Proposal for updated Filmic curve parameterisation in Darktable

[alpinist]

@anon41087856

@alpinist

I agree that replacing the dumb splines by something that matches the problem better (one could even call it a model), is more elegant. As is often the case, elegant rhymes with simple/intuitive also here.

The dumb splines ensure the first and second order continuity of the pice-wise functions while converging to the desired values.

Agree about the importance of continuity. Did not look at @Mister_Teatime’s curves under this aspect.

@anon41087856

@alpinist

In current filmic, contrast is simply the steepness of the linear section of the spline as shown in filmic’s plot. The problem is that the axes of the plot change meaning based on other parameters.

Contrast is a mid-tones preservation strategy across the log tonemapping liked to film gamma, there is no such thing as true or false contrast. That said, I don’t really understand your scaling.

My claim is that the following works (derivation here):

• Set up filmic parameters in an arbitrary way.
• Set the contrast parameter to the following value: b / 2.44 * log(w/b + 1), where w and b are the absolute values of the white and black points as set in filmic. For example, for filmic’s default values (w=4.40, b=7.75) we have to set contrast to 1.43.
• Observe in filmic’s “Ansel Adams zones plot” that the two vertical lines around the gray point are now parallel.

We have just set what I call “midtone contrast” to one. Setting it to, say, 1.3 would require multiplying the result of the above expression by this value.

My point is that I feel that this “midtone contrast” is a more natural parameter than what filmic calls contrast currently. When “midtone contrast” is one, said lines are parallel. When it’s greater than one, the lines open up V-like, and vice versa.

In contrast ;-), currently when the contrast parameter is fixed, the two lines around middle gray in the “Ansel Adams zones plot” can be converging, or diverging, or parallel, depending on other parameters.