Rec. 709 curves
For me, high dynamic range (HDR) video is a much more xciting development than 3D or resolutions greater than 4K. When properly filmed, graded, and viewed on
an HDR monitor, viewing so much detail in the highlights of HDR
video is a visual treat.
The best part about HDR content is that the workflow doesn’t
need to be overly complicated, thanks to the implementation of
the Hybrid Log Gamma (HLG) that was codeveloped by the BBC in
the U.K. and NHK in Japan. HLG is available on the Sony FS5, Z150,
PXW-Z90, AX700, and HXR-NX80; and on the Panasonic GH5.
HLG isn’t perfect, nor is it the best gamma curve for all productions. HDR video filmed using S-Log3, professionally graded, and
exported twice—once for HDR and once for SDR—can look better. But the benefit of using HLG is that you can use one video signal to support both HDR and SDR live, without having to grade or
use LUTs. Currently, over 400 HDTV and UHDTV models support
HLG playback, along with select projectors, Google’s Android TV
platform, and You Tube. Sooner or later, you might be asked to
produce a live event with an HDR viewing option, and this primer
on HLG workflows is a good place to start.
What Is HLG?
As a gamma curve, the HLG curve is very similar to the broadcast standard Rec. 709 curve from 0–70% (Figure 1). HLG gets
very aggressive with content that falls above 70% in order to be
able to record a larger dynamic range. The current Sony implementation allows for 10–11 stops of exposure, compared to 6 stops
in Rec. 709.