 1 EV on camera, you need to double the exposure time, on post-processing you need to double the linear RGB code values.

But pushing a slider on a computer screen doesn’t change the relative luminance of anything.

(Unless of course the computer is hooked up to physical lights or a camera, and changes the settings on those lights or camera. For example, darktable can control a tethered camera, and can control actual exposures made by that camera.)

So why bother pushing it then ?

It’s as if you said “paying goods with a credit card is like not spending money at all”. No actual bills left your pocket but your bank account sure depleted.

I think you are mistaking a symbolic effect connected to a real-life phenomenon, with no effect at all. When I push the exposure slider in software, my screen becomes brighter. So it does change the luminance.

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Ha, yes, good catch. Pushing the slider brightens the screen, increasing the relative luminance of that part of the screen, thus exposing our eyeballs to more light. So, yes, it increases an exposure.

Hmm.

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I think you got mistaken.

In camera, you expose to avoid highlights clipping, which usually leads to under-exposing, compared to what your lightmeter/auto-exposure wants you to do.

In software, the easiest way to recover midtones is push the whole picture in exposure module (thus, making highlights look temporarily clipped in the display output, because they are still there in the pipeline), then use filmic to recover the highlights with the shoulder part of its curve.

But, the other way to use filmic is to push the highlights just on the verge of clipping in the exposure module, so at filmic time, you know your theoritical standard grey is at 18% and the white is at +2.45 EV. Then, you fix the midtones by decreasing the reference grey value.

While both methods are 100% mathematically equivalent (adding +1 EV in exposure module, or dividing the grey reference by 2 is the same), the GUI might get easier to control with the first method.

But, keep in mind that the first method will push the top end of the RGB data possibly far above 100%, and since most of the UI controls in darktable stop at 100% (for parametric masking and such), some of your bandwidth will not be controllable anymore (for example in levels and tone curve). darktable doesn’t clip at 100%, so the data will still be there, but you won’t have dials to bend them.

That’s why tone equalizer and colour balance let you control parameters pretty much between 0 and +infinity, that’s why I don’t like levels and tone curves anymore.

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Thanks for the (re-)explanation @anon41087856. I think I’ve been mixing up your two methods - pushing exposure to recover the mid-tones and then starting filmic with 18% grey. I understand now :grinning:.

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@anon41087856 Please correct me if I’m wrong. I’m learning new the new pipeline and doing linear modules exercises. What I understand from your post is that:

  1. If there are no RAW clipping I can push data in exposure module so that DT warns about clipping and histogram depicts clipping.
  2. I can follow with the pipeline (lens correction, contrast equalizer, color balance) and then recover the clipped data in the filmic RGB module.

Is my understanding correct?

yes. Always remember that what you see in the global histogram and on screen, is the end product of a full pipeline.

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What about Lightness (An 18% Intensity would be 50% Lightness)? Why should photographers deal with non-linear brightness?

Unfortunately, there are several definitions of lightness. :stuck_out_tongue:  Terms are fine as long as they can be understood within the scope of the tool.

Sorry, I’m a little late on this, but what about “gray-scale shift” - isn’t that what this function really is about?