The American Society of Cinematographers

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Return to Table of Contents October 2008 Return to Table of Contents
Body of Lies
Production Slate
Post Focus
Key Terms
DVD Playback
ASC Close-Up
An Overview of the ASC CDL

The transformation of the traditional photochemical workflow into a film/digital hybrid workflow has complicated the path that features take from the set to the screen. The ASC Technology Committee was founded in part to respond to the problems created by this increasingly complex workflow, and one of the committee’s most significant achievements so far has been the creation of the ASC Color Decision List (CDL).

Analogous to the edit decision list, which lists the time code in- and outpoints of a video edit, the CDL is a list of the primary color corrections to be applied to an image. Four parameters constitute the CDL: slope, offset, power and saturation. While saturation has only one value, each of the other parameters comprises a red, green and blue value, making for 10 CDL numbers in total for each footage segment.

“The movement to create the CDL came from a knowledgeable user base that said, ‘Enough is enough! We can’t function this way!’” says Curtis Clark, ASC, chair of the Technology Committee. Among the many contributors to the development of the CDL were Lou Levinson, chair of the Digital Intermediate (DI) Subcommittee; Joshua Pines, vice chair of the DI Subcommittee; David Reisner, secretary of the Technology Committee; and Glenn Kennel, chair of the Display Subcommittee.

Pines, the vice president of imaging research and development at Technicolor Digital Intermediates, calls the CDL “a way of giving creative control back to the cinematographer,” adding, “Just like the director has first cut, the cinematographer should have first look.”

“At its core, the ASC CDL was designed to move a very simple set of color corrections across platforms,” says Levinson, a colorist at Post Logic Studios. “The question is: if you generate looks you like in the beginning, is there any way you can get them to propagate through the project, even though you’re going to different facilities that have different hardware and software? When you’re doing a feature, especially in an all-electronic post environment, dailies might be done in one place, preview screenings assembled somewhere else, and the final DI carried out in a third facility. Also, the dailies might be done on a Pogle, the preview screening on a da Vinci, and the DI on a Lustre. It’s hard enough to get look management exported across the hall in your own building!”

Levinson emphasizes that in order for the CDL to work, a production must decide on the project’s monitoring and color space. There are two main choices: 1) film emulation, a look-up table (LUT) used to simulate a film print on a video monitor, and 2) Rec 709, standard color space for a hi-def video signal.

The slope (S), offset (O) and power (P) parameters of the CDL are derived from the basic controls available on most telecine color-correction systems: lift, gain and gamma, respectively. Each of the CDL’s parameters (sometimes referred to collectively as SOP) converts an input to an output according to a transfer function that can be plotted as a line or curve.

If the video signal is in logarithmic (log) color space, the red, green and blue offset values are equivalent to printer points used in photochemical color timing. Some cinematographers elect to only work with offset. “If you want to go through calibration cycles with a particular film lab, you can correlate to printer lights fairly accurately,” says Levinson. Further, Pines notes that slope can be seen as the equivalent of the gamma of the film stock, but power has no easy photochemical equivalent.

The CDL is limited to correction of the three primary color pairs where 90 percent of a colorist’s work typically happens. “If you change red, you are also changing cyan,” notes Levinson, “whereas at a more exotic stage, you can change red without changing cyan.” The CDL does not address corrections to portions of the frame.

Noting that the CDL applies to both log and linear (lin) data, Pines likens the CDL to a black box “doing mathematical operations on data. It is signal-agnostic, leaving the interpretation of the number and how to display it to someone else.”

Clark, Levinson and Pines emphasize that the ASC CDL can only work properly with rigorous testing and supervision of the entire post workflow. During prep, says Levinson, the production must run test footage through every facility that will be involved in post to ensure the compatibility of the CDL data. “You need all the vendors who are going to be a part of [the production] to prove it will work before you start [shooting],” he says.

“We keep reminding people the CDL does less than they think it does — there are a lot of other things that go along with it that the facility has to take care of,” he continues. “If you’re doing a DI in 709 space, it’s up to you to keep all the monitors calibrated, and if you’re doing film emulation, you have to have it correct at every stage of the game. There’s a lot of housekeeping that falls to the user.”

The CDL validates two separate strategies for storing its 10 numbers in a file that follows the image throughout the post pipeline: piggybacking onto existing file formats or creating a new one in XML. Clark notes, “Everyone is migrating to being XML-compliant, and down the road, it may be the common file format, but you’ve got to accommodate the file formats in use now. If you can’t associate the color correction of shots on the set with an EDL, then they become lost.”

According to Levinson, most productions are inserting ASC CDL data into existing FLEX, ALE or CMS files, and FLEX is the preferred format for CDLs that start in dailies. Pines notes, “The CDL is physically attached to source material in FLEX or ALE files before editorial, and it is attached in the CMX EDL after editorial. The idea is that eventually the XML will be extended, adopted and supported.” However, current XML implementations are often incompatible between vendors, and to date, there has been little dialogue with the makers of Final Cut Pro and Premiere.

Levinson surmises that most applications of the CDL have begun with telecine dailies of film projects, though a few pioneering projects have also used the CDL on digital and film productions with on-set previsualization boxes (such as those offered by Assimilate, FilmLight, Gamma & Density, Iridas, Panavision and Technicolor). Many of these on-set solutions include a 3-D LUT (or viewing transform), giving a visual indication of the look of the final print during the shoot. Pines stresses, however, that the CDL does not do film emulation. “If you want to use the CDL for dailies applications, you’d better put it through the same transforms as those used on set,” he says. He adds that a standard has yet to emerge for getting the CDL from an on-set box to dailies, but several companies are working on such a transmission.

Visual effects comprise another possible area of future growth for CDL applications. “What you think of as a simple effect is incredibly dependent on people at both ends of the chain, in different facilities, seeing exactly the same thing,” says Levinson. “We could see more exotic previz coming out of applications such as Maya, which could potentially export CDL information.”

The CDL is an example of metadata — data about data — and this means that while the color-correction data is associated with the image data, the image itself is not necessarily corrected. “The CDL was designed as an inherently non-destructive process,” notes Levinson. “You can choose to bake [the corrections] in for a particular delivery, but it doesn’t happen automatically.” Clark adds, “You can bake the corrections into Avid or Final Cut Pro files, but you still want to have a separate capability to associate that color-correction data with its corresponding image and alter it later — for example, for previews or the DI.” (According to Levinson, discussions are ongoing with Avid regarding the potential for assembling the CDL color corrections for display within editorial — in other words, having a CDL-conform button in the Avid that could turn the corrections on and off.)

Detailing the possible application of the CDL workflow for HD preview screenings, Clark hypothesizes, “If you want to take the CDL color corrections done for the dailies and make adjustments for shot-to-shot variances in the cut, you can use the same CDL functionality in compliant applications. Then, in principle, the information associated with those images and that preview cut will make its way to the final color-correction session.”

The CDL should save valuable time in the DI suite, but Levinson says he sees it “more as a control for your own look management. But if you give me raw shots and a CDL, when I punch conform on a Baselight or Lustre, for example, all the images for a particular film reel will, in theory, come up in the right place with their color corrections. So you’re not working backwards; you’re picking up where you left off.”

To date, the CDL has proven to be “a robust foundation for color-management workflow,” says Pines. Looking forward, Levinson says, “We hope it will become a functional piece of a larger, unified metadata scheme. It will include a way of exchanging LUTs without giving away intellectual property.

“The CDL is a fairly simple thing, and it’s doing what we intended it to do,” continues Levinson. “We put it out there as a good idea, and a number of vendors have already adopted it, which is wonderful. The game now is to get it out there so people can use it easily.”

The ASC is working closely with the Academy of Motion Picture Arts and Sciences and the Society of Motion Picture and Television Engineers to continue to address post problems, and Clark says they are turning their attention to defining standards for LUTs and a post file format. “I see the ASC CDL as the first wave,” Clark opines. “The LUT and the file format, both equally important, are the second and third phases of regaining some kind of coherence in this increasingly complex and frustrating workflow.”

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