D.1 Comparable quality viewport switching latency
26.1183GPPRelease 17TSVirtual Reality (VR) profiles for streaming applications
This sub-clause illustrates how the weighted average QR value and the effective resolution can be calculated.
The quality level of each region is determined with its respective quality ranking (QR) value. A viewport can be covered with multiple regions. A quality level value for the viewport can be derived as weighted average of the QR values of the regions covering the viewport. The weight of each region is defined as the percentage of the viewport area covered by the corresponding region. The viewport quality level can be calculated by the following equation.
: Number of regions covering the viewport
QR value of i-th quality ranking region
The viewport coverage value (in percent) of i-th quality ranking region
Figure D.1-1: An example of a viewport covered by four quality ranking 2D region
Figure D.1-1 is an example of a viewport covered by four quality ranking regions. The quality of the viewport is equal to the weighted sum of the quality ranking value and the coverage percentage value of each quality ranking region.
The resolution of each region is determined by its respective width and height values in pixel which are available in the quality ranking box under the name orig_width and orig_height. Note that these values are already normalized to represent the full-sphere resolution you would get if the resolution of this region would be used for the full sphere.
The effective resolution (i.e. the total number of original pixels) for the content visible in the viewport can be derived as the weighted average of the resolution of each region covering the viewport. The weight of each region is defined by the percentage of the viewport area covered by the corresponding region. The effective viewport resolution can be calculated by the following equation.
: Number of regions covering the viewport
: The width component of the original source pixel resolution for the i-th quality ranking region
: The height component of the original source pixel resolution for the i-th quality ranking region
The viewport coverage value (in percent) of i-th quality ranking region
Coverage [1]
Coverage [3]
Coverage [4]
Region [1]
Region [2]
Region [4]
Region [3]
Coverage [2]
Figure D.1-2: An example of a source packed image with four quality ranking 2D regions with different resolutions
Figure D.1-3: An example of a viewport covered by four different quality ranking 2D regions
Resolution [3]
Resolution [2]
Resolution [4]
Viewport
35%
15 %
40 %
10%
Coverage [1]
Coverage [2]
Coverage [3]
Coverage [4]
Resolution [1]
Figure D.1-2 is an example of a source with four regions with different resolution. Figure D.1-3 represents an example of a viewport which is covered by the four quality ranking 2D regions. The effective viewport resolution is equal to the weighted sum of the resolution for each quality-ranking 2D region and its corresponding viewport coverage percentage value.
Figure D.1-4: Comparable quality viewport switching latency measurement example
Figure D.1-4 presents an example of the metric measurement operation. The viewport quality is evaluated at time t0, and then again at time t1. The media playback module renders the high-resolution sub-picture #1 at time t1. The user viewing orientation is gradually changing from sub-pic#1 to sub-pic#2 as the time progresses.
At time t2, the media playback module starts to render the buffered low-quality representation of sub-pic#2 as the viewport moves into sub-picture #2. At time t2, the viewport quality drops in values as compared to the viewport quality at time t1, and a new sub-picture (sub-pic #2) is rendered. A viewport switching event is identified at time t2.
The viewport quality values evaluated at t1 identifies the first viewport. The viewport position and viewport quality level list are assigned to the attributed Position and QualityLevel of the firstViewportItem.
An effective viewport resolution and viewport QR quality value for the new viewport that is comparable to that of the firstViewportItem after viewport switching time is logged at time t4. The new viewport position identifies the Position of the secondViewportItem. The corresponding QualityLevel list for the secondViewportItem is assigned.
The associated viewport values stored for the worst viewport quality during the switch is assigned to the field Position of the worstViewportItem. The corresponding QualityLevel list for the worstViewportItem is also assigned.
The comparable-quality viewport switching latency is measured as the time interval between the logged times for firstViewportItem (t1 in this example) and secondViewportItem (t4 in this example).