color grading

What color grading is

Color grading is the stage of post-production in which the color and tonal values of a film’s images are adjusted after shooting. Its purposes are twofold. The first is technical consistency: individual shots are recorded at different times, under changing light, and sometimes with different cameras or lenses, and grading brings them into agreement so that a sequence reads as a continuous whole. The second is expressive: the overall balance of color and brightness can be shaped to establish mood, direct attention, or give a production a distinctive look. The work is performed by a colorist, generally in consultation with the director and the director of photography, so that the graded images realize the intentions established during cinematography and lighting.

A distinction is sometimes drawn between color correction, the more technical task of neutralizing casts and matching shots, and color grading, the more interpretive shaping of a look, though in practice the two are carried out together and the terms are often used interchangeably.

Photochemical color timing

Before digital methods, the equivalent operation on color film was color timing (also called grading in British usage), performed by a laboratory technician known as a timer or grader. When a film was printed from the camera negative, the amount of red, green, and blue light used to expose each shot could be varied. These settings were expressed as printer points on a numbered scale, and adjusting them altered the density and color balance of the print. The timer worked shot by shot to correct exposure differences and balance color, judging the result from test prints. Because the adjustments acted on the whole image through the printing light, control was largely confined to overall density and color balance rather than to isolated regions of the frame.

The digital intermediate

Most contemporary color grading is carried out through a digital intermediate (DI), a workflow in which the film is scanned or captured to high-resolution digital files, graded on a computer with a calibrated reference display, and then output — to a digital cinema master, to broadcast formats, or back to film for printing. The DI became widespread in feature production in the early 2000s and gave the colorist far finer control than photochemical timing allowed.

Grading operations are commonly divided into two kinds. A primary correction acts on the entire image at once, setting overall exposure, contrast, white balance, and the color of the shadows, midtones, and highlights. A secondary correction is confined to a selected part of the image — a range of color, a region isolated by a mask or power window, or an object tracked across the frame — allowing, for example, a single hue to be shifted without affecting the rest of the picture. The greater dynamic range and bit depth of digital capture leave more latitude for such adjustment than a finished print retained.

Log footage and LUTs

Digital cameras frequently record in a log (logarithmic) encoding, which distributes the available data so as to preserve detail across a wide range of brightness, from deep shadow to bright highlight. Log images appear flat and low in contrast when viewed directly, because they are intended as a malleable starting point for grading rather than as a finished picture.

To interpret such footage, colorists and other crew use a lookup table (LUT), a stored mapping that converts one set of color values into another. A viewing or display LUT can be applied on set or in editing to show approximately how log material will look once graded, without altering the recorded file, while other LUTs standardize the conversion between color spaces or approximate a chosen look. A LUT applies a fixed transformation and is generally used alongside, not in place of, the shot-by-shot decisions that make up the grade itself.