What’s In a Name?
On the differences among grading controls and operational ordering
By Alexis Van Hurkman | May 20, 2013
In this week's somewhat detail-oriented "Answers Occasionally Given," I start out tackling the differences among color and contrast controls with similar names; in particular, the difference between Lift, Shadow, and Offset controls. Then, I move on to a reader's question about the best stage of a multi-operation color adjustment in which to adjust Luma. It's GRIPPING, I tell you…
Remember, I'm only as interesting as the questions you ask me. If you've got a question about grading, postproduction workflow, or any interesting intersection of production and post, send me an email at firstname.lastname@example.org and I may be able to answer it during one of my weekly updates.
This week, colleague Scott Simmons asks:
Some grading applications say "lift, gamma, gain." Other are "shadows, midtones, highlights." Some throw "setup" in there. Is there really any difference in the terms or is that different developers using different terms?
I get this question a lot, in different forms, depending on who's asking and what application they're referring to. There's a bit of confusion mainly because not every application or plug-in capable of color correction uses the terminology that is more or less standardized among most dedicated color correction workstation applications from companies like DaVinci, FilmLight, Quantel, and SGO.
In general, there are three mathematical schemes used to adjust color and contrast:
Each of these describe a different set of interactions among the traditional three sets of controls that govern the darkest part of the image, the average part of the image, and the lightest parts of the image, respectively. Each has its uses, and I'll describe these controls both in terms of how they affect contrast, and color.
Lift, Gamma, and Gain are the principal controls used by DaVinci Resolve, by Baselight's Video layer controls, and by the color correction features in both Final Cut Pro 7 and Final Cut Pro X (although in FCP these controls are misnamed). Each pair of contrast (or master) and color balance controls have the most influence in a particular portion of the signal; Lift affects the darkest parts of your image the most, Gain affects the brightest area the most, and gamma affects the middle tones the most. However, while each of these regions of adjustment falls off towards the area of the signal they don't affect, they still overlap broadly so that each control influences the neighboring control to a lesser degree. These overlapping regions of tonality can be seen in the graph below.
This graph is only an approximation, as not every grading application's math for defining the overlap of Lift, Gamma, and Gain adjustments is the same. This results in a slightly different "feel" for each application's controls, which will take some getting used to when you switch from one grading app to another.
When adjusting contrast, Lift, Gamma, and Gain controls can be used together to scale the darkest and lightest parts of the signal relative to one another. Comparing two waveform analyses, using the Lift control to lighten the darkest portions of the image keeps the white point of the video signal exactly where it is, and squeezes or stretches everything in between.
The Gain control works opposite Lift, darkening the lightest parts of the image while keeping the black point of the signal where it is. Gamma, for its part, redistributes the entire signal between the black and white points in order to brighten or darken the middle values of your image while leaving the brightest and darkest parts of your signal mostly alone.
The whole point of the Lift/Gamma/Gain scheme of controls is to provide targeted control over specific regions of tonality. You can apply a subtle adjustment to the darkest regions of your image without affecting highlights, or you can apply a creative color adjustment to the highlights or middle tones of your image without affecting its darkest blacks.
Some applications, like Adobe Speedgrade, use Offset, Gamma, and Gain as their primary adjustment controls. Others, like Resolve and Baselight, have a separate Offset control that's available in another mode. When used to adjust color balance, an Offset control raises and lowers the entirety of every channel, resulting in an adjustment throughout the entire tonal range of the signal, as seen below. Meanwhile, Gamma and Gain controls work the same as those in a Lift/Gamma/Gain system.
When used to adjust image contrast, an Offset control raises or lowers the entire signal by a single amount. This is also sometimes referred to as a Setup control by applications that reference the terminology of an older generation of analog signal processing equipment (such as the TBC or time-base corrector). Since the signal is not compressed in any way, lifting the signal too high can push the lighter areas of the signal out of bounds, shown below.
To account for this, Offset controls are usually accompanied by Contrast controls, that let you stretch or squeeze the video signal around a central pivot point. This provides an ability to expand or compress image contrast that's similar to using Lift and Gain together, but using different (and sometimes more convenient) math.
You'll typically adjust an offset control first, since that transforms the color and/or contrast of the entire signal and sets your starting point. Offset is great for dropping the black point when the overall signal would benefit from being similarly darker, and for adjusting color balance on images with significant color casts when you want the result to affect the darkest shadows through the lightest highlights. Then, you can use either Contrast/Pivot or Gamma/Gain controls to adjust other parts of the signal as needed.
Incidentally, there's a mathematical definition of Offset/Gamma/Gain, codified by the ASC CDL. CDL-compliant controls work according to the following equation—
output = (input x slope + offset)power
This math maps to your controls as follows—the Gain control sets a slope by which the input signal is multiplied, the Offset control sets a value that is added to that result, and the Gamma control applies a power function to the overall result. CDL-compliant controls are very specific, and applications that don't adhere to this math must of necessity apply a mathematical transformation when either importing or exporting CDL adjustment data to take how their own controls work into consideration.
Bottom line, using Offset rather then Lift, or vice versa, requires a slightly different way of thinking depending on what you're used to, but you can get where you need to go in the end.
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