Screen-related content

The screenRef parameter appears in audioBlockFormat with typeDefinition 'Objects' and audioBlockFormat with the typeDefinition 'HOA' and the audioPackFormat with the typeDefinition 'HOA'. It indicates whether the content is screen-related or not. The audioProgrammeReferenceScreen parameter appears in audioProgramme and describes the screen which was used during production.

The screenRef flag can be used by a renderer for special processing of all screen-related objects taking into account the size of a local reproduction screen compared to the production screen-size.

If a renderer uses the screenRef flag to enable a special processing, it should use the reference/monitoring/production screen-size of the currently rendered audioProgramme as the reference screen.

If the flag is set and no audioProgrammeReferenceScreen element is included in the corresponding currently rendered audioProgramme, the default reference production/monitoring screen is implicitly defined on the basis of Recommendation ITU-R BT.1845 – Guidelines on metrics to be used when tailoring television programmes to broadcasting applications at various image quality levels, display sizes and aspect ratio.

Property	Value
Azimuth of left bottom corner of screen	29.0°
Elevation of the left bottom corner of screen	−17.3°
Aspect ratio	1.78 (16:9)
Width of the screen	58° (as defined by image system 3840 x 2160)

These spherical values can be transferred to Cartesian coordinates assuming a reference distance of 1.0 by first transferring the values above to the “standard” azimuth/elevation convention (0° azimuth is in front of the right ear, positive values are counted counter-clockwise; 0° elevation is directly above the head, positive values are counted downwards to the front) and then using the trigonometric functions to gain the Cartesian coordinates. The screen is assumed to have its corners touching the unit sphere. This results in the following values (orientation of the Cartesian coordinate axes):

Property	Value
X-coordinate of the centre of the screen	0.0
Y-coordinate of the centre of the screen	0.8438
Z-coordinate of the centre of the screen	0.0
Aspect ratio	1.78
Width of the screen	0.9354

Coordinate conversion

The maths to convert from the polar coordinate screen to Cartesian coordinates is

$$d = \frac{1}{\sqrt{\frac{1}{a^2}\tan^2\left(\frac{w}{2}\right) + 1}},$$

where $d$ is the Y-coordinate of the centre of the screen, $a$ is the aspect ratio, and $w$ is the polar angle of the screen width.

$x = 2d \tan⁡\left(\frac{w}{2}\right)$, where $x$ is the Cartesian screen width, and $w$ is the polar angle of the screen width.