Vulkan on Android 6 — VR Stereo Rendering Part 6: Visual Comfort

Photo by Priscilla Du Preez on Unsplash

• Part 1
• Part 2
• Part 3
• Part 4
• Part 5
• Part 6

In this post, we are going to discuss what makes our eyes feel comfortable when we put HMD on.

Parallax

Our eyes produce two different images because they view the same object from two different positions. When we focus on an object, we can image plane perpendicular to our line of sight and tangential to the surface of the point of focus. And such a relationship exist:

Fig. 1

If we are focusing on the middle triangle, the triangles behind and front the middle one appear to be coupled. You can see why on the figure since they are not projected to the same point. If you are not convinced, you could experiment with your fingers or pens.

Fig. 2

As figure 2 shows, we put our two hands thumbs up in front of us with different distance.

First, we focus on the closer thumb, and we notice the further one appears to be coupled(stay focus on the closer one). Now close your left eye. You’ll find your left eye sees the further left one since the further right one disappears. Likewise, you’ll find your right eye sees the further right one since the further right one disappears when you close your right eye. We can see the further thumb projected to the near plane produces the splitting thumbs vision with the same side of each eye.

Next, we focus on the further thumb, and we notice the closer one appears to be coupled(stay focus on the further one). Now close your left eye, this time you’ll find your left eye sees the closer right one since the further left one disappears. Likewise, you’ll find your right eye sees the closer left one since the closer right one disappears when you close your right eye.

Relation to VR

Visual Effect

Fig. 3 Zero parallax has object looks like on the screen surface.

Fig. 4 Positive parallax has object looks like inside the screen surface.

Fig. 5 Negative parallax has object looks like out of the screen surface.

Effects to Eyes

We know from part 1 of the series that accommodation bends our eye lens to focus on an object. And convergence controls our both eyes to focus on the same object. The natural way of a human eye is that the distance of accommodation and convergence should be the same. However, in VR our screen is fixed at the distance. Unlike our eyes, we change our focal length as we focus on another object. So it’s easy to confuse our brain because of the distances of accommodation and convergence do not match. When conflict happens, our eye will strive to adjust its muscles to match accommodation and convergence, causing high eye pressure and make us sick.

Fig. 6 zero/positive/negative parallax

Fig. 7 no convergence

In most cases, negative parallax is worse than positive parallax since we’ve converged our eyes so much in our daily life watching near objects instead of far ones like trees or mountains in distance to relax our eyes. Divergent parallax which is caused by exceeding distance between both eyes makes our brain hard to fuse the images. That is unnatural to normal human eyes which needs to avoid. So the best scene arrangement is to allow only zero and positive parallax, making the best experience.

Reference

• https://en.wikipedia.org/wiki/Parallax
• http://docs.autodesk.com/3DSMAX/16/ENU/Stereo-Camera/index.html?url=files/GUID-B12EEC7E-F1E1-491E-9088-9D8D7059B27B.htm,topicNumber=d30e279
• http://www.motionesque.com/stereography-and-the-cardboard-effect/
• http://manual.reallusion.com/iclone_6/enu/pro_6.0/09_3D_Vision/Settings_for_Pop_Out_and_Deep_In_Effect.htm