Color grading

Color grading is the process of altering and enhancing the color of a motion picture or television image, either electronically, photo-chemically or digitally. The photo-chemical process is also referred to as color timing and is typically performed at a photographic laboratory. Modern color correction, whether for theatrical film or video distribution, is generally done digitally.

How color timing works

Traditional color timing was done by passing various intensities of red, green or blue light through a color film negative onto an intermediate positive stock to create a new image. A "white" light is passed through a series of dichromatic filters which splits the light into its various primary colors (red, green and blue). By incorporating a physical shutter system into the light path to vary the intensity of each primary color, the color timer could selectively alter the color results on the positive print.

Timers usually worked on a scale of 50 "lights" or "points", which are 50 steps of light intensity values for each of the primary colors. A neutral light setting would normally be 25-25-25 (red, green, blue), middle of the scale.

Additive Coloring

Because of the nature of color negative film, the coloring process is inverted. Adding a "point" of blue does not result in adding blue to the positive image, but actually results in an addition of yellow light (the opposite of blue). Adding a point of green results in more magenta light and adding a point of red results in more cyan light. The higher the "points", the darker the image (the more light exposed creates a darker image), the lower the lights the lighter the image. If a cinematorgrapher wanted to make a scene more blue - while not necessarily darkening it - the color timer would have to add "points" of red and green together while reducing points of yellow equally to maintain the overall density of light striking the film print.Probst, Christoher (May 1997). "Color Conumdrum". American Cinematographer Magazine, pp. 79-85.

Generally speaking, each laboratory calibrated their timers differently, thus a 25-25-25 at one lab might be the same as a 32-32-32 at another.

One point of light generally was equal to .025 Log E of density on the resultant positive print. At such a density, eight points is equal to one camera stop (doubling) of light exposure (.65 gamma).Holben, Jay (April 2000). "Densitometer Readings" American Cinematographer Magazine, p. 121f (insert) This style of color timing was done through the use of a Hazeltine, a color video analyzer, named after its manufacturer, that projects a positive image onto a screen from a negative to allow the timer to make adjustments in density and color for printing.Konigsberg, Ira (1987). The Complete Film Dictionary. Meridian/New American Library. ISBN 0-452-00980-4

Subtractive Coloring

An older system of color timing that was re-adopted by a few modern labs (including Technicolor in Studio City, CA). The system utilized transparent cyan, magenta and yellow color filters inserted into the path of the main light beam. These filters were placed in a "pack" in various densities to create the correct color biased light to pass through the negative and expose the positive film. Neutral density filters were also added to the pack to control overall intensity of light without biasing color.Happé, Bernard (1974). Your Film and the Lab (2nd ed.). Focal Press. ISBN 0-240-51212-x.

Telecine

With the advent of television, broadcasters quickly realized the limitations of live broadcasts and they turned to utilizing film transferred to video to broadcast already produced movies as well as photographing new shows that could then be aired at different times in different time zones. The heart of the system was the Kinescope, the device on which a film image was transferred to video.Kallenberger, Richard H., Cvjetnicanin, George D. (1994). Film into Video: A Guide to Merging the Technologies. Focal Press. ISBN 0-240-80215-2

The early telecine hardware was the "film-chain" which utilized a film projector connected to a video camera. As explained by Jay Holben in American Cinematographer Magazine, "The telecine didn't truly become a viable postproduction tool unitil it was given the ability to perform color correction on a video signal."Holben, Jay (May 1999). "From Film to Tape" American Cinematographer Magazine, pp. 108-122.

Today, telecine is synonymous with color timing as tools and technologies have advanced to make color timing (color correction) ubiqutious in a video enviornment.

How telecine coloring works

In a CRT system, an electron beam is projected at a phosphor-coated envelope, producing a beam of light the size of a single pxel. This beam is then scanned across a film frame from left to right, capturing the "vertical" frame information. Horizontal scanning of the frame is then accomplished as the film moves past the CRT's beam. Once this photon beam passes through the film frame, it encounters a series of dichroic mirrors which separate the image into its primary red, green and blue components. From there, each individual beam is then reflected on to a photomultiplier tube (PMT), where the photons are converted into an electronic signal to be recorded to tape.

Early color correction on Rank Cinetel MkIII telecine systems was accomplished by varying the primary gain voltages on each of the three photomultiplier tubes to vary the output of red, green and blue, respectively. Further advancements converted much of the color-processing equipment from analog to digital and then, with the next-generation telecine, the Ursa, the coloring process was completely digital in 4:2:2 color space. The Ursa Gold brought about full 4:4:4 color space.Holben, Jay (May 1999). "From Film to Tape" Part 1 of 2 American Cinematographer Magazine, pp. 108-122.

Color correction control systems started with the Rank Cintel TOPSY (Telecine Operations Programming System) in 1978. In 1984 da Vinci Systems introduced their first color corrector, a computer-controlled interface that would manipulate the color voltages on the Rank Cintel MkIII systems. Since then, technology has improved to give extraordinary power to the digital colorist. Today there are many companies making color correction control interfaces including da Vinci, Pogle, and more.

Some of the main functions of electronic (digital) color grading:

Reproduce accurately what was shot
Compensate for variations in the material (i.e. film errors, white balance, varying lighting conditions)
Optimize transfer for use of special effects
Establish a desired 'look'
Enhance and/or alter the mood of a scene - the visual equivalent to the musical accompaniment of a film.

Note that some of these functions are contrary to others! For example, color grading is often done to ensure that the recorded colors match those of the set design. In music videos however, the goal may instead be to establish a stylized look.

Traditionally, color grading was done towards technical goals. Features like secondary color correction was originally used to establish color continuity. The trend today is increasingly moving towards creative goals- improving the aesthetics of an image, establishing stylized looks, and setting the mood of a scene through color. Because of this trend, some colorists suggest the phrase "color enhancement" over "color correction".

Primary and secondary color correction

Primary color correction effects the whole image utilizing control over intensities of red, green, blue, gamma (mid tones), shadows (blacks) and highlights (whites). Secondary correction brings about alterations in luminance, saturation and hue in six colors (red, green, blue, cyan, magenta, yellow). The main objective of secondary controls is to adjust values within a narrow range while having a minimum effect on the remainder of the color spectrum.Using digital grading, objects and color ranges within the scene can be isolated with precision and adjusted. Color tints can be manipulated and visual treatments pushed to extremes not physically possible with laboratory processing. Special digital filters and effects can also be applied to the images.

Power Windows

Evolution of digital color correction tools advanced to the point where the colorist could use geomteric shapes to isolate color adjustments to specific areas of an image. Called Power Windows by da Vinci Systems, these tools can highlight a wall in the background and color only that wall - leaving the rest of the frame alone - or color everything but that wall. Subsequent color correctors (typically software-based) have the ability to use spline-based shapes for even greater control over isolating color adjustments.

Inside and outside of area-based isolations, digital filtration can be applied to soften, sharpen or mimic the effects of traditional glass photographic filters in nearly infinite degrees.

Motion Tracking

When trying to isolate a color adjustment on a moving subject, the colorist traditionally would have needed to manually move the mask to follow the subject. In its most simple form, motion tracking automates this time-consuming process using algorithms to evaluate the motion of a group of pixels. These techniques are generally derived from match moving techniques used in special effects and compositing work.

Motion tracking can be combined with other techniques to add light to subject's eyes or even to 're-light' a scene!

DI

The evolution of the telecine device into film scanner allowed the digital information gathered from a film negative to be of sufficient resolution to re-export back to film. In the late 90s, films like Pleasantville and then O Brother, Where Art Thou? pushed the technology to create the digital intermediate, which allowed all of the power of the telecine colorist in a traditional film world. Today, many 'A' list feature films go through the DI process. Traditional photochemical processing is happening less and less.

In Hollywood, O Brother, Where Art Thou? was the first film to be wholly digitally graded. The negative was scanned in with a Spirit Datacine at 2K resolution and then colors were digitally fine-tuned using a Pandora MegaDef color corrector. The process took several weeks. The resulting digital master was output to film again with a Kodak laser recorder to create a master internegative.

Hardware-based versus software-based systems

Hardware-based systems (da Vinci 2K, Pandora, etc.) have historically offered better performance and a smaller feature set than software-based systems (i.e. Discreet Lustre). While hardware-based systems always offer real-time performance, software-based systems need to render as the complexity of the color grading increases. On the other hand, software-based systems tend to have more features such as spline-based windows/masks and advanced motion tracking.

The lines between hardware and software is blurring as many software-based color correctors use the workstation's GPU (graphics processing unit) as a means of hardware acceleration. As well, some newer software-based systems use specialized hardware to improve performance (i.e. da Vinci Resolve).

Significant Colorists

A short list of some noted color timers:

Film

Dan Muscarella, CFI (Terminator 2, Lost Highway, Titanic)
Phil Hetos, Deluxe (Flatliners, Last of the Mohicans, Independence Day)
Lou Levinson, Post-Logic (associate ASC member) (E.T. the Extra-Terrestrial, The Royal Tenenbaums)

Digital^{Probst, Christopher (June 1999). "From Film to Tape" Part 2 of 2 American Cinematographer Magazine, pp. 102-117.}

Stefan Sonnenfeld, Company 3 (Man on Fire, Domino, Miami Vice)
Mike Pethel, Company 3
Beau Leon, Syndicate
Jais Thierry Lamaire, 525 Studios

References

External links

"It's all in the timing", an article from a Starwars.com newsletter about the use of color timing in Attack of the Clones

Color | Film and video technology

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