This may not specifically help with RT, but it illustrates what’s going on with the black-white point reconciliation. In that respect, my rawproc software may not be ‘production quality’, but I’ve found it to be a decent platform for tutorial development.
So, starting with an innocuous color negative/positive image I found on the internet:
First, you can see the color shifts in the individual channels of the histogram, but they’re not really indicative of the image proper until we crop to it:
Now, the nature of the color cast becomes evident, particularly in the intensity offset of the red channel. The black and white limits of each channel are also evident; for this particular image, all three blacks appear to be at about 40 on a 0-255 scale (internally, rawproc is working on 0.0-1.0 float, but the histogram and tone operators work on 0-255 for convenience). With rawproc, we’re going to now add three curve tools, one for each channel. First, red:
Note what I’m doing is to scooch the black and white control points along the bottom and top bounds of the curve box until the data starts to pile up at the respective side. Now, green:
And blue:
Finally, the rgb curve to invert the negative to positive:
There’s a lot of writing on the internet about knowing the film stock to do this conversion, but the compelling thing about just normalizing the black and white points is that it doesn’t depend on such consideration. Also, once you become familiar with the dynamics of the adjustment, you can also handle white balance in the same operations; however, I’ve found that just adding a white balance tool after the conversion is more intuitive.
I used curves in the above missive to illustrate, but rawproc has a separate black/white point tool with an auto-calculate option that i would use instead. With that tool, I can do color negative conversion with my img command line program like this:
$ img neg.jpg blackwhitepoint:red blackwhitepoint:green blackwhitepoint:blue curve:255,0,0,255 pos.jpg
I’ve got a box full of 35mm color negatives from my mother, pictures of people long gone whose names I don’t know, so I’ll be doing this in volume pretty soon in order to get images in front of folks who still know the names. My plan is to shoot the negatives raw with my D7000 and macro lens, and process the images with the above command.