One of the most surprising documents we’ve seen since I began covering HDR was the Technical Specifications for the Delivery of Content to Sky UK. The first of the specs that caught our eye were the minimum camera requirements for scripted HDR content, which stipulate 4:4:4 chroma subsampling, 12-bit color depth and a dynamic range of >14 stops – which few cameras are capable of achieving.
Like Netflix, Sky cautions against excessive noise, noting that, “The impact of the chosen Exposure Index (EI), in relation to aperture and sensor sensitivity, should be understood. Intentionally introduced noise, through low lit images and camera settings, should be avoided as it can impair the resolution and compromise downstream processes.”
Also like Netflix, Sky prefers an ACES workflow, but will accept the logarithmic color space of the camera, writing, “A scene-referred workflow, such as ACES, is recommended. However, it is acceptable to work within the colour space defined by the camera’s OETF and colourimetry when it results in a satisfactory grade with uncompromised quality.”
Regarding low luminance imagery, Sky is especially concerned about viewers at home being unable to see in the shadows:
“Whilst grading suites provided precisely controlled equipment and viewing environments, the same is not true for most home viewing. Low luminance images where the subject and relevant surroundings cannot be clearly distinguished cause frustration and complaints from viewers.
- Content should be suitable for viewing in ordinary home viewing environments, where controlled, low lighting cannot be guaranteed, and display technology will vary.
- Particular care must be given to shadow details, with consideration to what is inconsequential image information and what needs to be preserved through re-encodes and consumer TV presentation. Note that nuanced shadow detail can be lost through encoding and consumer TVs often emphasise objectionable banding artefacts in shadow detail, which can be more apparent where midtone information is scarce.”
Concerning graphics luminance levels, excessively bright levels aren’t tolerated,
“Subject to the context of what’s in the frame and the neighbouring frames, Reference White (AKA diffuse white or graphics white) should sit around 100% in SDR, 58% (203 cd/m2) in PQ and 75% in HLG, as per ITU-R BT.2408-3 (Table 2). This may differ where the established look and/or graphic colour justifies it.”
In addition, Sky provides the following grading guidance, beginning with prioritizing the HDR grade:
“Consideration must be given to providing the optimum HDR image.
To avoid an overly reserved approach to what HDR offers, the initial grade should be completed on the HDR version, with the SDR grade being completed afterwards.
The full luminance range of the HDR grading monitor may be used.
However, to reduce the risk of an unintended look on more advance displays, the grade should not exceed the capability of the grading monitor. Therefore, the luminance might not exceed 1,000 cd/m2 or the chromaticity might not exceed the gamut available within the P3 colour primaries (though within a BT.2020 container).
Note that consumer TVs will typically display SDR images brighter than the intended grade. Whilst the same TV will reserve some of its brighter luminance capabilities for the highlights in an HDR image. This can result in a perceived dimmer HDR image, particularly when comparing the midtones. The grade should not try to counter the limitations of current consumer TV’s – the grading monitor must be the trusted reference – however, an awareness of this presentation attribute may help inform a more appropriate HDR grade.
Unless the impact on the viewer is intentional, care must be applied to ensure:
Comfortable viewing, with appropriate use of the greater luminance available, for the duration of the title.
Subjects are rendered at a suitable luminance level.
Loss of detail in highlights and low lights is minimised where possible – care should be taken when grading within the confines of the display’s abilities and avoid pushing the signal to a point where the grading software may adversely hard clips the signal (unless the resultant look is intended).
Bright elements do not cause discomfort to viewers, unintentionally distract, or compromise the perception of the main subject.
A measured approach when transitioning from bright scenes/shots to dark ones,as the viewer’s eye may require time to adjust.
A measured approach when transitioning from dark scenes/shots to bright ones, as a sudden jump in luminance could cause discomfort.
An appropriate reference white level is respected throughout. Subject to the context of what’s in the frame, the neighbouring frames and the established look, Reference White (AKA diffuse white or graphics white) should sit around 100% in SDR, 58% (203 cd/m2) in PQ and 75% in HLG, as per ITU-R BT.2408-3 (Table 2).
Inappropriate use of the dynamic range, without editorial justification, will result in content being rejected.”
The Daejeon Chronicles 
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