JVET-E0071 AhG8: Viewport-based subjective evaluation of 360-degree video coding [P. Hanhart, Y. He, Y. Ye (InterDigital)]
At the 4th JVET meeting, common test conditions and evaluation procedures for 360-degree video coding were established (JVET-D1030). Different viewports (VPs) were defined for each 360-degree video sequence for evaluation on 2D displays. This document reports results of two subjective evaluations comparing VPs rendered from 360-degree video sequences encoded with HM-16.14 and JEM-4.1 in the equirectangular projection (ERP) format at low bit rates. A preliminary experiment was conducted using QP=37 for both HM and JEM. To have a fairer comparison, a second experiment was conducted using QP=37 for JEM and floating QP for HM in order to match the JEM bit rate. In both experiments, the stimulus comparison method was used to compare the quality difference between HM and JEM. Two video sequences were presented side-by-side on a 4K TV and expert subjects were asked to rate which video has better overall quality. Results reportedly show that the JEM achieves some visual quality improvements over the HM at low bit rates. Finally, based on observations made during the subjective tests, this document discusses issues regarding viewport selection.
Only predefined static viewports were tested.
This coded the 8K/4K ERP original format.
Another proposal is “bullet time dynamic viewport”, with frozen frame.
Some observations:
- Viewports with stitching artefacts should be avoided.
- Sequences with fast camera motion may be critical to view.
- Dynamic viewport should be used carefully for sequences with camera motion or fast object motion.
Sequences with the most visible differences: Harbour, SkateboardInLot
4K sequences are not as useful for visual testing, because the viewport is too small and the compression artefacts are less visible.
In our investigations, we should concentrate more on video, because frame freezing can be quite different in terms of compression artefacts.