HDR Remastering with the Academy Color Encoding System

HDR Remastering with the Academy Color Encoding System

Written by Julian Pössnicker on May 2021

After writing my Guide for YouTube HDR delivery I decided to remaster a two-year-old film project “Der Kelch des Pius”. My team and I colored the film only for theatrical DCI-P3 projection and later for a BT.709 grade.  For each format, we repeated the whole color grading process which was very time-consuming. For the HDR remaster I wanted to test a new method that would be transformable to multiple formats. Beforehand I used the Academy Color Encoding System (ACES) only for 3D-Animations and DaVinci Resolve for editing. So, I knew a portion of it, but I did not fully understand its full potential. After more in-depth research about ACES, I decided to test its abilities and therefore this project started.

 

Production

We had a good time making this film. The look of the film was clear early in production. The film should be in a 4:3 aspect ratio and the camera angles are only locked in 90 degrees rotations and the shots should mainly be static with no movement. So, a little bit like a Wes Anderson film.

We shot with the original Blackmagic Ursa 4K. This is an excessively big and heavy camera, but it was a perfect fit for the film at that time. Because of the limited movement in the shots, the heaviness of that camera was not a big problem. The 10-inch display and the 4:3 guides were perfect for focusing and framing the shots. The biggest advantage of the Ursa 4K was the recording directly to the ProRes format and the ability to use a logarithmic encoding, which are important requirements for the ACES workflow.

 

Remastering with the Academy Color Encoding System

Changing the non-color-managed workflow to the ACES color management was quite easy. We used ACEScct for our project which gives us a more “cinematic” shadow luminance compression. But the difference between ACEScc and ACEScct is not overly dramatic. We set the ACES Output Transform to DCI-P3 for correct color reproduction on our P3 calibrated display. For HDR delivery the ACES Output Transform can be set to PQ BT.2020 after the grade. All color corrections and grades will be still there after the change to another output format. The only drawback was that the color grade in the non-color-managed workflow is not compatible with the ACES workflow anymore. So, the coloring had to be done again. But ACES already got us a great grading starting point after setting up the input transform and output transform which you can see in the following picture.

 

For the input transform, we used the Blackmagic Ursa 4K IDT of DaVinci Resolves color management. That straight-out of ACES look was great, but a few corrections had to be made. Changing the exposure was only a little offset change. Because ACEScc and ACEScct are logarithmic or quasi logarithmic transfer functions changing the offset value changes the luminance values such as the iris or iso in the camera does. The white balance corrections were only a few color curve tweaks.

 

The look was particularly important on the first day of production. Its look should be pleasing but realistic. So, the tweaks are very minimal but effective. The following picture shows the difference from the color corrected picture to the result.

 

ACES HDR Output

This part is only about testing a possible way to grade and export an HDR video because we do not own or have access to a professional calibrated HDR display. But the outcome speaks for itself. Setting the ACES Output Device Transform to an HDR format such as smpte 2084 with the BT.2020 color space the colors in the viewer inside DaVinci Resolve got unsaturated and low contrast. This behavior is intentional and not a bug. Because HDR videos contain a bigger latitude of light the encoded luminance has to be sorted differently. The camera logarithmic profiles are comparable to the HDR PQ and HLG standards which are using similar logarithmic transfer functions to encode the full latitude. The HDR displays will be interpreting and display such HDR videos. The outcome should be comparable to the DCI-P3 grade displayed on the P3 calibrated display. But with much higher max luminance and wider color space.

DaVinci Resolve has a few tools for checking the HDR exposure. DaVinci Resolve`s “Parade” Scope can be set to display the output nit value. This tool will help to judge the HDR video even without an HDR display. After the luminance corrections, the HDR video was ready for upload.

 

Summary

In summary, I can say that the ACES workflow is powerful and quite easy to use. Eventually, I will be using ACES for all my productions. The HDR output option is interesting but with my current non-HDR-setup not usable for my professional productions. But if I had the opportunity to use an HDR display in a future project I will be investigating ACES` HDR capabilities again. For now, ACES is a perfect SDR workflow which helps me get a great look faster and better. Especially for CGI with realistic light values. But this is a topic for future posts.

 

Academy Color Encoding System ACES Blackmagic BT.2020 DaVinci Resolve Der Kelch des Pius HDR PQ SDR

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