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Using Color Curves

Those of you who work with Adobe Photoshop and other image editing applications are probably already familiar with curves. Meanwhile, colorists and color timers who’ve been working in other applications specific to video correction and film grading may ask, “Why should I use curves when I’m already used to using color balance controls?”

The simple answer is that while the color balance controls shown previously let you adjust the red, green, and blue components of an image simultaneously, the red, green, and blue curves let you adjust the corresponding color components of the image individually. This opens up additional creative and utilitarian vistas with a specificity that the color balance controls simply aren’t capable of.

In most color correction applications, the red, green, and blue color adjustment curves are located alongside the luma curve we saw in the previous chapter (Figure 4.95).

Figure 4.95

Figure 4.95 RGB curves lie next to the luma curve in most applications.

Each color curve controls the intensity of a single primary color component of the image. In some applications, these curves are locked together by default, enabling RGB contrast adjustment using the curves, as you saw in Chapter 3.

However, if you uncouple the curves from one another, you can make changes to specific color channels within as broad or as narrow a range of image tonality as you like. In fact, with a single curve, you can make as many specific adjustments to narrow portions of image tonality, from the shadows through the midtones through the highlights, as you can place control points onto the curve. Figure 4.96 shows a rough breakdown of which parts of the default slope of the curve’s interface correspond to which tonal areas of the image. Bear in mind that since the practical definitions of shadows, midtones, and highlights overlap considerably, this is only an approximation.

Figure 4.96

Figure 4.96 This image shows which parts of the curve adjust which tonal regions of the image, approximately.

In most color correction applications, adjustments to the color channels work identically to those made using a Luma curve, as covered in Chapter 3. Click a curve to add as many control points as you need to modify its shape, dragging each control point up or down to change the level of that color channel to different values at the corresponding region of image tonality.

In Figure 4.97, you can see that four control points have been added to the curve, raising the amount of red at the top of the midtones while simultaneously lowering the amount of red at the bottom of the midtones. This is a far more specific adjustment than can be made using the color balance controls.

Figure 4.97

Figure 4.97 Adjusting the red channel within two different areas of image tonality.

The following section demonstrates this principle in greater detail.

Making Tonally Specific Color Adjustments with Curves

Let’s take a look at how we can affect tonally specific regions of an image using curves. Figure 4.98 shows a low-key night shot with a cool blue cast in the highlights (the top of the blue waveform in the parade scope is taller than the red and green). Otherwise, it exhibits fairly neutral color throughout the midtones, with deep, neutral shadows (evidenced by the relatively equal bottoms of the three waveforms in the parade scope).

Figure 4.98

Figure 4.98 A neutral image with balanced graphs in the parade scope.

The client has expressed a desire for a bit more zest in the highlights, particularly in the lighting that can be seen though the doorway. One way we could accomplish this is using the color curves.

  1. To simply add more red to this image using the curves, click the middle of the Red curve to add a single control point, and then drag it up to raise the amount of red, as shown in Figure 4.99.

    Figure 4.99

    Figure 4.99 Raising the Gamma of the red channel, along with most of the shadows and highlights.

    As you can see in Figure 4.100, this adjustment boosts the amount of red throughout the image.

    Figure 4.100

    Figure 4.100 The result of the correction shown in Figure 4.99. Notice that the red channel is elevated in its entirety.

    When you make an adjustment with only one control point, it results in a fairly extreme overall adjustment to the image, since it pulls nearly every part of the curve upward. This creates a reddish color cast over the entire scene.

    You should note that the initial two control points that the curve starts out with at the bottom left and upper right partially pin the darkest and lightest parts of the red channel in place. With ordinary adjustments of modest scale, these two original control points at their default position help preserve the neutrality of the darkest shadows and the brightest highlights in the image.

    Figure 4.101 compares the unadjusted and adjusted red graphs of the parade scope from the previous image. If you look closely at the top and bottom, you can see that the highlights and midtones of the red channel have been stretched by a greater amount than the shadows.

    Figure 4.101

    Figure 4.101 Comparing the unadjusted red channel at the left to the curve-adjusted red channel at the right.

  2. If you add a second control point to the Red curve (Figure 4.102), you can return the shadows of the red channel to their original state by dragging the new control point down until the bottom of the curve intersects the diagonal grid.

    Figure 4.102

    Figure 4.102 Adding a second control point to keep the shadows and darker midtones neutral, indicated by the proximity of the bottom of the curve to the intersection of the horizontal and vertical gridlines.

    The diagonal of this grid indicates the neutral state of each curve. Wherever a curve intersects this diagonal, the values of the image at that zone of tonality are as they were in the original image (Figure 4.103).

    Figure 4.103

    Figure 4.103 Our modified curve, compared to the diagonal intersection of gridlines that represents the neutral détente position of the curve, along which the image remains unaltered.

    With this last adjustment, the control point to the upper right continues to boost the highlights of the red channel. Meanwhile, the new control point you’ve added to the lower left pins the Red curve at a more neutral diagonal in the highlights. The curve from one control point to the other keeps this transition very smooth, producing a gradual transition from the unaffected shadows through the affected highlights, as shown in Figure 4.104.

    Figure 4.104

    Figure 4.104 Boosting red in the highlights (especially as seen through the doorway) while keeping the darker midtones and shadows neutral.

    The result is that the shadows of the room and the darker midtones of the woman and man remain neutral, but the brighter highlights, especially the highlight seen through the doorway, have the new insidious red cast you’ve introduced.

This is the power of curves. They give you specific, customizable control over the color in different tonal regions of an image that can sometimes border on secondary color correction.

Making Controlled Curve Corrections to an Image Using the Parade Scope

If you want to use curves as a corrective tool to neutralize color casts, one of the best ways to spot which curves need to be adjusted is to use the RGB parade scope.

As you’ve already seen in the previous example, the graphs for each of the three color channels in the parade scope correspond perfectly to the three available color curve controls. Since color casts generally reveal themselves in the parade scope as an elevated or depressed graph corresponding to the channel that’s at fault, these waveforms provide an instant guide to show you which curves you need to adjust and where to place the control points you need to use to make the adjustment.

The following example was originally shot with an extremely warm cast. As is so often the case, the director decided to ease off this bold decision in post. Large color casts like this are often ideal candidates for curve correction, since you can easily make very targeted corrections to the specific color channels that are at fault.

  1. Examine the RGB parade scope for the image in Figure 4.105. This shows an image with a red channel that is obviously too high relative to the rest of the picture, and it’s throwing off the highlights of the shot.

    Figure 4.105

    Figure 4.105 The original, unaltered image. The parade scope reveals that the red channel is too high and the blue channel is too low.

    The parade scope indicates that your first adjustment should be to lower the midtones of the red channel relative to the green and blue channels. To figure out where to put a control point to do what you want, you need only compare the height of the waveform you want to adjust to the height of the curve (Figure 4.106).

    Figure 4.106

    Figure 4.106 To affect the spike in the red channel waveform that just touches the dotted line, put a control point where the dotted line intersects the curve.

  2. Now, place a control point at the top third of the curve, and drag it down to lower the red channel midtones until the middle of the red channel (which is the portion of the waveform that corresponds to the wall) is only just a little higher than the middle of the green channel (Figure 4.107).

    Figure 4.107

    Figure 4.107 The red channel curve adjustment described in step 2.

    This neutralizes the highlights, but now you’ve traded an orange color cast for a greenish-yellow one (Figure 4.108). Next, you need to raise the blue color channel.

    Figure 4.108

    Figure 4.108 The result of the adjustment made in Figure 4.107.

  3. Now, place a control point at the bottom third of the blue color curve, at a height that corresponds to the top of the blue midtones, and drag it up until the blue midtones are closer to the same height as the green curve. You’ll know when to stop by keeping your eye on the monitor. Once the image looks neutral, you’re done.

    The resulting correction works well for the midtones, but the shadows now look a little weak. To fix this, add another control point near the bottom of the curve and drag it down to lower the bottom of the blue waveform (Figure 4.109).

    Figure 4.109

    Figure 4.109 The blue channel curve adjustment described in step 3.

    This last adjustment brings the image to a more neutral state (Figure 4.110).

    Figure 4.110

    Figure 4.110 The final result, after adjusting both the red and blue curves.

    At this point, it’s easier to introduce a more subtle warmth using the color balance controls that will complement rather than compete with the image.

As you can see, there is a fairly direct correspondence between the values displayed in the three graphs of the parade scope and the three color curve controls.

Which Are Faster, Color Balance Controls or Curves?

Unlike the color balance controls, which simultaneously adjust the mix of red, green, and blue in the image, each of the color curves adjusts just one color component at a time. This means that sometimes you have to adjust two curves to make the same kind of correction that you could achieve with a single adjustment of the appropriate color balance control.

For example, in Figure 4.111, the parade scope indicates a color cast in the shadows of the image, via a blue channel that’s too high and a red channel that’s too low.

Figure 4.111

Figure 4.111 A fluorescent green color cast.

To correct this using the curves controls, you’d have to make three adjustments, to the red, green, and blue channels. However, to make the same adjustment using the color balance controls, you would only need to drag up the Gain control toward magenta (Figure 4.112). Both adjustments result in nearly the same correction.

04fig112a.jpg

Figure 4.112 Two ways of neutralizing the green—one using curves, the other using a single Gain color balance operation—produce nearly identical results.

Which way is better? Well, that’s really a matter of personal preference. The best answer is whichever way lets you work faster. In a client-driven color correction session, time is money, and the faster you work, the happier your client will be.

Both controls have their place, and my recommendation is that if you’re coming to color grading from a Photoshop background, take some time to get up to speed with the color balance controls; you may be surprised at how quickly they work. And for colorists from a video background who haven’t used curves that much before, it’s worth taking the time to learn how to make curve adjustments efficiently, as it may open up some quick fixes and custom looks that you may have wrestled with before.

DaVinci Resolve Curves and Lum Mix

One of the interesting differences between DaVinci Resolve and other applications is the YRGB image processing Resolve uses in order to maintain image luminance while you make adjustments to individual color channels. This feature is most noticeable when you unlink curves in order to make the kinds of adjustments shown in the previous few sections. In DaVinci, lowering one color channel using either curves or individual RGB Lift/Gamma/Gain controls automatically raises the other two color channels in such a way as to preserve the overall lightness of the image. In this scheme, raising one color channel also lowers the other two, maintaining a kind of symmetry.

This type of image processing can take some getting used to if you come from an application where each channel is totally independent, but Resolve lets you modify this behavior using the often misunderstood Lum Mix parameter. When set to 100 (the default), Lum Mix maintains this symmetrical relationship between all color channels, as shown in Figure 4.113.

Figure 4.113

Figure 4.113 With Lum Mix set to 100, lowering the green channel in DaVinci Resolve results in the red and blue channels being raised to maintain image lightness.

To disable this behavior, all you need to do is to lower Lum Mix to 0, and all per-channel operations within that node will have no effect on the other channels of the image.

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