Before the digital photography age, film was king. Of course that is obvious, but now that digital cameras have been around for many years, some people have never exposed film. Before digital became useful and prevalent professional photographers used film. Color negative and transparency (also known as slides in 35mm format) and of course black and white. Today I’m going to leave out black and white and talk about color film cross processing.
In this photo of the glass bottles you can see that the highlights on the clear glass bottles have a cyan or blue green coloring. The coloring on the background and foreground reflections have a magenta and lavender coloring. The color on the background was actually red and the bottles were colorless. The developing of the film for the cross process effect changes actual into something different and really unpredictable.
Essentially cross processing is the act of taking one type of film, such as negative film and developing it in chemicals for developing transparencies or vice versa. Negative film has specific chemicals (C41 process) to process the negative correctly. The same holds true for transparencies or slides (E6 process). When taking one film and processing in the chemicals of another, the results can be very interesting. The results are often outstanding and sometimes not so desirable.
So what you say, nobody uses film any more. Cross effects can be achieved with digital images also.
Here is a nice photo of a daisy straight from the digital camera with a vignette around the edges. The vignette is the darkening around the edges for framing the subject and draw the viewer’s eye to the subject and away from the background.
And here is a cross processed effect using software to give the look of cross processing from the old days of film. You can see the cyan or blue green coloring in the petals of the daisy and the obvious coloring shift of the green to a reddish orange. When using software for the effect, you can see the effects live, so the mystery or surprise results are gone. But now you can create and alter the effects to your liking.
Other characteristics of cross processing can be increase or decrease of contrast, increase in grain, and color shifts. When using film, the different manufacturers had different chemical make up of their films, so each manufacturers film (i.e. Kodak, Fuji, Agfa) had different results when comparing side by side tests.