|Cross-processing is a analog processing technique where slide film (normally developed thanks to an E6 solution) is processed in chemicals used for processing print film (C41). The resulting images get skewed colors usually a cyan hue and increased contrast and saturation.||
The standard way for doing digital cross-processing is to use a channel curve tool but darktable lacks this tool for the moment and another way to accommodate the effect is used.
This procedure uses tone curve, channel mixer and splittoning modules.
Prepare the image for the cross process steps by adjusting the base settings such as exposure, whitebalance etc. for a correctly looking image.
Select the medium contrast curve preset for tone curve module (Section 188.8.131.52, “Tone curve”) to boost the overall contrast in the image. You might later return here to tune the curve for better result.
This step changes the color cast as the base for the effect using the channel mixer module (Section 184.108.40.206, “Channel mixer”). You might later again return to this and finetune the colorcast of the final result.
Enable the channel mixer module
Select the blue channel and set the blue color value to 0.8
Select the red channel and change the blue color value to 0.1
Select the green channel and change the blue color value to 0.1
We use splittoning (Section 220.127.116.11, “Splittoning”) to add some more coloring to the result for cyan/blue shadows and yellow highlight.
Enable the splittoning module
Select a cyan/blue tone for shadows and set saturation around 50%
Select a yellow/orange tone for highlights and set saturation around 70%
Set compression to 10%
Use the balance slider to tune the splittoning effect. This differs on every image due to it's exposure, motive etc.