Figure 4

When the angle θ between the x-axis and z-axis is not the same as in the Cartesian coordinate system, the spatial relationships among these axes and visual perception are also different. (A) The included angle cos\mathit{\alpha }=cos\left(\mathit{\theta }‒{90}^{\circ }\right)=cos\left(30^{\circ }\right)=\sqrt{3}/2=0.866, angle θ from 120° (a) to 90° (e). In case (e), there is no stereoscopic perception. (B) The included angle θ from 90° a to 120° e. In case a, there is no stereoscopic perception. (C) The included angle θ from 90° e to 120° a, obtained by rotating (A) 90° in the vertical direction and turned 30° in the horizontal direction. In case (e), there is no three-dimensional perception.