ABSTRACT
Color is an essential aspect of a picture which conveys to the viewer many emotions and symbolic meanings. Adjusting the color grade of pictures is therefore an important step in professional photography. This process is part of the larger activity of grading in which the color and grain aspects of the photographic material are digitally manipulated. The term color grading will be used specifically to refer to the matching of color. Color grading is a delicate task since the slightest of color variations can alter the mood of a picture. For instance, changing the white point of a picture can make a picture look warmer or more
metallic. Refer to Chapter 10 for details on white balancing. Increasing the contrast can give a sharper aspect to the picture. Color grading is an even more critical step in movie postproduction. Like with photographic stills, it gives the movie a unique artistic signature. For instance the movie Ame´lie (2002) has a look of its own. Bright colors, deep black levels and saturated colors all contribute to create an artificial expressionistic world. The main problem of grading a movie is to adjust the color consistently across all the shots, even though the movie has been edited with heterogeneous video material. Shots taken at different times under natural light can have a substantially different feel due to even slight changes in lighting. Currently in the industry, color balancing (as it is called) is achieved by experienced artists who use very expensive edit hardware and software to manually match the color between frames by tuning parameters. Typical tuning operations comprise adjusting the exposure, brightness and contrast, calibrating the white point or finding a color mapping curve for the luminance levels and the three color channels. For instance, in an effort to balance the contrast of the red color, the digital samples in the red channel in one frame may be multiplied by some gain factor and the output image viewed and compared to the color of some other (a target) frame. The gain is then adjusted if the match in color is not quite right. The amount of adjustment and whether it is an increase or decrease depends crucially on the experience of the artist. This is a delicate task since the change in lighting conditions induces a very complex change of illumination. It would be therefore beneficial to automate this task in some way.
