I’m sure others are asking the same questions!
- No, the generalized skew log-logistic function that the sigmoid module is based on can not be expressed by the filmic spline setup. I would rate the difference between the two as about as large as between base curve and filmic.
- Not possible due to the above.
- Simpler is kind of the first argument because it is the most obvious difference when you first look at the two. The non UI advantages include better roll-off/convergence in highlights and shadows, higher-order smoothness, more robust (all user inputs are valid up to the precision of the floating-point power function), and always supports -inf to inf EV dynamic range. The last couple of posts from me have been about researching if these mathematical properties also propagate to subjectively nice-looking pictures. I let you be the judge on that.
- Yes, as good or possibly even better by being well defined for up to infinite dynamic range!
- Tricky one and I do not have the authority to answer. This module started as a way for me to learn the darktable source code and it is thus not me who decides how or even if it should be included or not. That will be a discussion and decision for the darktable maintainers to make. I had personally liked to see a wealth of options for display transforms in darktable. One way of organizing that would be to collect them all in one parent module called f.ex. “display transform” and then have options for what method to use. Would make sense to me as you can only use one display transform and it has to happen at a very specific point in the pipeline.
A closer look at the difference
I actually developed a full tool for exploring the difference between a couple of different tone curves! I encourage everyone to try out the online version by themselves over here:
https://share.streamlit.io/jandren/tone-curve-explorer
I think the log-log view is especially interesting as all other methods have this linear section while the filmic does not. This is also the easiest way to show that filmic can’t produce the same result as the sigmoid as there is not setting the returns a linear slope for filmic in the log-log view. Filmic is linear in gamma-log instead if anyone wonders!
X axis is in EV with middle grey = 0.1845 = 0 EV
Y axis is the log2 off the linear display intensity. So display = 100% = 0