Any interest in a "film negative" feature in RT ?

Software RawTherapee
5

Here is an improvement on the G’MIC pipeline. I added a rudimentary form of auto WB, because you have to manually white balance the resulting image anyway.
Basically i’m reading the median for each individuall channel, and using them as coefficients to keep the median values roughly aligned. This way the resulting image will be almost balanced, and yow only have to make slight adjustments by hand.
This way, there’s no need to have the coefficients as parameters, so a film type will be characterized by just 2 exponents.

film_neg:
  # Calculate formula with Kodak ColorPlus 200 parameters
  -fill [(R^-1.57969792927765),(G^-1.15851358827476),B^-1]
  # Split the image in separate channels and read channel medians
  --split c rk={ic#1} gk={ic#2} bk={ic#3}
  # Discard single channel images
  -rm[1,2,3]
  # Divide each channel by its median, take them all roughly in the same ballpark
  -fill[0] [(R/$rk),(G/$gk),(B/$bk)]
  # Clip away outliers; threshold arbitrarily chosen as 6 times the global median
  -cut 0,{ic*6}
  # Gamma and contrast to taste
  -apply_gamma 2.2
  -adjust_colors -10,10
  # Normalize to 16bit range in case you want to save the result
  -normalize 0,65535

This is more or less what i plan to implement inside RawTherapee. Turns out G’MIC is a perfect tool for prototyping!

I’ve tried digitizing some very old Kodak negatives from 1969; the color cast is different from more modern film. Using the exponents calculated for the ColorPlus 200, the result is quite bad, and of course i can’t get a colorchecker shot on a 1969 film because i don’t have a time machine :smiley:
BUT… i noticed you can still get decent results by using the beginning of the film as white/black patches to calculate the coefficients. You know, the part of film that sticks out of the roll when you buy it. That part has been outside and is overexposed. The part inside the roll, before the first frame, is completely blank. For example, using this (they cut the dark part very short in the lab, and put a sticker on it) :

start

…yields this:

neg2

Far from perfect of course, but not too bad for a starting point.

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