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Building RAW Negative Inversion Tools

Ah, ok, that makes sense, thanks for the explanation. I guess I'll have to do something more sophisticated than just inverting the wb coefficients, because the output channels are getting cut off on the high end. Thanks for explaining that!

I want to try to invert the RAW data, rather than the processed data, for a few reasons. The first is that I eventually want to make a tool that outputs a RAW file (using the DNG SDK). While it is possible to invert a negative in most RAW editors by inverting the tone curve, it makes the resulting image difficult to work with, and being able to work with the inverted image in RAW would make things a lot easier.

The second reason is a bit more abstract, and is somewhat of an experiment. The way that the different color emulsion layers work in a negative—specifically the orange mask that negative the color leakage in the magenta and cyan channels—means that for any operation on those channel to work correct, you need to operate in the negative's native color space. The matrix multiplication that converts the RAW data into a color space alters the curves in the individual color channels in a way that makes inverting and cancelling the effect of the orange mask difficult. I'm guessing that the demosaic process might screw with the color channels from the negative as well.

Finally, the ultimate goal of all of this is building a tool that will take 3 RAW exposures—one of each color channel—created by taking a photo of the negative with red, green and blue light, and compositing them together. The tool will then invert the image, white balance and output to RAW.

A lot of this process is derived from the way film scanners work. Most high end film scanners user RGB light sources and black and white sensors to read the three color channels and account for the orange mask by adjusting the analog gain per channel.