TechTalk Interview: Understanding .Look Files

In 2003 IRIDAS introduced the first non-destructive color grading technology in the film industry with the launch of the first version of SpeedGrade. Within this application was a technology that has generated great excitement – and sometimes a little confusion: the .Look file format.

In this TechTalk we interview IRIDAS founder and CEO Lin Kayser about the concepts behind .Look and why IRIDAS made specific design choices.

IRIDAS Magazine: With so many LUT formats out there already, why has IRIDAS created yet another file type?

Lin: When we began work on the color pipeline in 2002, our goal was to create a simple means of communicating the creative look to all of the players in a movie production. We wanted to to offer a way of transporting the actual parameters that went into a color correction algorithm.

– But that would have meant that everyone who wanted to support the format would have to implement the IRIDAS algorithms.

True. While it is great to be able to see, for example, that somebody adjusted the saturation or turned the temperature towards the “warm” end of the scale, we cannot reasonably expect that every tool in the pipeline will support our algorithms. [** That may be possible one day if enough vendors join us in supporting the ASC’s Color Decision List **]

We wanted to go further though and give people access to all the creative tools in SpeedGrade. That’s why a 3D Look Up Table is included within each .Look file.

– So a .Look file contains both a set of parameters and a 3D LUT?

Exactly. The 3D LUT shows the results of all the color adjustments, even exotic ones like Bleach Bypass or Cross Processing. A 3D LUT is a substitution list for colors: for every color in the spectrum, it defines a replacement color.

– Why are you including the parameters then, if the 3D LUT already represents the same color transformation?

A 3D LUT is useful to a point, but it does not tell us which operations were used to create it. I cannot analyse the operations, tweak them or undo any of them. If I want the look to become the basis of the final grade, I need the actual parameters. That way I can look at it in the DI suite and adjust the settings.

– Couldn’t you use the 3D LUT as the basis and then grade on top of it in the DI suite?

This is done quite often but this approach has problems. Applying a LUT is a destructive process. Once I have replaced one color with another, there is no way I can get back to the original data. At the risk of oversimplification: If I replace green with blue, how do I tell which blue area was green before?

In addition there is a technical limitation inherent in any 3D LUT operation: 3D LUTs become very big very quickly. A 3D LUT for 8 bit color depth means 16.8 million entries (8 bits have 256 shades of color, and I have three colors: R,G,B. The result is 256 to the power of 3 = 16.8 million). If we move up to 10 bits of color information, the minimum for serious color grading work, we have 1024 to the power of 3: one billion entries. That is more data than even a high-end workstation can load into RAM. 3D LUTs attempt to solve this problem by dividing the color space into smaller segments. Values between these segments are interpolated.

The bottom line is that a 3D LUT is only an approximation of the result of the color grading algorithms. By grading on top of a 3D LUT, you are sacrificing quality.

– So how does the IRIDAS approach work in practice?

We store the actual SpeedGrade color grading algorithm parameters in the .Look file. All of our applications, including FrameCycler only ever look at the parameter section of the .Look file – the adjustments which have been applied to create the look. Other applications and devices that support .Look files use the 3D LUT section of the file, which approximates the result of these adjustments.

– So a third party vendor can support .Look without having to understand the algorithms?

That’s the advantage of our approach. We prefer an algorithmic representation of the color transformations, but we wouldn’t expect other vendors to keep up with all the algorithms we develop – and we don’t want to share all of our secrets!

Summary

A SpeedGrade .Look file is more than just another 3D LUT format. It contains two – or three – sections, all of which reflect the same color transformations. The parameters section reflects the individual color grading algorithms which are used by all IRIDAS applications. The 3D LUT is used by 3rd party “monitoring-only” devices and applications for reference and review. In addition, if users choose to work with the ASC Color Decision List subset of color correction tools, the .Look file will also include a section with the CDL parameters.

A reference JPEG image accompanies every .Look file as a quick visual reference. The JPEG can be distributed independently of the .Look file

Thus the SpeedGrade .Look file format provides a simple means for communicating artistic intent throughout production, post-production and DI.


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