Day 22: 25 Insights in 25 Days 2019 New Year Marathon!
Meet the ASC-CDL (Color Decision List): The Theory
In a previous Insight, colorist Rob Bessette talked about his experience coloring dailies, after a career in commercial color grading. One of the comments he made was about restricting his color corrections to be CDL-compliant. What precisely did he mean, being ‘CDL compliant’? Why does is it important for dailies colorists to restrict their toolset? I’ll explain the theory behind the Color Decision List (CDL). And at the end of this article we look at why DPs typically combine a LUT with a CDL to achieve their final looks. But before we get there, we need to understand what precisely a CDL is designed to do.
What is a CDL?
The ASC-CDL, more commonly expressed and known simply as the CDL, was created through a joint operation between the American Society of Cinematographers’ Technology Committee, production / post production vendors, and color scientists to standardize the exchange of primary color adjustments. The specification was first released around mid-2007 and is now widely adopted among several software and hardware manufacturers.
The CDL stands for Color Decision List. Just like how an EDL communicates basic edit data, a CDL carries a subset of color-correction data.
Although all color-correction systems offer their own basic grading controls, ‘Lift,’ ‘Gain’, and ‘Gamma’ are implemented differently from vendor to vendor. Their definitions of LGG can vary in sensitivity and response. The strength of a CDL- based workflow is its software-agnostic nature. CDL compliant hardware and software can create, modify, and reproduce consistent color results exported from any other supported platform. In other words, it’s all about the perfect interchange of color modifications.
The parameters of a CDL replace the traditional Lift, Gamma, and Gain you’re probably used to manipulating. However, each set of CDL controls behave very differently from one another!
- Slope is to Gain
- Offset is to Lift
- Power is to Gamma
You may be wondering why the ASC-CDL sequence is expressed as ‘S-O-P’, as opposed to ‘O-S-P’ to match the more commonly known ‘LGG’. That’s because in CDL math, the arithmetic of SOP-S are calculated in that precise sequence. This is extremely important, if you think back to high school math class, you’ll remember that even the slightest change of order of operations will result with something else entirely different!
Slope, Offset, and Power (referenced from here as SOP), are transfer functions applied to all three independent channels of R, G, and B. The end result is a group of nine separate operations in a single CDL; since each SOP operation has three color components.
Those with attentive eyes will notice an extra feature in the above screenshots. That’s right, a tenth designation that solely controls Saturation was added in Version 1.2 of the ASC-CDL, and affects the entire combined result of RGB after all SOP transfer functions have been applied. That’s why it’s called SOP-S.
We’ll have a closer look at these individual operations in a few moments.
Less Is More – What a CDL Isn’t
Just like LUTs, CDLs do not affect any kind of spatial or temporal adjustments – power windows, tracking, secondaries, and all plugins are out of scope. The key takeaway with CDLs is that they are primary-only color corrections, with the added saturation slider.
This primary-only set of controls may initially feel like a restriction, but it is absolutely deliberate. Remember, a CDL is not meant to compete with a full 5 or 10 node color grade. It does not replace a 3D LUT. If anything, it’s designed to side-step proprietary color grade exchange formats such as the .drx (DaVinci Resolve Grade) or .blg (BaseLight Grade). Any software that offers CDL compliance will all generate the same precise results when reading a CDL. What makes CDLs so vastly compatible? Its simplified design. The conversion between SOP and LGG can be easily reverse engineered by software engineers.
However, as powerful as CDLs can be (they are a bit of metadata themselves) they are quite featureless when it comes to communicating other pieces of information. A CDL knows nothing and transmits nothing about the recording format, exposure, and color space of input data. Since the CDL does not communicate any other important details, clear communication and precise look management is essential. Otherwise, matching pictures between post-houses, vfx artists, and finishing artists – and all the different software they use – is nearly impossible. But even with its simplicity the key to success in a CDL workflow: Test, test, test!
When to use a CDL?
Some of you may go through your entire color grading career and not use a single CDL! The reason for this is because not every workflow requires CDLs to begin with. To really appreciate the problems that CDLs solve, we first look at the context you see them being used.
Production is where most CDLs are born. They are created while live grading on set by the DIT while collaborating with the DP. Or while grading dailies at a post house the evening or day after the shoot. At its best, on-set / near-set / dailies grading preserves the cinematographer’s intent as well as protects his or her creative ideas and intellectual property, down the line. Prior to any kind of systematic look management, there were written notes, photos taken on set, and magazine cut-outs to refer to. CDLs are one of the few modern ways to close this creative gap. As with solving other technical problems, CDLs are used to avoid unintentional changes to creative color choices introduced by filters, lenses, and light fixtures.