Off The Charts

CHARTS

A couple of columns ago, I answered a question for someone who wanted to be properly prepped for an upcoming color-grading project. I mentioned that it was important to make sure the colorist had the right information about the footage. This led to a discussion of formats, camera types, profiles, etc. What I didn’t mention was that you can prepare a visual reference so the colorist has an idea of what the camera captured. One way of doing this is by shooting a chart.

Charts have been around for a long time. They can have a number of different uses and with those different uses come different designs—some highly specific and some more general purpose. Maybe you’ve seen charts in the plethora of camera comparison tests that are almost ubiquitous on the web. These may be custom-made or not. Less prevalent are charts that are relegated to a setup bench or lab.

Historically, charts have some connection to test signals like color bars and the test patterns that are alluded to in older television shows when TV stations “ended their broadcast day.” That connection is about trying to measure or evaluate the production chain. You can measure the fidelity of the entire process by putting a known good signal at the beginning of the process and comparing the resulting signal to the original.

Test signals have become more and more complicated—or maybe the word is clever—in order to deal with more complex signal paths. Just think of the dizzying number of recording formats, compression schemes, resolutions and delivery codecs. Charts can also be used in a manner similar to test signals. If you think about it, where is the beginning of the cine-production process? The lens and camera. I’ve written previously about DSC Labs’ ChromaDuMond Billups VF/X chart (dsclabs.com). I talked about how it can be used to evaluate chroma processing and how it can show changes in subsampling (going from 4:4:4 to 4:2:0).

There’s another use for charts: communication. A properly lit, appropriate chart, captured and traveling with the footage, can be used to communicate with your colorist. Cinematographers shooting film have been shooting charts for decades. They know that the charts are critical to communicating with the graders so grading can accurately represent the scene captured. Today’s digital cinematographer has a daunting number of choices for capture: S-Log, C-Log, Log-C and RAW are just a few of the growing number of options. (I won’t go so far as calling them digital film “stocks,” as that really depends on a camera’s image sensor, processing and setup.)

While a person might drill down into the metadata to figure out how the camera was set up, they wouldn’t get the full picture on exposure. Shooting a chart can be helpful in filling in that picture. The need for charts to help fill in the picture is something manufacturers have recognized. DSC Labs has their One Shot™ Plus chart, Datacolor has their SpyderCHECKR 24 chart (spyder.datacolor.com), and X-Rite has their ColorChecker Video chart (xritephoto.com). I’ll examine the anatomy of one of these charts, the ColorChecker Video.

The ColorChecker Video chart has a series of black-and-white and color patches on one side and a white balance target on the other. The color side is where this “communication” can happen. In the center of the chart are four large grayscale chips ranging from full white to a high-gloss black. The two gray values are dark gray and 40% IRE gray. Complementing them is a linear grayscale series. With these grayscale values you can “communicate” your setup’s dynamic range. The values will also help indicate any changes in color reproduction throughout light level changes—like darker tones-going-green or brighter tones-going-yellow.

On the color side of things there’s a series of color patches from light to dark that helps evaluate possible reproduction of skin tones. On the other side of the chart there are saturated and desaturated chromatic colors. (Chromatic colors are colors that have a dominant hue. Gray, black and white are achromatic, while red, green and blue are chromatic.) The particular colors chosen for this series line up with the markings on a vectorscope. Additionally, to help make sure the chart is evenly lit, there are two sets of black-and-white chips to measure for even illumination—one pair on the lower left and one pair on the upper right.

A properly lit chart is no small matter. You have to make sure the chart is lit evenly. Falloff of light on one side of the chart or from top to bottom will skew how the grayscales are represented. Other charts have similar features, some with more capabilities and some with less. Once a frame is captured at each camera setup, a colorist now has data to start building a base grade. But there are also options for automation here. More and more color-grading software packages (like Resolve and Color Finale) can recognize the color patches on many of these charts and can automatically bring the shot into a neutral starting point. While this won’t take the footage to a final grade, it can be a quick way to bring similar shots closer to a base.

One last word: The usefulness of a chart is dependent on its proper use. It would be easy to say that the chart should be placed to fairly represent the scene. But that can be a tricky statement to make. For example, when you shoot a scene lit by a bonfire, if the chart simply represents that scene, the illumination of the chart will be from the very warm light of the fire. If the colorist sees this chart out of context, he or she will proceed to neutralize all that color. Instead, you should light the chart for the color balance the camera is set to.

AUTOMATIC “EXPORTATIONS”

Q My edit software of choice has a growing set of options for exporting my edit. When I was first doing projects, I was sort of able to just set it and forget it. Now I find I need to create different exports for use on one platform or another. When I look at some of the specs for upload, they don’t much match what I see in the presets. Without going into a laundry list of different options, is there some trick to picking the right one, something to look for?
Dave. C.
Via email

A I think your last four words are key to answering your question. First, there are a few simple things that can be obvious, but need to be stated. For certain, you’ll need to pay attention to resolution and frame rate specifications. Those are usually spelled out pretty simply. Then there are the issues of file size and bit-rate. Fortunately, software is capable of estimating file size. Remember, however, that an estimate is just an estimate. You can do some test exports early in the project, overnight or over lunch, to gauge actual file size. An export test run will also give you a handle on how long the compression will take. That can be a useful piece of information if you’re working on long-form programs and/or if you have a tight deadline.

Sometimes the bit-rate found in presets can be generous. For example, the bit-rate might be twice the size that you really need. While there won’t be a problem as long as the bit-rate doesn’t exceed the platform’s specs, it will increase upload time and storage availability. And this gets back to your last four words: “something to look for.” It’s important to realize that presets are just a starting point. More importantly, compression isn’t a “set it and forget it” process. You have to watch the compressed file to make sure it’s of the quality you need. Just watching the first few seconds and making sure there’s sound doesn’t do it. Watching and listening from beginning to end may be bothersome because you’ve lived this show for the last days, weeks or months, but it’s necessary.

This final check is where you might notice things that you would never see when editing in a higher-quality codec. For example, I saw a piece recently that was edited with graphics utilizing a 4:4:4 codec, but the interim preview codec was 4:2:2. For some reason, this wasn’t an issue during the edit, but when it was exported for online use to a file format with a different codec, the background exhibited errors attributed to chroma subsampling. Without watching the piece after export and before uploading, the error would have been in the online product. While presets are a good starting point, testing and watching are a better method to get to the finish.

To have your video production technical questions answered, send an email to [email protected]

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