American Cinematographer - October 2008 - (Page 74)

Post Focus An Overview of the ASC CDL by Benjamin B 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.” 74 October 2008 “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 corre- late 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

Table of Contents Feed for the Digital Edition of American Cinematographer - October 2008

American Cinematographer - October 2008 Contents Editor's Note Short Takes: DirecTV's "Fourth Wall" Campaign Production Slate: Prison Break Production Slate: Need for Speed: Undercover Middle East Intrigue Kingdom of Light Memoirs of a Spook Landmark Gains Post Focus: An Overview of the ASC CDL Filmmakers' Forum: Roger Deakins, ASC, BSC New Products & Services International Marketplace Classified Ads Clubhouse News ASC Close-Up: Ross Berryman

American Cinematographer - October 2008

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