One aspect of filming and viewing HDR content that initially surprised me—but in fact makes
perfect sense—is that regardless of whether you
are showing content in HDR or SDR, your main
content has the same brightness on the delivery
end. This is especially true for your talent; you
would not expose them in the highlights. This creates challenges when filming with different Log
and Gamma curves because you need to expose
differently on the acquisition end of the workflow, depending on the Log or Gamma curve.
Setting Exposure, Resolution,
Bit Depth, and Bitrate
In general, when filming with HLG, you should
expose a bit darker than you normally would,
in order to make sure that you don’t push your
talent into the highlight end of the curve where
there isn’t as much information recorded per
stop. You also won’t be able to do much grading
to HLG footage because the slope of the curve
changes dramatically from the lows and mids to
the highlights. This shouldn’t be a big issue for
camera operators and technical directors who
are used to producing live content, as they are
accustomed to having to nail the exposure for
live content already.
When it comes to resolution and color bit
depth, shooting with HLG is no different than
shooting with any Log or Gamma curve. If your
video camera supports 4K UHD only in 8-bit color, then your footage won’t grade as well as HD
footage filmed in 10-bit color. Also, most HDR
monitors are capable of supporting 10-bit color,
so if you output 8-bit color, you’re limiting playback to only 256 values for each of red, blue, and
green, instead of 1,024 values with 10-bit.
When filming with HLG, it’s especially im-
portant not to get stingy with the codec and
bitrate choice on the recording end. In fact, you
may want to add an external recorder with a
DNxHD or ProRes Codec as opposed to the long
GOP interframe in-camera codecs your camera
supports. Just be very careful about which video
output you choose, as the HDMI and HD-SDI out-
puts on cameras like the Sony FS5 have different
maximum resolutions and color bit depth.
Choosing a Color Space
Choosing a color space can be a bit confusing. When shooting with HLG, you can choose
between Rec. 709 and Rec. 2020 color. HDR TVs
prefer 2020, and SD monitors prefer 709, so one
of your workflows is going to have hue and saturation levels slightly off.
If you’re shooting with HLG exclusively for
SDR delivery and you want to protect your highlights, use Rec. 709 color. If your shoot requires
live HDR delivery, then you might want to use
Rec. 2020 color and let the SDR look slightly
off. If this doesn’t sound like a perfect workflow,
that’s because it isn’t. It is a hybrid workflow with
attendant trade-offs. Finally, if you’re delivering
SDR live and HDR for later on-demand viewing,
you may want to shoot with Rec. 709 color and
do some light grading for the HDR version.
Alternatively, you might want to consider one
of the other Log or Gamma curves and apply
live LUTs. You can also perform a proper color
grade for each of your HDR and SDR outputs. At
the end of the day, most live producers will need
to test each workflow and decide for themselves
which one best suits their needs.
Shawn Lam ( firstname.lastname@example.org) is an award-winning video
producer and technical director. His Vancouver-based video
production company, Shawn Lam Video Inc., specializes in
corporate and event video production, including online video,
video switching, webcasting, and video SEO.
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SDR and HDR
versions of the