Visualising higher-dimensional space-time and space-scale objects as projections to ℝ3

Objects of more than three dimensions can be used to model geographic phenomena that occur in space, time and scale. For instance, a single 4D object can be used to represent the changes in a 3D object's shape across time or all its optimal representations at various levels of detail. In this paper, we look at how such higher-dimensional spacetime and space-scale objects can be visualised as projections from R4 to R3. We present three projections that we believe are particularly intuitive for this purpose: (i) a simple 'long axis' projection that puts 3D objects side by side; (ii) the well-known orthographic and perspective projections; and (iii) a projection to a 3-sphere (S3) followed by a stereographic projection to R3, which results in an inwards-outwards fourth axis. Our focus is in using these projections from R4 to R3, but they are formulated from Rn to Rn-1 so as to be easily extensible and to incorporate other non-spatial characteristics. We present a prototype interactive visualiser that applies these projections from 4D to 3D in real-time using the programmable pipeline and compute shaders of the Metal graphics API.3D Geo-Informatio