Tangent screen with 2D Cartesian coordinates

Using the conventions of the **RUF** cartesian system (Rightward-Upward-Forward) presented earlier, a Tangent Screen can be simply defined as all the points with a constant Z coordinate (0.30 m in Figure 1). Only one point "P" is shown in Figures 1 and 2. It is usually assumed that the subject's eyes are lying on the X axis on each side of the Origin, so that the viewing distance in this experiment is 0.30 m (such a short viewing distance is commonly used with low vision patients).

Figure 1: Static representation of a 2D flat tangent screen displayed in a 3D cartesian coordinate system. All points lying on this tangent screen have the same Z coordinate of 0.30 m. The viewing distance is thus said to be 0.30 m. (SO : Screen Origin)	Figure 2: Same as in figure 1 with the viewing distance varying from 0.20 to 0.40 m. (SO : Screen Origin)

In sum, Virtual Reality can be used to simulate previous experiments whose stimuli were displayed on a tangent screen.

With a 3D cartesian system, one can define a/ the viewing distance of the tangent screen and b/ two "rectangular" coordinates (X and Y in meters) for each point on this tangent screen. Of course, this is not the best choice for vision experiments where using angular values corresponding to the retinal projections of points is usually needed. This is discussed in more details in later sections.

😉 Remark: One advantage of using the RUF (Rightward-Upward-Forward) coordinate system is that its X and Y axes are actually oriented in the same way as many important open-source programs designed to program experiments on 2D monitors - see notably the great PsychoPy.

The remark above suggests that users of programs such a PsychoPy should feel quickly at ease to specify the 3D positions of objects in PTVR.