How to test the quality of a LUT

Look-up tables or LUTs are color-correction transforms that have existed for a long time in the post-production industry. They are used for display profiling, field monitoring, and color correction. In color correction, there are technical LUTs and artistic LUTs. Technical LUTs transform camera Log color space into a desired color space according to the technical data. Artistic LUTs can alter colors in a certain way based on particular aesthetics. They can be camera-specific or generic, meaning they don’t change the color space and can be used with any source. 

Why LUTs can be problematic?

Creating a LUT is easy and it can be done in free software such as LUTCalc. Because of this, the web is flooded with different LUT packs of questionable quality, promising to transform any shot into a movie. The situation has improved over the years and while there are still a lot of bad products sold, there’s an easy way to test the quality of a LUT.

LUT Resolution

Look-up-table visualisation in 65x points

The most common LUTs used are 3D LUTs in cube format. They come in  17, 33, and 65 points or samples, corresponding to their resolution. Most field recorders such as Atomos and mirrorless cameras only support up to 33x size. Technically, a higher resolution LUT 65x65x65x should be less prone to banding or image artifacts but if created correctly it shouldn’t matter. Most of the official manufacturer’s LUTs are in 33x resolution.

Testing a LUT

A simple way to test a LUT is to apply it to an image with smooth color gradients. The image can be generated in Photoshop using the gradient tool or downloaded from the web. In this example, we’re using True Color Stress Image which is available for download here.

Here’s an example of one of the popular film-emulation LUTs applied to a studio image and the test image. 

Film emulation LUT applied to Arri studio image

Film emulation LUT applied to stress test image

You can see the issues with color transitions and color separation, which will affect the image quality and cause banding. The LUT might look normal in a studio setting but will cause problems in other situations. That’s why using stress tests and having a large dataset of images is important for the LUT development process. A high-quality LUT from a reputable source shouldn’t cause any deterioration when applied to a test image.

Here’s an example of Cinema Tools Nikon N-Log film emulation LUT stress test image.

Cinema Tools Ehnance C N-Log LUT applied to stress test image

Even some official manufacturer’s LUTs can cause issues. Here’s an example of Nikon’s first-generation N-Log LUT applied to a greyscale gradient.

Official Nikon N-Log LUT applied to the greyscale gradient

Official Nikon N-Log LUT waveform monitor

The most apparent issue here is the aggressive tone curve with abrupt highlight clipping but when we examine it we can also notice “steps” on the waveform monitor. Nikon fixed the issue with updated versions of LUTs, although they still look harsh. 

How to reduce LUT resolution

If you need to reduce the LUT resolution from 65x to a size suitable for use in a camera or external recorder, you can do it in LUTCalc. Upload the LUT using the Choose File button, check the resolution you want to save, and click Generate LUT.