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Color Correction Handbook for Video and Cinema: HSL Qualification and Hue Curves

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This chapter covers two of the principle ways of creating targeted secondary color corrections—that is, a correction to a specific object or subject within the frame. You can use HSL Qualification to isolate regions of the image for secondary correction or hue curves to accomplish similarly targeted corrections in a different way entirely.
This chapter is from the book

There are many instances where, instead of making a primary color correction to an overall image, you instead need to make an adjustment to a specific object or subject within the frame, leaving everything else alone. For example, you may want to make the sky more blue, the grass a bit darker, or tone down an actor's aggressively loud shirt.

These kinds of adjustments are referred to as secondary color corrections, since they're generally made after the primary color correction that you made to fix any issues with the overall color or contrast of the shot. Secondary color corrections are an essential part of any colorist's toolkit, and every single professional color correction application provides a variety of ways to accomplish this important task.

This chapter, in particular, covers two of the principle ways of creating such targeted corrections. Most of the chapter is dedicated to how you use HSL Qualification to isolate regions of the image for secondary correction. At the end of the chapter, you'll also see how to use hue curves (and the slightly different user interface [UI] of vectors) to accomplish similarly targeted corrections in a different way entirely.

Many of the techniques in this book rely upon secondary color correction to make adjustments to very specific regions of the picture, but it's a mistake to be too reliant on secondary corrections to deal with simple issues. It's good to get in the habit of checking whether or not you can make the same correction through careful manipulation of your primary color correction controls, instead of wasting time using qualifiers for operations that might be just as effective using the tonally specific Shadows/Lift, Midtones/Gamma, and Highlights/Gain color balance controls.

However, with time you'll start to get the knack of when you can accomplish what's necessary via primary correction, and when it's fastest to throw on a secondary correction to make that special tweak.

HSL Qualification in Theory

Before we launch into how to use these controls, let's first have a look at how HSL Qualification works. Essentially, what you're doing is pulling a chroma key to isolate part of the image based on a specific range of color, or a luma key to isolate part of the image based only on a specific range of lightness. Those of you who are familiar with compositing software know that keys are usually used to define areas of transparency—for example, making a greenscreen background transparent so that an actor appears to be floating in space.

However, when doing color correction, the key you're pulling creates a matte which is used to limit which part of the image is being affected by the color balance, contrast, or other controls, and which part is not (Figure 4.1).

Figure 4.1

Figure 4.1 The original image, an HSL Qualification matte, and the resulting secondary color correction. An example of why the costume department sometimes hates colorists (this is a deliberately extreme example, for illustration).

Once you've defined a matte using your qualifier's color sampling controls and refined it using the individual H, S, and L color component controls (along with any blur or edge manipulation parameters), you can then choose whether you want to correct the region of the picture inside the matte (in white) or the region of the picture that's outside of the matte (in black).

Once all this has been set up, it's a simple matter of adjusting the controls to manipulate the image.

Onscreen Interfaces for HSL Qualification

In most color correction applications, the onscreen interface for HSL Qualification looks pretty much the same, although the programs may differ in how much they rely on the handles of graphical hue, saturation, and luma controls, or whether they rely on sliders and numeric parameters.

Also, your results will vary due to the differences between various applications' under-the-hood image processing, and the different algorithms used for each keyer's implementation (Figure 4.2). Still, the fundamental principles of use remain largely the same across many different applications, so if you learn one, you'll know how to use them all.

Figure 4.2

Figure 4.2 HSL controls compared; from top to bottom: DaVinci Resolve, Autodesk Lustre, Iridas SpeedGrade, Assimilate Scratch.

Some brave UI designers have sought to develop this interface using a more graphical UI, a process I applaud when the result provides more obvious functionality in mouse-driven interfaces. For example, the Hue Angle keyer in FilmLight Baselight, while essentially an HSL keyer like those shown in Figure 4.2, presents a hue-wheel GUI for simultaneously adjusting hue and saturation in an intuitive way (Figure 4.3).

Figure 4.3

Figure 4.3 FilmLight Baselight's Hue Angle keyer is essentially an HSL keyer, but with an integrated Hue/Saturation control for adjusting and customizing the key.

Another example of an application with a forward-looking approach to keying UI is Magic Bullet Colorista II, a color correction plug-in for nonlinear editors (NLEs) and compositing applications, shown in Figure 4.4.

Figure 4.4

Figure 4.4 The graphical interface of the keyer in Colorista II departs from the traditional HSL interface in order to make the process of pulling keys more approachable.

Though a departure from conventional HSL keying UIs, Colorista II retains the individual hue, saturation, and luma controls of the traditional HSL keyer in the form of the cube interface at the upper right-hand side of the UI and luma selection at the lower right, although folks will likely begin their key adjustments using the wedge/vectorscope interface underneath.

Keep in mind that most professional color correction applications map the HSL Qualifier parameters to knobs on a compatible control surface, so the onscreen UI may be incidental to their use. Indeed, many colorists prefer "dialing in" modifications to a keyed matte to tighten it up.

However, if you're working with a system that has a solid onscreen UI, and you find yourself to be a mouse-oriented user, many onscreen UIs present options that aren't available via a control surface. At the end of the day, the best method to use is the one you find fastest.

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