Making The Match With 3D LUTs

Many people know AbelCine for the scene files we create for various cameras. Last year, I wrote a column called "Making The Match" (HDVideoPro, April 2013), and I outlined my process for creating matching scene files for various cameras. We received a lot of great feedback on that column and have received even more interest in matching files.

Recently, we received a request to make matching scene files between a Sony F55 and a Canon EOS C500. This could be done with the process I outlined in my previous column, but with some of the new features found in the Sony F5 and F55, my answer went in a whole different direction. As of version 4.0 of the firmware for the F5 and F55, the camera has the ability to load in 3D Lookup Tables (LUTs) for viewing on set. This function allows us to heavily modify the color rendition of the camera. Up until this point, 3D LUTs have been used primarily in postproduction applications, but now they can be used on set without any additional hardware. This is a great feature, and we’re seeing it appear in more cameras, including the ARRI AMIRA and Panasonic VariCam 35 and HS. This is a real game changer when it comes to on-set camera matching; keep reading to see how it will work.

Think of Lookup Tables, or LUTs, as a game of Bingo. A LUT is basically a table of values, stored as a basic text file.

3D WHAT?

If you’re unfamiliar with Lookup Tables, or LUTs, here’s a brief explanation of what they are and the different types. A LUT is literally a table of values, generally stored as a basic text file. They serve as a key, of sorts; given a set of input parameters, it specifies an output. Think of it as a game of Bingo; I’m sure the older readers of HDVP will really appreciate this analogy. The Bingo caller shouts out the input, and the Bingo card is your metaphorical LUT. If the caller says "B7" and you have that on your card, then you mark it out.

When you apply a LUT to an image, the individual color values of the image are your input, and they’re "looked up" in your LUT. Say an individual pixel of color comes in with a red value of 1023, or R1023, if you like our Bingo analogy. The LUT will specify that any pixel with that red value should become a different value. For instance, it might change red values at 1023 to a value of 940.

There are many different types of LUTs out there, but most can be broken down into 1D or 3D types. A 1D LUT specifies inputs and outputs for Red, Green and Blue independently of each other. So you can think of it as three rows in our Bingo card.

The three channels in a 1D LUT act independently of each other, which allows you to change things like white balance and contrast easily. However, since saturation changes require colors to be adjusted relative to each other, the 1D LUT can’t change color saturation—kind of limiting, but this is where the ASC CDL (Color Decision List) comes into play. The ASC CDL allows you to make all the adjustments of a 1D LUT along with Saturation changes. It allows for Slope, Offset and Power adjustments to each color channel—Red, Green and Blue. These adjustments change the contrast and brightness of the color channels, and the Saturation adjustment can be scaled up or down on a simple scale.

Both 1D LUTs and ASC CDL support can be found in many cameras today. The ARRI ALEXA supports a modified ASC CDL format, which ARRI calls a Look. Sony has support for 1D LUTs in the F5 and F55, as well as ASC CDL support in the F65. These are pretty powerful adjustments for most applications, but because the color channels are adjusted independently of each other, color can’t be heavily altered using them.

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