Have you ever wanted to move a primary grade from one system to another one. For example you are on set and do a basic grade on the footage using “Iridas Speedgrade On Set” just to get a look for the Dailies, but when you get to the final grade you are using another grading system which can’t read the Iridas project file. There is a solution for this, it’s the Color Decision List.
ASC CDL (American Society of Cinematographers Color Decision List) is a format for the exchange of basic primary color grading information between equipment and software from different manufacturers. The values that are transferred are Slope, Offset, Power, and Saturation. Secondaries are not included in the CDL, so you can’t transfer a final grade, just the basic levels.
The ASC CDL does not handle everything necessary to communicate a look. The dailies colorist and the final grade colorist must communicate information like color space, data representation format, display device, LUT and viewing environment in order to match the images between the different systems.
Pro / Con
Why would you want to transfer these settings between the offline and final grade?
For one it may save time in the expensive grading suite, just leveling out takes quite some time.
I have had some DOPs and Directors coming in to the grading suite telling me they want the look they had on the offline material, which takes some time to create. With a CDL I would have saved a lot of time which I could have used to do the creative stuff.
Many times the offline looks not so pretty, but they got used to it during the offline process, so wouldn’t it be nice if the offline material had a nice grade applied to it. The good thing is that the time spent on creating good looking dailies will shorten the time you need to level the shots when doing the final grade since the settings are transfered.
Depending on the monitoring, LUT etc. the offline look may not match the look you get when conforming the original material into the final grading suite and applying the CDL. This is why it’s very important to test the whole pipeline before going into production.
How does it work?
The three basic transfer functions – Offset, Slope, and Power – are applied in the order Slope, Offset, then Power and are sometimes referred to collectively as SOP. The transfer functions are in RGB color space and can be applied in the linear or log domains to each color component. The three transfer functions for the three color components (R, G and B) can collectively be described by nine parameters.
The tenth parameter is Saturation. Unlike the Slope, Offset, and Power functions, Saturation operates on all three color channels in combination. The ASC CDL uses a common industry definition for Saturation (common Rec. 709 weightings). Saturation is applied after Slope, Offset, and Power.
In a CMX EDL with CDL information you may find the following COMMENTS
*ASC_SOP (1.0 1.0 1.0)(0.0 0.0 0.0)(1.0 1.0 1.0)
The first row describes the Slope, Offset and Power for each R, G and B.
The second row describes the saturation.
Slope changes the slope of the transfer function without shifting the black level. The default slope value is 1.0
out = in * slope
Offset raises or lowers overall brightness of a component. It shifts the transfer function up or down while holding the slope constant.
If the underlying data is log, then Offset is an interpretation of printer points – the most common
method of color correction in film lab work. The default offset value is 0.0
out = in + offset
Power is the only non-linear function. It changes the intermediate shape of the transfer function. The default power value is 1.0
out = in ^ power (where ^ is “raised to the power”)
The saturation operation modifies all color components. Color components are weighted by the values most commonly used in Rec. 709 implementations of saturation. The default value is 1.0 and 0 is total desaturation (grey).
ASC CDL INTERCHANGE FORMATS
The CDL information can be transferred in a couple of different file formats already used in the industry. The formats that are supported are ALE, FLEx, CMX EDL and XML.
ALE and FLEx files are used to transfer information available at the time of dailies creation to the editorial database.
CMX EDL files are output from editorial and used primarily to “conform” the individual shots into the final edit. The COMMENT field is used to attach ASC CDL parameters to each “event”. There are two ways of specifying this in an EDL file – either “inline” or “via XML reference”. “inline” puts the SOP and SAT values directly in the EDL, where as “XML reference” points to an XML file with the SOP and SAT values.
The XML specification defines two types of files:
1. a Color Decision List (CDL) file that contains a set of color decisions (a color
correction with a reference to an image) and that may also include other project
2. a lightweight Color Correction Collection (CCC) file which solely contains one or
more color corrections. This is used in together with a CMX EDL
In the next article about CDL I will go through the file formats more in detail.
© 2011 Nikolai Waldman