The Curves Controls

The curves controls, located underneath the color controls in the Primary In room, provide an additional method for adjusting the color and contrast of your images. If you're familiar with image editing applications such as Photoshop, chances are you've used curves before.

Figure. Luma curve control.

The three main differences between the curves controls and the color balance controls are:

Color balance controls are usually faster to use when making broad adjustments to the shadows, midtones, and highlights of the image. Curves, on the other hand, often take more time to adjust, but they allow extremely precise adjustments within narrow tonal zones of the image, which can border on the kinds of operations typically performed using secondary color correction.

Important: While the power of curves can be seductive, be wary of spending too much time finessing your shots using the curves controls, especially in client sessions where time is money. It's easy to get lost in the minutiae of a single shot while the clock is ticking, and such detail work may be faster to accomplish with other tools.

How Curves Affect the Image

Curves work by remapping the original color and luma values to new values that you choose, simply by changing the height of the curve. The x axis of the graph represents the source values that fall along the entire tonal range of the original image, from black (left) to white (right). The y axis of the graph represents the tonal range available for adjustment, from black (bottom) to white (top).

Without any adjustments made, each curve control is a flat diagonal line; in other words, each source value equals its adjustment value, so no change is made.

Figure. Illustration of source vs. adjustment values in curve control.

If part of a curve is raised by one or more control points, then the tonal area of the image that corresponds to that part of the curve is adjusted to a higher value. In other words, that part of the image is lightened.

Figure. Raising midtones using luma curve.

If part of a curve is lowered with one or more control points, then the tonal area of the image that corresponds to that part of the curve is adjusted to a lower value. In other words, that part of the image is darkened.

Figure. Lowering midtones using Luma curve.

Curve Editing Control Points and B-Splines

By default, each curve has two control points. The bottom-left control point is the black point and the top-right control point is the white point for that channel. These two control points anchor the bottom and top of each curve.

Figure. Default control points on Luma curve.

Curves in Color are edited using B-Splines, which use control points that aren't actually attached to the curve control to "pull" the curve into different shapes, like a strong magnet pulling thin wire. For example, here's a curve with a single control point that's raising the highlights disproportionately to the midtones:

Figure. A single control point's influence on a curve.

The control point hovering above the curve is pulling the entire curve upward, while the ends of the curve are pinned in place.

The complexity of a curve is defined by how many control points are exerting influence on the curve. If two control points are added to either side and moved down, the curve can be modified as seen below.

Figure. Using multiple control points to create a complex curve.

To make curves sharper, move their control points closer together. To make curves more gentle, move the control points farther away from one another.

Figure. Comparison of sharp and gentle curves and their control points.

The following procedures describe how to create, remove, and adjust the control points that edit curves controls.

To add control points to a curve
  • Click anywhere on the curve itself.

To adjust a control point
  • Drag it anywhere within the curve control area.

To remove control points from a curve
  • Drag a point up or down until it's outside the curve control area.

To remove all control points from a curve
  • Click the reset button (at the upper-left side of each curve graph) for the curve from which you want to clear control points.

Using Curves to Adjust Contrast

One of the most easily understood ways of using curves is to adjust contrast with the Luma curve. The Luma curve actually performs a simultaneous adjustment to the red, green, and blue channels of the image (as you can see if you take a look at the Parade scope while making Luma curve adjustments), so the overall effect is to adjust the lightness of the image.

Note: Adjustments made to the Luma curve may affect its saturation. Raising luma by a significant amount can reduce its saturation.

You can draw a general correspondence between the controls described in Contrast Adjustment Explained and the black point, midtones, and white point of the Luma curve. For example, moving the black point of the curve up raises the black point.

Figure. Moving the Luma curve's black point.

Moving the white point of the curve down lowers the white point of the image.

Figure. Moving the Luma curve's white point.

These two control points roughly correspond to the Shadow and Highlight contrast controls. If you add a third control point to the Luma curve somewhere in the center, you can adjust the distribution of midtones that fall between the black and white points. This adjustment is similar to that of using the Midtone contrast control. Moving this middle control point up raises the distribution of midtones, lightening the image while leaving the white and black points pinned in place.

Figure. Raising midtones with a single Luma curve control point.

Moving the same control point down lowers the distribution of midtones, darkening the image while leaving the white and black points pinned in place.

Figure. Lowering midtones with a single Luma curve control point.

While these three control points can mimic the functionality of the Shadow, Midtone, and Highlight contrast controls, the true power of curves comes from the ability to add several control points to make targeted adjustments to the lightness of specific tonal regions in the image.

The Luma Curve Limits the Range of the Primary Contrast Sliders

One important aspect of the curves controls is that they can limit the range of subsequent adjustments with the primary contrast sliders in the same room. This can be clearly seen when you make an adjustment to lower the white point of the image using the Luma curve. Afterward, you'll find yourself unable to use the Highlight contrast slider to raise the image brightness above the level that's set by the Luma curve. You can still make additional contrast adjustments in other rooms.

An Example of the Luma Curve in Use

The following example illustrates how to make very specific changes to the contrast of an image using the Luma curve. In this shot, the sky is significantly brighter than the rest of the image. In order to bring viewer attention more immediately to the subject sitting at the desk, you need to darken the sky outside the window, without affecting the brightness of the rest of the image.

Figure. Example image needing luma adjustments.
To make adjustments to a Luma curve
  1. Before making any actual adjustments, pin down the midtones and shadows of the image by adding a control point to the curve without moving it either up or down.

    Figure. Adding a control point to lock part of the Luma curve.

    Adding control points to a portion of a curve that you don't want to adjust, and leaving them centered, is a great way to minimize the effect of other adjustments you're making to specific areas of an image. When you add additional control points to adjust the curve, the unedited control points you placed will help to limit the correction.

    Tip: When adding multiple control points to a curve, you can use the grid to identify where to position parts of a curve you want to be at the original, neutral state of the image. At its uncorrected state, each curve passes through the diagonal intersections of the background grid.

    Figure. Illustration of the neutral diagonal position of each curve.
  2. To make the actual adjustment, drag the white point at the upper-right corner down to darken the sky.

    You want to make sure that you don't drag the new control point down too far, since it's easy to create adjustments that look unnatural or solarized using curves, especially when part of a curve is inverted.

    Figure. Lowering the top of the Luma curve.

    That was a very targeted adjustment, but you can go further. Now that the sky is more subdued, you may want to brighten the highlights of the man's face by increasing the contrast in that part of the image.

  3. Add a control point below the first control point you created, and drag it up until the man's face lightens.

    Figure. Raising the midtones of the Luma curve.

    The man's face is now brighter, but the shadows are now a bit washed out.

  4. Add one last control point underneath the last control point you created, and drag it down just a little bit to deepen the shadows, without affecting the brighter portions of the image.

    Figure. Compressing the shadows of the Luma curve.

    As you can see, the Luma curve is a powerful tool for making extremely specific changes.

Using Curves to Adjust Color

Unlike the color balance controls, which adjust all three color channels simultaneously, each of the color curves controls affects a single color channel. Additionally, the red, green, and blue color curves let you make adjustments within specific areas of tonality defined by the control points you add to the curve. This means that you can make very exact color adjustments that affect regions of the image that are as narrow or broad as you define.

What Is Color Contrast?

Contrast in this documentation usually describes the differences between light and dark tones in the image. There is another way to describe contrast, however, and that is the contrast between different colors in an image. Color contrast is a complex topic, touching upon hue, color temperature, lightness, and saturation. To greatly simplify this diverse topic, color contrast can pragmatically refer to the difference in color that exists in different regions of the image.

In the following example, the image starts out with an indiscriminate color cast; in other words, there is red in the shadows, red in the midtones, and red in the highlights, so there aren’t many clearly contrasting colors in different areas of the image. By removing this color cast from some parts of the image, and leaving it in others, you can enhance the color contrast between the main subject and the background. In images for which this is appropriate, color contrast can add depth and visual sophistication to an otherwise flat image.

Correcting a Color Cast Using Curves

In the following example, you'll see how to make a targeted correction to eliminate a color cast from the lower midtones, shadows, and extreme highlights of an image, while actually strengthening the same color cast in the lower highlights.

The following image has a distinct red color cast from the shadows through the highlights, as you can see by the elevated red waveform in the Parade scope.

Figure. Sample image with red color cast.

Note: For clarity, Broadcast Safe has been turned off so you can better see the bottoms of the waveforms in the Parade scope.

In this particular shot, you want to keep the red fill light on the woman's face, as it was intentionally part of the look of the scene. However, to deepen the shadows of the scene and make the subject stand out a little more from the background, you'd like to remove some of the red from the shadows.

To make a targeted color cast correction
  1. Add a control point to the red curve near the bottom of the curve, and pull down until the red color cast becomes subdued.

    Figure. Lowering midtone reds using the Red curve.

    This should coincide with the bottom of the red waveform in the Parade scope lining up with the bottoms of the green and blue waveforms.

    Figure. The resulting red reduction in the sample image.

    This operation certainly neutralizes the red in the shadows; unfortunately, because this one control point is influencing the entire curve, the correction also removes much of the original red from the midtones as well.

    Tip: If you're wondering where you should place control points on a curve to make an alteration to a specific area of the image, you can use the height of the corresponding graphs in the Waveform Monitor set to either Parade (if you're adjusting color) or Luma (if you're adjusting the Luma curve). For example, if you want to adjust the highlights of the image, you'll probably need to place a control point in the curve at approximately the same height at which the highlights appear in the Waveform graph.

    Figure. Illustration superimposing a curve over a single color channel graph.
  2. Add another control point near the top of the red curve, and drag it up until some red "fill" reappears on the side of the woman's face.

    Figure. Adding red to the highlights of the Red curve.

    This adjustment adds the red back to the woman's face, but now you've added red to the highlights of the key light source, as well.

    Figure. The resulting change in the highlights of the sample image.

    Since the key light for this shot is the sun coming in through the window, this effect is probably inappropriate and should be corrected.

  3. Drag the control point for the white point in the red curve control down until the red in the brightest highlights of the face is neutralized, but not so far that the lighting begins to turn cyan.

    Figure. Reducing red at the top of the Red curve.

    At this point, the correction is finished. The red light appears in the fill light falling on the woman's face, while the shadows and very brightest highlights from the sun are nice and neutral, enhancing the color contrast of the image.

    Figure. The resulting change in the brightest highlights of the sample image.

    Here is a before-and-after comparison so you can see the difference.

    Figure. Before and after the Red curve adjustments.