Video Color Correction – Part 1 – LUTs

Part 1 – Look Up Tables (LUTs)

In the big world of filmmaking and photography, where every image tells a story and colors have the power to open up different worlds and evoke emotions, there is one tool that makes a significant contribution to making it all work – LUTs.

But what exactly are LUTs? And are there different types? These questions and many more will be answered here – I’ll answer your LUT questions!

LUTs briefly explained?

Let’s start with the basic question: What exactly are LUTs?

I’ll try to explain this as simply and as graphically as possible.

Let’s imagine we have a magical picture book. This book is full of different pictures and we can decide what each of them should look like. Let’s close our eyes and think of the colors or the mood we like best for our picture – perhaps a warm summer look or a dark autumn look? Most of us will already have certain colors in mind based on these small descriptions.

LUTs or look up tables are like spices. They specify how to change the colors to match our vision. It’s like “one-click color grading” for videos and photos. These types of LUTs are called creative LUTs. They help us to implement creative decisions quickly and easily.

But let’s assume that we want to share the pictures with others afterwards, but that they each have different types of picture books. This is where the conversion LUTs come into play. They act like translators. They make it possible for our images to look almost exactly the same in our friends’ picture books as they do in ours. Conversion LUTs are look-up tables that can convert our footage material to different image standards such as Rec. 709.

Unlike creative LUTs, conversion LUTs have the task of creating an image that is as “distortion-free” as possible. Therefore, there is usually no color shift within the footage material.

However, there are many other types of LUT and areas of application. However, we will concentrate on these two types as they are among the most important in post-production.

Creative LUTs – Looks with just one click

The application of creative Look Up Tables (LUTs) goes beyond simple color correction and enables a profound artistic influence on the image aesthetics. Creative LUTs are often used to create certain movie looks or emotional atmospheres. One example of this is the classic “orange and teal” look, which enhances contrasts between warm and cool tones and is often found in blockbusters. The color harmony is particularly emphasized in such a look. The fact that orange and teal (teal = blue-green hue) are on opposite sides of the color wheel and are therefore complementary colors, makes an image stand out very strongly when they are combined. Incidentally, this is a method that is very popular.

In addition, creative LUTs can introduce subtle nuances in the shadows, highlights and midtones of an image to enhance depth or create a

to emphasize a certain mood. A filmmaker or photographer can thus influence the visual presentation of their work with little effort.

In the creative industry, custom LUTs that are tailored to a specific personal style are also created often. This process requires a deep understanding of color theory and the technical application of such.

Example of a creative LUT

Creative LUTs – use with caution…

In general, creative LUTs should be used with caution, as many can have a negative impact on the quality of your own image. Most colorists work without any creative LUTs in post-production. This gives them full control over the image and allows them to see what happens to the material step by step.

Creative LUTs – Overview

• Can quickly give pictures a creative look
• Are easy to use
• Can influence hue, contrast, saturation and luminance

Risks:

• LUTs do not work equally well with every clip and every camera
• Can lead to artifacts, clipping and noise
• Can make videos look exaggerated and unprofessional if used incorrectly

Example of a problem when using a LUT – on the left side of the picture you can see on the gray gradient map that certain black values are already lost due to the LUT

Conversion LUTs – Application

Conversion LUTs (Look Up Tables) are used in the post-production of images and videos to perform color transformation processes. Unlike creative LUTs, which aim to create a specific visual style, conversion LUTs are mainly used to perform color space conversions and ensure that the visual material is displayed consistently across different devices and platforms.

1. Color space conversion

Top image, Sony SLog3, bottom image Rec.709 conversion

If different devices or software programs use different color spaces, color shifts can occur due to incorrect color space interpretation. Conversion LUTs help to translate the color values from one color space to another to ensure that the color representation remains correct.

Sony SLog 3 – you can see from the scope diagram that the colors are compressed (log) and do not use the full dynamic range.

Rec.709 Conversion. Das Scope-Diagramm ist nicht mehr so gestaucht, das Bild wirkt deutlich ansprechender. Der Dynamikumfang wird deutlich mehr genutzt.

2. Standards

In film production and photography, there are various color spaces such as DCI-P3 for cinemas, or Rec.709 as the most common video standard for web and television. When material is transferred between different production steps or platforms, a conversion LUT is often used to ensure that the colors are displayed correctly and meet the desired standard.

3. Log and RAW footage

When processing RAW files from cameras, conversion LUTs are used to convert the data into the desired color space and output color format. Most camera manufacturers have their own conversion LUTs for their various cameras and sensors. Since each sensor has a different color reproduction, these proprietary conversion LUTs (e.g. Sony SLog3 to Rec.709) are the easiest way to perform a conversion from a log profile to Rec.709.

Overall, conversion LUTs help to ensure that the color representation in post-production remains more accurate and consistent, regardless of the many technical differences between different recording devices and platforms on which the content is viewed.

Raw material from the camera

Conversion LUTs – Risks

Unfortunately, even conversion LUTs only provide a rough guide to “correct” colors. There are simply too many factors on set that a manufacturer cannot take into account when creating LUTs. A few examples are different lenses. Each lens has a slightly different color reproduction, which can have a strong influence on the image. The same applies to so-called ND filters, which are very often used in the video sector. Neutral density filters reduce the light falling on the sensor. These often create a color cast (usually slightly greenish or magenta) and thus also influence the tonality of the raw material.

Material converted to Rec.709 standard with Conversion LUT

The only way to achieve consistency between different cameras is to use color cards directly on set.

Every camera/lens/filter combination is that much different that a one-size-fits-all LUT solution often does not bring the result desired. Even changing the ISO value in the camera can lead to a change.

About the author – Marco Schreiber

Marco Schreiber is a professional videographer, cameraman, colorist and DaVinci Resolve trainer with more than 10 years of experience. Starting with TV productions, he also has extensive expertise and diverse experience with YouTube and social media. He currently works mainly for companies and in the sports sector.