Color Correction II




Most modern NLEs have installed some form of color correction and grading tools. These range from the controls that mimic TBC front panel controls all the way up to the full feature set of telecine-style grading systems.


Color grading tools can be broken down into these categories:


Proc amp controls




These offer basic control of video levels and generally include the coarse ability to change brightness, contrast, color saturation and hue. These controls may be arranged in aforementioned manner (like a TV set) or like a professional proc amp or TBC, with adjustments for pedestal (or black level), video gain and chroma saturation.


Primary color correction




This section or filter is used to correct the balance of an image. Usually primary controls offer either sliders or color wheel dials to shift color balance to red, blue or green. Color wheels offer greater precision, since you can hit the in-between color values of yellow, cyan and magenta. Color wheels are often referred to as Hue Offset controls. Most primary correction tools divide the video spectrum into thirds, controlling levels and balance for shadows, midrange and highlights. Depending on the system, color wheel controls are often labeled as lift/gamma/gain or shadows/midtones/highlights or pedestal/gamma/video. Some systems also include a fourth color wheel control that acts as a master balance/level control. Most of these tools offer the ability to raise or lower the luminance value of the portion you are controlling, as well as the color balance. This arrangement is similar to the tools that telecine colorists first started using with daVinci (and similar) telecine grading tools.


Luma Ranges




Generally, you don’t have control of where the crossover occurs – or how graceful this crossover is – between shadows and midtones, or midtones and highlights. If your particular NLE or correction filter offers a Luma Ranges control, you will be able to alter the preset crossover points for more precise grading.






In addition to Hue Offsets, some systems also include Curves controls. Curves display the red, blue and green (and often luminance or master) value of the image as a diagonal line on a graph from dark to light. A straight line represents no adjustments to the image. When making grading changes, the colorist add points along the line and shifts that point left or right from the line’s path. In doing this the path is curved, thereby amplifying or reducing the saturation of red, blue or green in that range of the image. Unlike Hue Offsets, which permit only one adjustment, Curves allow multiple small adjustments along the line. This level of precision is necessary for shots, such as a dark-skinned actor against a burned-out window, or to create looks designed to mimic various film stocks.


Secondary and Selective color correction




Primary correction is typically used by a colorist to get a neutral or at least overall balance to the image. Secondary correction is used to set a look. There are actually two applications of this term. The original basis for secondary correction was to specifically change saturation or hue of six portions of the color vectors, loosely corresponding to red, magenta, blue, cyan, green and yellow. For instance, a colorist could zero in on a pie slice portion of the vectorscope corresponding to red and increase or decrease the envelope around this color so that precisely the correct shade of red was affected without changing other red shades too drastically.


In many modern systems, this type of correction is referred to as Color Replacement or Selective Color Correction. Generally it relies on various types of keying technology to isolate a specific shade of a color. As an example, a colorist might isolate a pale blue sky, in order to enrich the intensity of the blue in only the sky. In addition to Selective Color Correction, there are other uses of Secondary Correction. Many systems permit the stacking of color correction filters or to add layers of correction within the same tool. In this case, Secondary Correction might be used to change the total image, not just selected colors. As this is a non-destructive process, a colorist might also use Secondary Correction to create several looks for client approval. Each layer can be enabled or hidden as the colorist reviews different possible looks.






Shapes, which are often called “windows” based on daVinci’s Power Windows feature, are another way to isolate portions of the image so that one portion can be graded differently than another. Some systems offer ovals, ellipses, circles, rectangles and squares as preset shapes that can be resized, moved and softened. Other systems also permit the creation of freeform drawn shapes. Even if your NLE doesn’t offer shapes, you will probably be able to use other drawing or shape tools within the effects palette to isolate a portion of the image and apply separate correction to the part inside or outside of the shape. This may require duplicating the clip onto several tracks of the timeline. Shapes, such as ovals are often used to isolate a people from their background. The actor’s grading is brighter, while the background is darker, creating a subtle spotlight effect to the shot. Shapes can also be used in the other direction to darken an object in the set that might be too bright and therefore distracting, like a bright table lamp in a living room set.






As you increase or decrease the video levels on most grading systems you are also affecting the apparent color saturation. This is especially true when gamma (midrange) levels are adjusted. For example, as you increase video or gamma on a person’s face, the highlights on the cheeks become flatter and more cartoonish in appearance. This happens because you have fewer steps in the video signal to define the chroma detail of that portion of the image. When doing this it is useful to decrease chroma saturation in the highlight section of the image. Likewise, when you lower black levels (“crush blacks”), chroma in dark areas, like dark blues and dark reds, appear more saturated. In this example, you should decrease the chroma saturation in just the shadow portions of the image to get a more natural appearance.


Not every NLE has grading tools that permit selective saturation adjustments in portions of the image; however, some offer separate filters labeled as Desaturate Highlights. Often these filters have controls for both shadows and highlights that include not only saturation adjustments, but also the video values where the effect starts and the softness of the ramping for the beginning of the effect. Essentially this type of control gives you a type of Luma Range control without having the specific Luma Range tool.


For precise color grading on NLEs that don’t have an all-encompassing single grading toolset, I often find that it is best to apply several specific filters in a stack of effects. For example, if I’m grading a complex shot in Final Cut Pro, I may start with the basic 3-way Color Corrector for my overall shot balance. Next, I will apply several instances of the 3-way Color Corrector, but use it with its Limit controls. This turns the 3-way into a secondary color correcting tool. Now I can zero in on the sky, flesh tones, etc. to give the shot more punch. If I feel I have driven the levels to extremes, I will apply the Desaturate Highlights filter and enable both high and low values to decrease color saturation at both ends of the range. Finally, if I’m concerned about broadcast levels, I’ll apply a Broadcast Safe filter as the last effect in the chain.


© 2007 Oliver Peters