A Guide to Working with LUTs
In recent times, you’ve probably heard the term “LUT” (short for Look Up Table) used ambiguously. There are often misunderstandings and assumptions about LUTs, how they work, and their usage. Here’s what you should know.
Here, we’ll discover how proper use of LUTs can benefit the production and color grading workflow. In order to understand this article, we suggest you be familiar with or at least have a basic understanding of the following terms:
1D and 3D LUT
Colorspaces such as Rec 709
Logarithmic Curves/Log profiles
There are two primary uses for LUTs.
Technical colorspace transformations convert an image from one color space to another — for example, LogC to Cineon/REDlogFilm or Rec.709 to DCI-P3.
More relevant to this discussion are creative transformations, which are purely creative and intended to produce a subjective look. Any transformation that converts from camera log (or scene-referred) to Rec.709 (or output referred) falls in this category. Creative looks can be created in just about any color correction software, including Resolve, Baselight and Speedgrade among others.
There’s no magic in LUTs.
LUTs aren’t smart. There’s nothing dynamic or changing about them. LUTs contain pre-baked transformations, but the files themselves are ignorant of your source and destination colorspace. They are not algorithms that interpret your footage dynamically.
The mystery and misunderstanding of LUTs seems to have led to high demand of creative LUTs and thus creating a strong marketing strategy for many small companies and colorists to sell grades and looks.
Whether you choose to use a LUT or not depends on your personal choices and workflow as well as the needs of the project.
Whether you choose to use a LUT or not depends on your personal choices and workflow as well as the needs of the project. Typically these LUTs are nothing more than just a basic grade, but they can sometimes be more involved, such as with Film Print Emulations (FPEs). FPEs are very difficult to replicate from scratch. In general, creative LUTs can be useful in a pinch but really aren’t something you necessarily need to spend your money on
Camera manufacturer supplied conversions, such as Log to Rec. 709
These are often free and are released from each camera manufacturer for their log profiles.
ARRI has a LUT generator that’s accessable here(link is external).
RED has released a few transforms from REDlogFilm to REDgamma3 and REDgamma4, while a third party has reverse engineered the complete set and produced inverses as well, which can be downloaded here(link is external).
These LUTs are purely creative and are basically the camera company’s guess at what they they think will make the footage look best. Many colorists and DP’s reject manufacturer supplied Rec. 709 LUTs because it’s not how they want the final image to look. In the same vein, the common use of these LUTs are why many people are under the impression that each make and model of camera has a specific, inherent “look” – but the purpose of the raw or log capture is to record the scene's exposure information as accurately as possible.
Scene Referred vs Output Referred
Before going any further, let’s take a look at the differences between scene referred and output referred workflows, two generalized ways of encoding image data.
Scene referred imagery is essentially anything that has a direct correlation to the way light behaves in the real world. However, this is not to say that it is how our eyes respond to and interpret light — only how light itself is measured. If an image is encoded in RAW (literal sensor data) or Log (a compressed and efficient encoding which gives about the same information as RAW) then it is scene referred.
Output referred imagery does not carry a relation to real world exposure values, meaning that it’s an interpretation of what is happening.
Two Ways to Grade
Now let’s look at two ways of grading an image:
In Diagram 1, we are taking a LogC image and viewing in a Rec709 colorspace. The final output is Rec. 709 even though no LUTs were used. This is a perfectly acceptable way of doing things but can limit what you do if you ever have to master in a larger colorspace, and if nothing else, can cost you some time if you ever have to master in another format, especially when other colorists are involved or you don’t have access to the original project files.
In Diagram 2, we have a LogC to Rec709 transform LUT at the end of the pipeline, and we are making grades before (under) this LUT. What’s important to note is that from here you can then turn off your LUT and export a log master, which can help you down the line.
The rest of this article is available on Videomaker magazine's website. Please give it a look!
Published in the May 2017 Issue of Videomaker.