Inside a VR Video "Viewbox"

XR Lab

What's the "Ames Room" technique and what does it have to do with improving VR video quality?

VR video "viewboxes" give the illusion that the viewer is in the center of a large sphere.

In the video above, frames from our Sizzle shoot are paused and the room geometry is highlighted with red lines to explore the distortion and illusion of virtual reality video environments.

Today (Fall 2016), almost all VR video delivery is done by taking a "latlong" mapping of the sphere (also called equirectangular) and projecting that image onto the inside of a spherical geometric shape, as if it was projected onto a large dome movie theater with the viewer at the exact center.

This approach does not provide enough resolution in front of the viewer to saturate the head-mounted display screen, resulting in soft images. At Pixvana, we use "viewboxes" to stream higher resolution directly in front of the viewer's field of view.

 

A VR “Viewbox” is the projection model used to represent a video stream as if it where a spherical image surrounding the viewer
A VR “Viewbox” is the projection model used to represent a video stream as if it where a spherical image surrounding the viewer 

By borrowing a technique from "Ames Rooms", we can greatly increase the quality of VR video.

The wikipedia entry on Ames Rooms describes them as:

"...a distorted room that is used to create an optical illusion. Likely influenced by the writings of Hermann Helmholtz, it was invented by American ophthalmologist Adelbert Ames, Jr. in 1934."

Photo by Ian Stannard

 

2000px-ames_room-svg

Nice diagram. How does it relate to virtual reality?

  • VR video players render a viewing room of arbitrary geometric shape (eg, sphere, cube, frustrum, octagon, pyramid).
  • VR videos can be pre-processed with this geometric room shape in mind, when projected onto the walls of the viewing room.
  • When the viewer watches the video from the center of the viewing room, they perceive the room as a sphere.

vr-video-ames-room-image-1

In this figure:

  • Figure 1-A is the traditional Latlong of this frame of a VR video. The entire video is represented as a texture that will be projected onto a spherical dome.
  • Figure 1-B is a pre-computed Viewbox, which has the textures needed to reconstruct the scene when the video is wrapped onto a six-sided room with unequal wall dimensions, called a frustum shape.

In both cases, the viewer perceives that they are in the middle of a spherical room when watching the VR video.

vr-video-ames-room-image-2

 

However, if we were able to pause the video and move our viewpoint around the room, we would be able to perceive the illusion of the Ames Room at work. The six-sided walls of the room are highlighted by red-lines (indicating the geometry of the room). As we walk closer/farther to the corners of the room, we can see the optical-illusion at work.  It appears as if wallpaper had been made with the distorted texture, and wrapped onto the walls of the room. As long as the viewer's head remains in the middle of the room, it is impossible to see the effect.

 

vr-video-ames-room-image-3

Experience the Ames Room effect in our Pixvana SPIN Technical Preview on the HTC Vive

We've integrated an easter-egg PAUSE capability in our Pixvana SPIN Technical Preview. Pause the video and move your head about the room when running the preview on a HTC Vive. Simply hit the "spacebar" on your keyboard to pause/resume playback. Note that when the video is paused, we change the size of the room to make it small so that walking around the room is possible. During normal playback, the room is actually very large in order to remove perceived parallax as you move your head around (eg, the illusion is maintained). For this reason, a slight shift in the image occurs while pausing/resuming.

Our HTC Vive Technical Preview will be released soon in the Steam Store. Sign up to get a link for our SPIN Player Technical Preview on Steam. You'll also be the first to know when our beta program opens.

To see some real clever mastery of this POV Ames Room magic at work, check out this OK-GO music video "Writing on the Wall" that plays extensively with POV based alignment of objects on the physical set, iteratively!

XR Lab