Displaying 3D images

Let's dive more deeply into the last three points in the optics part.

First is the automatic pupillary distance calibration. Let's be honest--no device except for a 3D printer can generate true 3D objects. It will always be a two-dimensional graphic, but presented in such a way that our brain gets tricked into seeing the missing third dimension. To achieve this, the device creates two slightly different images for each eye. We will get into that in later chapters, but for now let's just take this for granted. Our brain will see the difference between those two images and deduce the depth from that. However, the effect of this depends on one big factor--the images have to be exactly right. The position of each pixel has to be at exactly the right spot.

This means that we cannot just put a pixel in a X-Y coordinate on the display--a pixel that is meant to be in the middle of our view has to be presented right at the center of our pupils, and since no two eyes are the same, the device has to shift the logical center of the display a bit. In the early prototypes of the HoloLens, this was done manually by having a person look at a dot on the screen and then adjust the dials to make sure that the subject only sees on a single, sharp defined point. The automatic pupillary distance calibration in HoloLens now takes care of this, ensuring that every user has the same great experience.

The holographic resolution and density also need a bit more explaining. The idea here is that the actual number of pixels is not relevant. What is relevant is the number of radiants and light points. A light point is a single point of light that the user can see. This is a virtual point we perceive floating somewhere in mid-air. In reality, pixels are made out of these light points. There are many more light points than pixels. This makes sure that the device has enough power to produce pixels the person can actually see. The higher the number of light points you have, the brighter and crisper each pixel seems. The radiant is the number of light points per radian. As you probably know, a radian is a measure of angles, just like degrees. One radian is about 57.2958 degrees. Then, 2,500 radiants mean that for each radian, or 57.2958 degrees angles, we will have 2,500 light points. From this, you can deduce that objects closer by will have a better density of light points than objects far away.