All of the concepts in modern digital color correction stem from the film lab process known as color timing. The term “timer” originally described the person who knew how long to leave the negative in the chemical bath to achieve the desired result. Once the industry figured out how to manipulate color in the negative-to-positive printing process, the “color timer” was the person who controlled the color analyzer and dialed in degrees of density and red/blue/green coloration.
Various color wheel systems use alternate color science.
Electronic video color correction started with early color cameras and telecine (film-to-tape transfer or “film chain”) devices. These were based on red/blue/green color systems, where the video engineer (or “video shader”) would balance the three components, along with exposure and black level (shadows). He or she would adjust the signal of the pickup systems, including tubes, CCDs and photoelectric cells.
Electronics manufacturer RCA added “chroma proc” circuitry to its cameras that divided the color spectrum according to the six divisions of the vectorscope: red, blue, green, cyan, magenta and yellow. The chroma proc let the operator shift the saturation and/or hue of each one of these six slices. For instance, you could desaturate the reds within the image. Early color correction modules for film-to-tape transfer systems adopted this same circuity. The “primary” controls manipulated the actual pickup devices, while the “secondary” controls were downstream in the signal chain and let you further fine-tune the color according to this simple six-vector division.
Primary controls let you balance the basic RGB components of the signal. Blackmagic DaVinci Resolve features both sliders and wheels for alternate approaches.
These early color correction systems were built to transfer color film for live broadcast or to videotape. They were part machine control and part color corrector. Modern color correction systems for postproduction came to be because of three key advances: memory storage, scene detection and signal decoding.
Memory storage. Once it was possible to store and recall color correction settings, then it was easy to go back and forth between camera angles or shots and apply a different setting to each. Or you could create several looks and preview them for the client. The addition of this technology was the basis for a seminal patent lawsuit, known as the Rainbow patent suit, as the battle ranged over who had ownership of this technology.
Scene detection. Film transfer systems had to play in real time to be recorded to videotape, which meant that shot changes had to trigger the change from one color correction setting to the next. Early systems did this via the operator manually marking an edit point (called “notching”), via an EDL (edit decision list) or through automatic scene detection circuitry. This was important for the real-time transfer of edited content, including film prints, cut negative and eventually videotape programs.
Avid Symphony’s secondary color controls divide color according to six vectors.
Signal decoding. The ability of color correction systems to decode a composite or component analog (and later digital) signal through added hardware transformed color correction, shifting it from camera shading and film transfer to a general tool available at a post facility. The addition of a signal decoder board in a Da Vinci color corrector unit split the input signal into RGB parts and enabled the colorist to enhance the correction of an already edited master using the “secondary” signal electronics of the system. This enabled “tape-to-tape” color correction of edited masters.
Fast Forward A Few Years
Modern color correction software tends to divide its controls into primary and secondary functions. There’s really no reason to do that, since we are no longer controlling the pickup devices within a camera or telecine. Nevertheless, it’s terminology we seem to be comfortable with. Often secondary controls enable masking and keys to isolate color—not because it has to be that way but because engineers at Da Vinci Systems originally included these features in the secondary control set. In modern correction tools, any function could happen on any layer, node, room, etc.
The Dale Grahn Color iPad application is designed to teach color principles using controls similar to those of a film color timer.
The core language for color manipulation still boils down to the early film controls. A signal can be brighter or darker, more or less “dense” (contrast), and have its colorimetry shifted by adding or subtracting red, blue or green; these adjustments may take place on the image overall or only in the highlight, midrange or shadow portions of the image. (If you’d like a refresher course, the CrumplePop iPad app Dale Grahn Color offers a tutorial on color timing principles and techniques.)
These adjustments may be made via sliders, knobs, color wheels and other user interfaces. Different software applications and plug-ins get to the same point through different means. Since some of these tools represent different color science and math, controls may be equivalent in their effect, though not necessarily identical in how they affect the image.
Contrast and temperature controls are considered a photographic approach to correction. When you adjust contrast, the image levels expand or stretch as viewed on a waveform monitor. Highlights get brighter and shadows deepen. This contrast expansion centers on a “pivot point,” which by default is at the center of the signal. If you change the pivot slider, you are shifting the center point of this contrast expansion. In one direction, the contrast control will stretch the range below the pivot point more than above it; shift the pivot slider in the other direction for the opposite effect.
Color temperature and tint (also called magenta) controls balance the red/blue/green signal channels in relation to each other. If you slide a color temperature control while watching an RGB parade display on a waveform monitor, you’ll note that adjustments shift the red and blue channels up or down (they move in opposing directions) while leaving green unaffected. When you adjust the tint (or magenta) slider, you are adjusting the green channel. As you raise or lower the green, the red and blue channels move together in a compensating direction.
Adobe SpeedGrade CC is one of the color correction tools to offer contrast and pivot controls for photographic-style grading.
The SOP model is used for CDL (color decision list) values and breaks down the signal according to luma (master), red, green and blue. These are expressed in the form of plus or minus values for slope, offset and power. Assimilate Scratch Play’s color adjustments are a good example of the SOP model in action. Slope is equivalent to gain. Picture the waveform as a diagonal line from dark to light. As you rotate this imaginary line, the higher part becomes taller, which represents brightness values. Think of the slope concept as this rotating line. As such, its results are comparable to a contrast control.
The offset control shifts the entire signal up or down, similar to other shadow or lift controls. The power control alters gamma. As you adjust power, the gamma signal is curved in a positive or negative direction, effectively making the midrange tones lighter or darker.
Temperature and tint controls are used to balance overall red, blue and green channel levels against each other.
The LGG model is the method used in most three-way color wheel-style correctors. It effectively works in a similar manner to contrast and SOP, except that the placement of controls makes more sense to most casual users. Gain, as the name implies, increases the signal, effectively expanding the overall values and making highlights brighter. Lift shifts the entire signal higher or lower. Changing a lift control to darken shadows will also have some effect on the overall image. Gamma bends the curve and effectively makes the midrange values lighter or darker.
A common misconception is that shadow/mid/highlight controls on a three-way color corrector will divide the waveform evenly into three discrete ranges. In fact, these are very large, overlapping ranges that interact with each other. If you change the color balance for the midrange tones, the color shift will also contaminate shadows and highlights. The extent of the portion that is affected is controlled by a luma range control. Some of the few color correction applications that give you control over shifting the crossover points of these luma ranges include Avid Symphony, Synthetic Aperture Color Finesse and Adobe SpeedGrade. Each offers curves or sliders to reduce or expand the area controlled by each luma range and effectively tightens or widens the overlap or crossover between the ranges.
Channels or Printer Lights
Printer light controls mimic film adjustments by altering the intensity of the red, blue or green channel.
Video signals are made up of red, blue and green channel information. It is not uncommon for properly balanced digital cameras to impart a green color cast to the overall image, especially if log profile recording was used. It’s best to simply balance the overall channels first in order to neutralize the image, rather than attempt to do this through color wheel adjustments. Some software uses actual channel controls, so it’s easy to make a base-level adjustment to the output or mix of a channel. If your software uses printer lights, you can achieve the same results. Printer lights harken back to lab color timing, using point values that equate to color analysis values. Dialing in a plus or minus red/blue/green printer light value effectively gives you the same result as altering the output value of a specific color channel.