Choosing a Region to Correct Using the HSL Qualifiers

One of the most common ways of isolating a feature for targeted correction is to use the HSL qualifiers (so named because they qualify part of the image for correction) to key on the portion you want to color correct. HSL stands for hue, saturation, and lightness, which are the three properties of color that together define the entire range of color that can be represented digitally.

HSL qualification is often one of the fastest ways to isolate irregularly shaped subjects, or subjects that are moving around in the frame. However, as with any chroma or luma key, the subject you’re trying to isolate should have a color or level of brightness that’s distinct from the surrounding image. Fortunately, this is not unusual, and reddish skin tones, blue skies, richly saturated clothing or objects, and pools of highlights and shadows are often ideal subjects for secondary correction.

If you’re familiar with the Limit Effect controls of the Color Corrector 3-way filter in Final Cut Pro, you’ll find that the Secondaries room HSL controls work more or less the same way.

Figure. HSL qualifiers.

The HSL controls work as a chroma keyer. By selecting ranges of hue, saturation, and lightness, you create a matte that is then used to define the region to which corrections are applied. Everything outside the matte remains unaffected (although you can also specify which portion of the matte you want to adjust, the inside or the outside).

Figure. Original image, HSL qualifier settings, matte, and corrected image compared.

The HSL Qualifier controls always sample image data from the original, uncorrected image. This means that no matter what adjustments have been made in the Primary In room, the original image values are actually used to pull the key. For example, even if you completely desaturate the image in the Primary In room, you can still pull a chroma key in the Secondaries room.

Tip: It is not necessary to use all three qualifiers when keying on a region of the image. Each qualifier has a checkbox and can be turned on and off individually. For example, if you turn off the H (hue) and S (saturation) controls, you can use the L (lightness) control by itself as a luma keyer. This is a powerful technique that lets you isolate areas of an image based solely on image brightness.

Creating Fast Secondary Keys Using the HSL Eyedropper

The eyedropper, at the top-left corner of the Basic tab, provides a quick and easy way to sample color values from images you’re correcting.

To use the eyedropper to pull a secondary key
  1. Click the eyedropper.

    Figure. HSL eyedropper.

    The eyedropper becomes highlighted, and crosshairs appear superimposed over the image in the preview and broadcast monitors. You use these crosshairs to sample the HSL values from pixels in the image.

  2. Move the mouse to position the crosshairs on a pixel with the color you want to key on, then click once to sample color from a single pixel.

    Figure. Crosshairs used to select a range of color.

    The crosshairs disappear, and the HSL controls are adjusted to include the sampled values in order to create the keyed matte. In addition, the Enabled button turns on automatically (which turns on the effect of the secondary operation in that tab). The Previews tab becomes selected in the middle of the Secondaries room, showing the keyed matte that’s being created by the HSL qualifiers. (For more information, see Controls in the Previews Tab.)

    Figure. Matte resulting from eyedropper use.

    Once you’ve created the keyed matte, the next step is to use the color correction controls at the top of the Secondaries room to actually make the correction. For more information, see The Primary In Room.

    In addition to sampling individual color values, you can also use the eyedropper to sample an entire range of values.

To use the eyedropper to sample a range of values
  • Click the eyedropper, then drag the crosshairs over the range of pixels you want to sample.

    The HSL controls expand to include the entire range of hues, saturation, and lightness in the pixels you sampled. As a result, the keyed matte in the Previews tab is much more inclusive.

    Figure. Matte resulting from dragging a range of pixels with the eyedropper.
To expand the HSL selection using the eyedropper
  • Click the eyedropper, then hold down the Shift key and either click a single pixel or drag over a range of pixels with the crosshairs.

    The crosshairs disappear, and the HSL controls are expanded to include the range of sampled values you dragged on to expand the keyed matte in the Previews tab.

    Note: When selecting a range of multiple HSL values, you can only select a contiguous range of values. You cannot, for example, exclude yellow if you’ve included both red and green, since yellow falls in between. If you need to select noncontiguous HSL ranges, you should use multiple secondary operations. For example, choosing red with Secondaries tab 1, and choosing green with Secondaries tab 2.

The HSL Controls

You don’t have to use the eyedropper to select a range of HSL values. You can also use the HSL controls at the top of the Basic tab to select specific ranges of hue, saturation, and lightness directly.

Each of these qualifiers can be turned on and off individually. Each qualifier that’s turned on contributes to the keyed matte. Turning a qualifier off means that aspect of color is not used.

Each qualifier has three sets of handles—center, range, and tolerance—which correspond to three knobs on compatible control surfaces. These handles can also be manipulated directly onscreen using the mouse.

Figure. Center, Range, and Tolerance handles of the HSL qualifier controls.

HSL Qualifiers Explained

To make HSL adjustments efficiently, you should have an in-depth understanding of the nature of each type of adjustment.

  • H (hue): Defines the range of colors that contribute to the key. Using hue by itself to define a keyed matte can yield similar results to using the Hue, Sat, and Lum secondary curves. Because the visible spectrum is represented by a wraparound gradient, the H handles are the only ones that wrap around the ends of this control, allowing you to select a complete range of blue to green, when necessary.
  • S (saturation): Defines the range of saturation that contributes to the key. Using saturation by itself to define a keyed matte can be effective for manually limiting oversaturated colors. Using saturation and hue, but excluding lightness, lets you manually limit specific colors throughout the image regardless of their lightness.
  • L (lightness): Defines the range of lightness that contributes to the key. Using lightness by itself to define a keyed matte is an extremely powerful technique that lets you quickly isolate regions of the highlights, midtones, or shadows to perform specific adjustments such as increasing or reducing the specific lightness of shadows, or manipulating the color within highlights.
  • Reset button: Resets all three qualifiers to the default state, which is an all-inclusive selection.

HSL Qualifier Controls

This section describes the HSL qualifier controls.

  • Center: A single handle defines the middle of the selected range of values.
  • Range: An inner pair of handles to the left and right of the center handle defines the initial range of values that contribute to the keyed matte. These are the solid white pixels seen in the matte.
  • Tolerance: An outer pair of handles defines a range of values that surround the range values to create falloff, giving a soft edge to the keyed matte. These are the lighter gray pixels seen in the matte.

Adjusting the HSL Controls

This section explains how to adjust the HSL controls.

To adjust the center point for any qualifier
  • Drag anywhere within the center of the two Range handles.

    Figure. Before and after adjusting the HSL center handle.
To make a symmetric adjustment to the Range handles
  • Drag the Range handles directly, or drag anywhere between the Range and Tolerance handles (if the tolerance is wide enough) to widen or narrow the range.

    Figure. Before and after making a symmetric Range handle adjustment.
To make an asymmetric adjustment to the Range handles
  • Hold down the Shift key and drag the handle you want to adjust; the opposing handle remains fixed in place.

    Figure. Before and after an asymmetric adjustment to the Range handles.

    When you make an asymmetric adjustment, the center point also readjusts to match the new range.

    Note: You cannot make asymmetric adjustments using knobs on a control surface.

To adjust the Tolerance handles
  • Drag anywhere outside of the Center, Range, and Tolerance handles to widen or narrow the tolerance.

    Figure. Adjusting the Tolerance handles.

    You can also make asymmetric adjustments to tolerance by holding down the Shift key while dragging.

The Color Swatches

A set of six swatches underneath the HSL qualifiers lets you automatically set the Hue qualifier to a narrow range that’s centered on one of the primary red, green, and blue, and secondary cyan, magenta, and yellow colors.

Figure.  HSL color swatches.

The swatches can be useful when you need to quickly make a hue selection for a feature in the image that corresponds to one of these colors. When you choose one of these swatches, the Saturation and Lightness controls remain completely unaffected.

To adjust the Hue qualifier using one of the color swatches
  • Shift-click any of the swatches.

    The Hue qualifier resets itself to select the corresponding range of color.

Key Blur

The Key Blur parameter lets you apply a uniform blur to the keyed matte in order to soften it. This can go a long way toward making an otherwise noisy or hard-to-pull key usable. This parameter defaults to 0, with a maximum possible value of 8.

Note: You can manually set the key blur to even higher values by typing them directly into the Key Blur field.

Figure. Before and after using key blur on a matte.

One of the nice things about keying for color correction is that, unlike keying to create a visual effects composite, you don’t always have to create keyed mattes with perfect edges or completely solid interiors. Often an otherwise mediocre key will work perfectly well, especially when the adjustment is subtle, so long as the effect doesn’t call attention to itself by adding noise, or by causing vibrating “chatter” around the edges of the matte.

For example, holes in a keyed matte often correspond to shadows that are falling on the subject you’re isolating. If you’re making a naturalistic adjustment to the highlights of the image, you probably don’t want to include such shadowed areas in the correction, so there’s no need to make further adjustments to the matte.

Check Your Secondary Keys During Playback

It’s always important to double-check to see how the secondary keys you pull look during playback. Sometimes a secondary operation that looked perfectly good while you were making the correction exhibits flickering or “chatter” at the edges that is the result of noise, or of including a range of marginal values that are just at the edge of the selected range. (This happens frequently for “hard-to-key” features in an image.) In these cases, additional adjustments may be necessary to eliminate the problem.

Also, secondary keys that work well in one part of a shot may not work as well a couple of seconds later if the lighting changes. Before moving on, it’s always a good idea to see how a secondary operation looks over the entire duration of a shot.