CDL 101

The ASC CDL is something a lot of people have heard of. A lot of people use them. Very few people seem to really understand how the format came to be, or how it’s generally used in the industry. Nor do they understand its importance in the production and post pipeline.

Color grading systems have relied upon some generally agreed upon controls for many years now. Foremost among them is the concept of lift, gamma, and gain. A lift control is essentially a level and balance control that applies primarily to the dark areas of an image. The gain control is similar, but applied primarily to the brighter areas. And the gamma control adjusts the midrange color and contrast. At least this is the way most people see it. In fact, one of the problems has always been that these generalized descriptions allow for a lot of interpretation. If we think of a signal as having values from 0 to 1, a “lift” control would be a level control that pivots around 1 and has a limited range. That means that as the control is varied, it affects the lowest values quite a bit, and the highest values very little. A gain control would be the opposite: a level control that pivots around 0, thus affecting the higher values while having little effect on the lower values. And a gamma control would, in effect, “stretch” the center point of the signal, “bending” it up or down. The problem is that all manufacturers have their own interpretations of all of these controls. Some change the pivot points, others limit the control range, and all of them utilize different sensitivities for the controls they provide. A lift control on, say, a DaVinci Resolve system, can feel very different than a lift control on a Filmlight Baselight system. A gain control on a Nucoda Film Master might yield a different “feel” than a gain control on an Autodesk Lustre. These differences led the American Society of Cinematographers to develop a set of mathematical transforms that would manipulate images in a predictable way, but be different than any of the controls being implemented at the time by the various grading system manufacturers, in the hope that all vendors would support the new transforms and allow for an independent, unambiguous way of describing color manipulations through the post process. The result was the ASC CDL system that is commonly used today for color manipulations that can travel through the production and post process and ensure that their results will be interpreted in exactly the same way and yield the same result regardless of what software is being used to implement them.

Rather than use lift, gamma, and gain, the ASC CDL system uses three transforms: offset, power, and slope. While for some these controls might seem similar to lift, gamma, and gain, they are in fact noticeably different, particularly the offset control. While a lift control has a pivot, an offset control does not. It is simply a control that adds or subtracts the same value to or from each original value, thus moving the entire signal up or down in value. In effect, it acts more like an “exposure” control than a range limited control. In many ways, the use of an offset control makes signal manipulation by the ASC CDL more analogous to a “contrast and exposure” control system- which, interestingly enough, is the type of control combination used by many Digital Intermediate systems for a number of years now. It is not illogical for this to be the case, seeing as how many high end cameras today record their images in a log encoded format, much like that of the film scans used for Digital Intermediate work since the 1990’s. Which brings us to the more general subjects of log and video style color grading. I’ve touched a bit on log grading in previous posts, but the discussions of ASC CDL and that of log grading concepts actually go hand in hand. And that will be the subject of my next post.