Film colorisation

Film colorization (American English; or colourisation [British English], or colourization [ Canadian English and Oxford English]) is any process that adds color to black-and-white, sepia, or other monochrome moving-picture images. It may be done as a special effect, to "modernize" black-and-white films, or to restore color films. The first examples date from the early 20th century, but colorization has become common with the advent of digital image processing.

Early techniques

Hand colorization

The first film colorization methods were hand done by individuals. For example, at least 4% of George Méliès' output, including some prints of '' A Trip to the Moon'' from 1902 and other major films such as ''The Kingdom of the Fairies'', ''The Impossible Voyage'', and ''The Barber of Seville'' were individually hand-colored by Elisabeth Thuillier's coloring lab in Paris. Thuillier, a former colorist of glass and celluloid products, directed a studio of two hundred people painting directly on film stock with brushes, in the colors she chose and specified; each worker was assigned a different color in assembly line style, with more than twenty separate colors often used for a single film. Thuillier's lab produced about sixty hand-colored copies of ''A Trip to the Moon'', but only one copy is known to still exist today. The first full-length feature film made by a hand-colored process was '' The Miracle'' of 1912.

The process was always done by hand, sometimes using a stencil cut from a second print of the film, such as the Pathécolor process. As late as the 1920s, hand coloring processes were used for individual shots in ''Greed'' (1924) and ''The Phantom of the Opera'' (1925) (both utilizing the Handschiegl color process); and rarely, an entire feature-length movie such as ''Cyrano de Bergerac'' (1925) and ''The Last Days of Pompeii'' (1926).

These colorization methods were employed until effective color film processes were developed. During the late 1960s and the early 1970s, black-and-white '' Betty Boop'', '' Mickey Mouse'', and '' Looney Tunes'' cartoons were redistributed in color. Supervised by Fred Ladd, color was added by tracing the original black-and-white frames onto new animation cels, and then adding color to the new cels in South Korea. To cut time and expense, Ladd's process skipped every other frame, cutting the frame rate in half; this technique considerably degraded the quality and timing of the original animation, to the extent that some animation was not carried over or mistakenly altered. The most recent redrawn colorized black-and-white cartoons are the Fleischer Studios/ Famous Studios' ''Popeye '' Merrie Melodies'', and MGM's '' The Captain and the Kids'' cartoons, which were colorized in 1987 for airing on the Turner networks. With computer technology, studios were able to add color to black-and-white films by digitally tinting single objects in each frame of the film until it was fully colorized (the first authorized computer-colorizations of B&W cartoons were commissioned by Warner Bros. in 1990). The initial process was invented by Canadian Wilson Markle and was first used in 1970 to add color to monochrome footage of the moon from the Apollo program missions.

Digital colorization

Computerized colorization began in the 1970s using the technique invented by Wilson Markle. These early attempts at colorization have soft contrast and fairly pale, flat, washed-out color; however, the technology has improved steadily since the 1980s.

To perform digital colorization, a digitized copy of the best black and white film print available is used. With the aid of computer software, technicians associate a range of gray levels to each object and indicate to the computer any movement of the objects within a shot. The software is also capable of sensing variations in the light level from frame-to-frame and correcting it if necessary. The technician selects a color for each object based on common "memory" colors—such as blue sky, white clouds, flesh tones, and green grass—and on any information about colors used in the movie. If color publicity stills or props are available to examine, authentic colors may be applied. In the absence of any better information, technicians may choose colors that fit the gray level and are consistent with what a director might have wanted for the scene. The software associates a variation of the basic color with each gray level in the object, while keeping intensity levels the same as in the monochrome original. The software then follows each object from frame to frame, applying the same color until the object leaves the frame. As new objects come into the frame, the technician must associate colors to each new object in the same way as described above. This technique was patented in 1991.

In order to colorize a still image, an artist typically begins by dividing the image into regions, and then assigning a color to each region. This approach, also known as the segmentation method, is laborious and time-consuming, especially in the absence of fully automatic algorithms to identify fuzzy or complex region boundaries, such as those between a subject's hair and face. Colorization of moving images also requires motion compensation, tracking regions as movement occurs from one frame to the next.

Several companies claim to have produced automatic region-tracking algorithms:

* Legend Films describes their core technology as pattern recognition and background compositing that moves and morphs foreground and background masks from frame to frame. In the process, backgrounds are colorized separately in a single composite frame which functions as a visual database of a cut, and includes all offset data on each camera movement. Once the foreground areas are colorized, background masks are applied frame-to-frame.
* Timebrush describes a process based on neural net technology that produces saturated and crisp colors with clear lines and no apparent spill-over. The process is cost effective because it relies on computers rather than human effort, and is equally suitable for low-budget colorization and broadcast-quality or theatrical projection.
* A team at the Hebrew University of Jerusalem's Benin School of Computer Science and Engineering describe their method as an interactive process that does not require precise manual region detection, nor accurate tracking; it is based on the premise that adjacent pixels in space and time that have similar gray levels should also have similar colors.
*At the University of Minnesota, a color propagation method was developed that uses geodesic distance.
* A highly labor-intensive process employed by the UK-based film and video colorization artist Stuart Humphryes, in conjunction with video restoration company SVS Resources, was employed by the BBC
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