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Using Log Color Controls

The color balance control functionality described earlier for adjusting lift, gamma, and gain is a classic set of controls for manipulating normalized video images in a video grading environment. The origins of these controls lay in telecine and online tape-to-tape color correction, with the assumption of a BT.709 color space and BT. 1886 gamma profile. In this environment, these controls provide a lot of specific control over the image.

However, as digital intermediate film grading workflows emerged, they were accompanied by the need to grade logarithmically (log) encoded digital film scans, typically in the Cineon and DPX formats, in such a way as to fulfill two distinct requirements:

  • First, it was necessary to map a set of image adjustment controls to the mathematical requirements of log-encoded media formats, with their compressed distribution of color and contrast image data.
  • Second, it was necessary to limit the colorist to using only image-adjustment operations that matched what could be done by optical film printers. Imposing this restriction ensured that the digital colorist couldn’t make adjustments that weren’t compatible with those made by the color timer in projects that mixed digital grading with the photochemical process of color timing.

One of the pioneers of log grading is Lustre, itself originally a product named Colossus, developed by Colorfront and later acquired by Autodesk; it was used extensively on The Lord of the Rings trilogy, among many other films. Lustre has a dedicated Log grading mode that sets the Lustre interface to use these controls exclusively.

Another pioneer of grading using Log controls is FilmLight’s Baselight, which has two different types of grading layers available: video and film. The Video layer exposes the lift/gamma/gain-style controls expected by the telecine professional. However, the Film layer exposes the Log grading controls of Exposure, Contrast, and Shadow/Midtone/Highlight (Figure 4.83). These film-style controls were first developed as part of an in-house compositing and finishing tool at the Computer Film Company, developers of which went on to found FilmLight and create Grader2, an early version of Baselight, to support work on Chicken Run in 2000.

Figure 4.83

Figure 4.83 Primary controls found in the Film Grade operator of FilmLight Baselight. The basic Exposure/Contrast/Saturation controls are shown, but log-oriented Shadow/Midtone/Highlight controls are also available on a second tab.

The original point of Log-style grading was to emulate, with digital tools, the color timing process in such a way as to enable digital colorists to create grades that wouldn’t stray too far from a color-timed result. Even as digital intermediate grading went from being the exception to the rule, Log controls capable of working with native log-encoded media remained valuable for workflows where film output was expected.

Now, of course, many other grading applications including DaVinci Resolve and SGO Mistika support Log-style grading, though it’s tempting to wonder why, given that film acquisition has become less and less common and true digital intermediate workflows for film print distribution are going the way of the dinosaurs.

The reason Log grading controls are still relevant is the increasing number of log-encoded camera acquisition formats, including camera raw formats that are debayered to a log-encoded result. It turns out that logarithmic encoding is still quite useful for efficiently moving a wide latitude of image data into a grading application’s image processing pipeline in order to achieve a reasonable balance between image quality and data throughput/processor performance.

Additionally, when used in conjunction with true log-encoded media, Log controls encourage a very specific grading workflow that, while limiting in one sense, enable a creative aesthetic that’s tied to the history of cinema.

Setting Up a Log Grade

As discussed in Chapter 3, Log controls are designed to work with the peculiarly compressed mathematical distribution of log-encoded image data. To work in this way, it’s important you use the log-style Shadow/Midtone/Highlight color balance controls to adjust the prenormalized state of the image, before the normalizing LUT or adjustment that you’re applying in a second operation. Otherwise, your Log control adjustments won’t work the way they should. For more information, see Chapter 3.

Adjusting Offset Color Balance

As always, you want to make sure you adjust the contrast of an image prior to adjusting its color, and this is even more true of Log controls. Then, the foundation of your log grade as far as color goes is a simple Offset adjustment. This may seem too good to be true, given everything you’ve learned about making color balance adjustments within specific zones of image tonality, but when working on competently shot images, a simple offset adjustment can give you a good, clean result that cures color imbalance from the shadows through the highlights.

Another reason to start with Offset first is that it’s more creative in nature. Veteran colorist Mike Most, who’s written of the advantages of log grading online and who was generous enough to discuss Log grading with me at great length, suggests that beginning your grade on a foundation of log-style controls may yield more inherently cinematic results. The reason given for this is simple: You can create nonlinear signal adjustments with Lift/Gamma/Gain controls that would never happen to a traditionally color-timed film, which is a visual cue the audience can spot.

The reason for this difference is that the principal controls of Offset (master color balance), Exposure (master offset), and Contrast/Pivot make linear adjustments that affect all three color channels evenly throughout the entire tonal range of the signal. This reflects the relatively straightforward adjustments that are made using a color analyzer’s red, green, blue, and density controls but still gives you more control then any color timer ever had via Contrast and Pivot.

The following example shows this workflow in the context of a moodily lit and shot image of a woman contemplating the choices she’s made. The client would like a naturalistic treatment in keeping with the project’s “70s independent film” aesthetic, which you might take to mean no crushed blacks, no harsh whites, and a fairly linear color balance throughout the tonal range, where possible.

  1. As usual, you’ll normalize the log-encoded clip using a LUT, or manually, in order to get the desired starting point for your grade (Figure 4.84). You’ll want to do this in a layer or node after or on top of one or two initial adjustments you’ll use for grading, depending on how your application is set up.

    Figure 4.84

    Figure 4.84 A CinemaDNG clip shot with the BMD Cinema Camera debayered as log-encoded media and after normalizing with a corresponding LUT from Blackmagic Design.

  2. Make any necessary contrast adjustments as an operation before the normalization operation, in this case lowering Master Offset to the desired black point and using the Contrast and Pivot controls to expand contrast to push up the highlights of the signal to indicate sunlight streaming in through the window (Figure 4.85). On some control surfaces, such as the DaVinci Control Surface used for Resolve, Master Offset is mapped to a ring control around a fourth trackball.

    Figure 4.85

    Figure 4.85 The image after making master Offset and contrast adjustments to a correction inserted prior to the LUT operation.

  3. In this case, stretching contrast has made the image extremely warm, but this is supposed to be a noon-day image. Consequently, the client would like a more neutral treatment, with natural skin tones. This can be achieved by making an adjustment to the Offset color balance control (using a remapping of one of the existing trackballs, a fourth trackball, or an onscreen control). When making this kind of adjustment, a tip is to make your adjustment so that the dominant subject of the scene—a person’s skin tone, the blue of a sky, or the green of foliage—looks the way you want it to look (Figure 4.86).

    Figure 4.86

    Figure 4.86 The image after a simple color balance correction pulling the Offset control toward blue to neutralize the extreme warmth.

    Because the Offset control simply raises or lowers each of the three color channels in their entirety to rebalance the image, the theory is that once you correct a known feature, such as skin tone, the rest of the image will likely fall right into line (Figure 4.87).

    Figure 4.87

    Figure 4.87 A set of nodes explicitly organized and labeled to show you one possible way of grading log media to exercise exact control over the image, in a deliberately restrained manner.

    This order of operations is illustrated via a series of individually labeled nodes in DaVinci Resolve. Keep in mind that you don’t need to create separate nodes or layers for each operation (unless you like being insanely organized). In particular, since in DaVinci Resolve the LUT is last inside of a node’s internal order of operations, you can apply a LUT and make the Offset Master, Contrast, and Offset Color adjustments all within a single node. Figure 4.87, however, shows the node’s internal order of operation artificially externalized.

The result, assuming you want a naturalistic grade, can be a simple color balance that lacks the kind of oversaturation in shadows and highlights that can give away a video image where you’ve independently adjusted the shadows and highlights. Again, Offset is similar to the printer points adjustments that color timers used for decades to balance films, and if you’re careful and grading competently shot material, the results can be remarkably cinematic in their simplicity.

Using Shadow/Midtone/Highlight Controls

Of course, sometimes you’ll end up contaminating the color of the highlights and shadows when making an Offset adjustment, especially when you deviate from a natural treatment of the image as it was, to create an exaggerated color balance. For example, if you’re grading an actress with very pale skin tone and you decide to add some life and saturation to her skin, you can end up exaggerating color throughout the rest of the image as well.

In the following example, rather than giving life to the skin tone, the client wants more of a deathly pallor to the lighting scheme illuminating the zombie attack (Figure 4.88). The media is log-encoded, and following the workflow of the previous section, you use the Offset color balance control to put some green into the lighting.

Figure 4.88

Figure 4.88 Before and after adding green via the Offset color balance control. The shadows are contaminated with green as a result.

As a result, there’s green in the shadows as well. However, in these cases, your grading application’s Shadow/Midtone/Highlight color balance controls are there to help you by allowing more specific adjustments that take into account the unique tonal characteristics of log-encoded media. Figure 4.89 shows an approximation of how the default ranges of the Shadow, Midtone, and Highlight controls divide the tonal range of a log-encoded image.

Figure 4.89

Figure 4.89 An approximation of how the Shadow, Midtone, and Highlight controls affect tonal zones based on the original log-encoded contrast for the purposes of color balancing.

As you can see, when used with a log-encoded image, the color interactions between each adjustment overlap softly. However, the changes you make are much more specific than those made using the Lift/Gamma/Gain controls, on the premise that you’ll want to be making narrow corrections to fix color contamination, while leaving the rest of the signal adjustment you’ve made as linear as possible (Figure 4.90).

Figure 4.90

Figure 4.90 The final grade, after pushing the Shadow color balance control toward magenta to neutralize the green in the darkest parts of the image.

Furthermore, as with Log contrast controls, the boundaries of color adjustment where the shadows end and midtones begin, and where the midtones end and the highlights begin, are adjustable using pivot, range, or band parameters (names vary by application) to change the center point of image tonality at which each adjacent pair of color balance controls overlap (Figure 4.91). This gives you added flexibility to apply more specific contrast and color adjustments.

Figure 4.91

Figure 4.91 The Shadows, Contrast, and Highlights pivot parameters, with their accompanying graphical controls that show the center point of each pivot as a dotted blue line on the LUT graph, which shows a graphical representation of the adjustments being applied.

Obviously, the Shadow/Midtone/Highlight controls are nonlinear in nature, since they allow differing color adjustments to the highlights and shadows independently of one another. As a result, this is still cheating if you’re looking to grade like the color timers do. Pragmatically speaking, since these controls are calibrated for log-encoded media, such targeted adjustments let you combine the best of both worlds, providing a cinematic base grade with digital refinements for fixing specific issues that need solving.

Continuing After a Log Grade

Once you’ve made an adjustment using Log mode controls along with a normalizing LUT or curve adjustment, you can always apply additional operations to the normalized image, using lift/gamma/gain, curves, and any other controls you like to make further alterations to the now normalized image.

In fact, you can also use the Log color balance controls on normalized images, but the results will be slightly different. Because the Log controls are calibrated to work on a very narrow tonal range, their effect on normalized pictures will be more highly specific than with log-encoded pictures. In Figure 4.92, you can see an image that’s already been normalized, with a wide range of image tonality from dark shadows to light highlights.

Figure 4.92

Figure 4.92 A normalized image with wide contrast.

Figure 4.93 shows the result of pushing the Highlights color balance control toward yellow on a normalized image. The resulting adjustment made to warm up the highlights of the image affects only the very brightest parts of the image (Figure 4.93).

Figure 4.93

Figure 4.93 Using the Highlight color balance control of the Log controls to add yellow results in a very specific adjustment when applied to a normalized image. While not necessarily suitable for general correction, this can be very useful for stylization.

Used in this way, the Log controls are very effective for inserting stylized color adjustments into very narrow zones of image tonality, especially when you take into account that most log controls can be altered using pivot or low/high range parameters, so you can customize the tonal range of the image you’re affecting.

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