Darktable, filmic and saturation

Software darktable
42

Dear all,

today I have been “dissecting” a bit the filmic code in DT, to better understand how it works, and I might have spotted a “conceptual” bug in the case when the “preserve chrominance” option is activated.

As far as I have seen, this option usually produces images with higher saturation. Such saturation is however strongly dependent on the “target gamma” of the display device. Below is one example of what I am talking about:

“preserve chrominance” activated, target gamma = 2.2:

15
152560×1550 2.69 MB

“preserve chrominance” activated, target gamma = 1.5:

49
492560×1550 2.44 MB

“preserve chrominance” de-activated:

54
542560×1550 2.26 MB

The problem I found lies in the sequence of operations that are applied when chrominance preservation is activated. Shortly speaking, the code follows those steps:

  1. computes the maximum of the RGB channels (max=MAX(RGB))
  2. computes the ratio between max and the RGB channels:
    ratios[c] = RGB[c] / max
  3. applies the filmic tone mapping to max:
    max' = TM(max)
  4. computes the new RGB channel values as
    RGB'[c] = ratios[c] * max'
  5. applies the inverse of the display power function, to get back to linear RGB:
    RGB''[c] = powf(RGB'[c], power)

In other words, the “luminance blend” at step 4 is applied using a max' value that is non-linearly encoded, and the individual RGB values are linearised afterwards. This means that the code applies a non-linear power transform to the RGB values, which in turn boosts the saturation but also introduces hue shifts due to its non-linear nature.

In my opinion, the correct way to proceed would be the following:

  1. computes the maximum of the RGB channels (max=MAX(RGB))
  2. computes the ratio between max and the RGB channels:
    ratios[c] = RGB[c] / max
  3. applies the filmic tone mapping to max:
    max' = TM(max)
  4. applies the inverse of the display power function to max':
    max'' = powf(max', power)
  5. computes the new RGB channel values as
    RGB'[c] = ratios[c] * max''

This way, the “luminance blend” at step 5 gets computed using linear quantities, as it should. This results in an output that has a saturation much closer to the case where “preserve chrominance” is de-activated, and also which is totally insensitive
to the “target gamma” setting.

“preserve chrominance” activated, target gamma = 2.2, proposed method:

06
062560×1552 2.23 MB

@anon41087856 what do you think?

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