Demystifying HDR Workflows: How to Use .Cube LUTs for High Dynamic Range Video

High Dynamic Range (HDR) has transitioned from a premium television feature to an industry standard. Whether you are delivering content for Netflix, YouTube, or Apple devices, mastering HDR is no longer optional for modern colorists and video editors.

However, transitioning your workflow from Standard Dynamic Range (SDR) to HDR introduces a steep learning curve. One of the most common points of confusion is how to properly use 3D Lookup Tables (LUTs)—specifically .cube files—within an HDR color-grading pipeline.

If you apply your favorite SDR LUTs to an HDR timeline, you will quickly find your highlights clipped, your shadows crushed, and your colors wildly distorted. Here is a comprehensive guide on how to properly leverage .cube LUTs in an HDR workflow without breaking your image.

The Core Difference: Rec. 709 vs. Rec. 2020

To understand why LUTs behave differently in HDR, we must look at the underlying color spaces and gamma curves:

• SDR (Rec. 709): Designed for older CRT monitors. It operates within a narrow color gamut and has a maximum peak brightness of 100 nits.
• HDR (Rec. 2020 / Rec. 2100): Features a massive color gamut and utilizes transfer functions like PQ (Perceptual Quantizer / ST 2084) or HLG (Hybrid Log-Gamma). HDR targets peak brightness levels from 1,000 nits up to 10,000 nits.

A standard .cube LUT is essentially a three-dimensional mathematical grid that maps input colors to output colors. If a LUT was designed for Rec. 709, it expects an input signal capped at 100 nits. Feeding a high-nit, wide-gamut HDR signal into that LUT will compress the dynamic range, defeating the entire purpose of shooting and delivering in HDR.

The Danger of Using SDR LUTs in HDR

Many editors have a library of creative “look” LUTs built for SDR. If you apply one of these Rec. 709 .cube files directly to your HDR timeline, two major issues occur:

1. Highlight Clipping: The LUT will hard-clip any highlight details sitting above 100 nits. Your beautiful cloud detail or specular reflections will instantly turn into flat, ugly white blocks.
2. Color Shifting: Because Rec. 2020 has a much wider color gamut than Rec. 709, the color coordinates mapped by an SDR LUT will result in oversaturated, unnatural skin tones and neon-like colors.

To use .cube LUTs in HDR safely, you must either use native HDR LUTs or manage your color space pipeline using Color Space Transforms (CST).

Step-by-Step: Setting Up an HDR LUT Workflow

The safest and most professional way to handle LUTs in HDR is within a non-destructive, color-managed environment like DaVinci Resolve. Below is a step-by-step workflow using the “Sandwich Method” with Color Space Transforms.

Step 1: Set Up Your Project Settings

Before importing footage, ensure your project is configured for HDR delivery.

• In DaVinci Resolve, go to Project Settings > Color Management.
• Set your Color Science to DaVinci YRGB Color Managed.
• Choose an HDR output color space, such as HDR ST2084 (PQ) with a limit of 1000 nits (a standard delivery target for most consumer HDR displays).

Step 2: Normalize Your Log Footage

If you shot your footage in a Log format (such as Sony S-Log3, Canon Log 3, or RED IPP2), do not apply a LUT directly to the raw clip.

• Use a Color Space Transform (CST) node as your first step to convert your camera’s native color space and gamma into your working space (e.g., DaVinci Wide Gamut / Intermediate).

Step 3: Implement the “Sandwich Method” for SDR LUTs

If you absolutely must use an SDR creative .cube LUT to get a specific aesthetic, you must “sandwich” it between two transform nodes:

1. Node A (Prep for LUT): Apply a CST to convert your wide-gamut working space (e.g., DaVinci Wide Gamut) down to Rec. 709 (Gamma 2.4).
2. Node B (The LUT): Apply your creative Rec. 709 .cube LUT on this node. Because the image has been temporarily scaled down to Rec. 709, the LUT will behave exactly as the creator intended.
3. Node C (Restore to HDR): Apply another CST to convert the image from Rec. 709 (Gamma 2.4) back up to your HDR working space or output space (e.g., Rec. 2020 / ST 2084).

Note: While this method works for styling, it is always preferable to use LUTs specifically mathematically calibrated for HDR.

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Best Practices for HDR LUT Management

To ensure your HDR grades look stunning and remain broadcast-compliant, keep these industry best practices in mind:

• Use High-Precision LUTs: Always use 65x65x65 (65-point) .cube files instead of 33-point LUTs. HDR’s wider color space and higher contrast ratios require finer interpolation to prevent banding and color artifacts in smooth gradients like skies.
• Grade “Under” the LUT: Apply your primary exposure, white balance, and contrast adjustments before (upstream of) the node carrying your LUT. This ensures you feed the cleanest, most balanced signal into the LUT’s mathematical matrix.
• Monitor on Hardware: You cannot grade HDR using your standard computer interface viewer. You must route your signal through an I/O device (like a Blackmagic DeckLink or UltraStudio) to a calibrated HDR display capable of displaying at least 1,000 nits of peak brightness.
• Trust Your Scopes: Keep an eye on your HDR Waveform set to Nits or ST 2084 scale. Ensure your peak highlights are hitting your target (e.g., 1000 nits) and that your creative LUT isn’t inadvertently clipping important highlight details.

Conclusion

Using .cube LUTs in an HDR environment requires a shift in mindset. You can no longer treat LUTs as a one-click fix. By understanding the math behind color spaces and utilizing tools like Color Space Transforms, you can safely integrate creative looks into your high dynamic range pipeline, resulting in rich, immersive, and visually stunning videos.