Cross-processing, also known as “Xpro”, is the procedure of deliberately processing film in a chemical solution intended for a different type of film. As the chemical mixture is optimized for a special kind of film, you will get unpredictable results when combined differently. The process is seen most often in fashion advertising and band photography, and in more recent years has become associated with the Lomography movement. High contrast and saturated colors is hat characterizes this technique.
There exists two kinds of film: Color negative film, also known as “slide film”, and color reversal film”, also known as “slides”. Different chemicals are used for both these types of film. Color negative film uses C-41 chemicals for processing. Slide film uses E-6 chemicals for processing.
When you cross-process, you use the chemicals for color negative film with color reversal films, and vice-versa. If you process negative film with the slide chemicals (E-6), you will get slides. Although the colors will not be as wacky; you will just get a kind of slide film with no big color shifts. If you process slide film in the negative chemicals (C-41), you will get negative film, but with the Lomographic burst of colors! This is because the color layers of the film were not optimized for this. As a result, the photos turn out saturated or grainy or with high contrast and you get all kinds of unexpected results. Different films have different characteristics when cross-processed. Some turn out more yellow or more green while others turn purple or red.
Remember that everything depends on how you process your film. The make and type of the film, the amount of light exposed on the film, chemicals and the developing process are responsible for what you get towards the end. When you have your film processed, you get different results from different labs because they do not always use the same chemicals and calibrations.
Cross processing effects can be simulated in digital photography by a number of techniques involving the manipulation of contrast/brightness, hue/saturation and curves in image editors such as Adobe Photoshop or GIMP; specific settings and/or commands for "cross processing" are found in several such programs, including Photoshop and Picnik. However, these digital tools lack the unpredictable nature of regular cross processed images.