Testing your matrix setup with the Oculus Rift

Working with the Rift requires per-eye manipulation of both the modelview and projection matrix.  The modelview matrix needs a horizontal translation that gives objects a sense of depth by providing a different viewpoint for each eye.  The projection matrix takes a different horizontal translation that provides the correct per-eye field of view and also serves to center the the scene under the lens axis, instead of in the viewpoint center.

Matrix manipulation can be a bit of a pain, though and getting the manipulation wrong is pretty easy to do.  For this reason my initial examples working with the Rift in any language or toolkit always involve rendering a small colored cube of a specific size very close to the viewer.  This type of scene serves as a useful test for a variety of ways it's possible to screw up the matrices or their application to the scene.

I'm going to look at why that is, and what the effects of incorrect matrix manipulation look like

Field of view in the Oculus VR

User Pogo on the Oculus VR forums asked the other day about why some applications didn't cover more of the available screen real estate on the Rift, and consequently occupied a smaller field of view than was possible.

Every pixel on the Rift display has a fixed relationship with the lenses (discounting the slight differences between the three sets of lenses that come with the Rift).  Each pixel has a specific θ (pronounced theta) which represents the angle between the lens axis and the line from the pixel through your eye.  Each pixel also has a specific 'perceived θ', which is the perceived angle between the lens axis and the where it appears to be because of the lens distortion.  

What this means is that if you're drawing on a smaller area of the Rift screen, you're essentially providing a smaller field of view.  

As an example of the issue Pogo posted the following images.  The first is from the Oculus configuration utility used to test the IPD and height settings a user has entered.

The second is from the lauded RedFrame environment demo.

And the last is from Half Life 2.  

All three applications are using native Rift support, but are rendering different fields of view.  But why?  Well, there are a number of reasons that this might happen.