Cross processing (sometimes abbreviated to Xpro) is the deliberate processing of photographic film in a chemical solution intended for a different type of film. The effect was discovered independently by many different photographers often by mistake in the days of C-22 and E-4. Color cross processed photographs are often characterized by unnatural colors and high contrast. The results of cross processing differ from case to case, as the results are determined by many factors such as the make and type of the film used, the amount of light exposed onto the film and the chemical used to develop the film. Similar effects can also be achieved with digital filter effects.
Cross processing usually involves one of the two following methods.
However, cross processing can take other forms, such as negative color print film or positive color reversal film in black and white developer.
Other interesting effects can be obtained by bleaching color films processed in black and white chemistry using a hydrochloric acid dichromate mixture or using potassium triiodide (KI3) solution. If these bleached films are then re-exposed to light and re-processed in their intended color chemistry, subtle, relatively low contrast, pastel effects are obtained.
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. However, these digital tools lack the unpredictable nature of regular cross processed images.