Better documentation (or HoloPlayCore property) needed for "cameraSize" constant

[mcclure]

The HoloPlayCore SDK ExampleProject contains an example of how to set up a virtual camera for a particular cell within a quilt grid. The method provided depends on two constants, a "camera size" (hardcoded to 5) and a field of view (hardcoded to 14°).

https://github.com/Looking-Glass/HoloPlayCoreSDK/blob/master/ExampleProject/src/HoloPlayContext.cpp#L667

There is a long, helpful comment explaining why 14° was picked. However the "camera size" is not explained at all. A comment in the header says it is the "size of the holoplay camera" and it is "changeable without limitation".

"Expected behavior:" I understand this is an arbitrary scale and you can tune it up and down. However, some guidance in the comments as to how one might go about picking a size or why 5 was picked in the example (does the "camera size" correspond to some useful practical value, like the radius of the imaginary holoplay camera "lens"?) would be helpful.

Since the holoplay screen effectively behaves like a plate of glass that exists in the rendered world, what would be especially helpful would be either an explanation of how to set the size so that the screen surface (what this document calls the "focal plane") is a particular known size in scene worldspace coordinates; or how to set the size so that scene worldspace coordinates can be placed in a relationship (such as 1 inch = 1 inch) with real-world space (in headset-based VR what we like to do is set our worldspace coordinates so that 1 unit = 1 simulated meter, but there are currently no properties in HoloPlayCore that allow an app to query the physical extent of the screen, only the pixel size of the screen).