In lorenzomorning/plotsphere: Plotting simple features on an interactive sphere

knitr::opts_chunk$set(
  results = "asis",
  collapse = TRUE,
  comment = "#>")
options(rmarkdown.html_vignette.check_title = FALSE)

library(rgl)
setupKnitr(autoprint = TRUE)
knitr::opts_chunk$set(results = 'markdown')

The package rgl is a 3D visualization system based on OpenGL. It provides a medium to high level interface for use in R, currently modelled on classic R graphics, with extensions to allow for interaction.

This section is a short introduction on useful information about rgl regarding this package plotsphere. For more detailed description of rgl see the documentation by main developer Duncan Murdoch.

While plot 3D is mainly used in this packages to produce 3D scatterplots we will rather plot graphical elements as spheres, points, arcs and triangles using xzy.coords into the 3D space.

1. Management of 3D plots

To deal with one or several 3D plots, these functions are the basic operators for their management:

open3d to open a new 3D plot window (prints id number). Use this function to open the first plot or new empty plots
close3d to close current 3D plot window
mfrow3d enables plotting of several devices next to eachother mfrow3d(row,col)
next3d jumps to the next plotting device when mfrow3d() is activated

More functions can be found in this rgl documentation chapter

2. Graphical Elements

There are many and specific graphical elements available in the rgl package. However, we will limit those to most necessary we need

Function | Description | Type of element | Geospatial geometry ----------------| --------------------|------------------|------------------- points3d: | adds points | primitive | point triangles3d: | adds triangles | primitive | polygon spheres3d: | adds spheres | constructed | The Earth arcs3d: | adds spherical arcs | constructed | line

Plotting our first element

Our first element will be a small sphere that is characterised by its center point and radius.

open3d()
# add sphere with center point and radius
spheres3d(x=0, y=0, z=0, radius = 50)

A first taste of geospatial

Within adding graphical elements, general arguments can be used to control the material. The most important ones as color, alpha and more can be found in this rgl documentation chapter. Additionally, advanced lightning of the whole plot could be set by the light3d function.

To demonstrate this functionality we plot a very primitive globe here! Note that our sphere here has the poles at the equators and edges along its surface. Therefore, use ps_globe() later on to create a nicer globe.

# add a first geospatial globe with some lightning settings
spheres3d(x=0, y=0, z=0, radius = 6350000, color = "royalblue3", specular = "royalblue3", shininess = 25, alpha = 0.7)

Why are we using a radius = 6350000? That is not the radius of the Earth Spheroid right?

True, the radius that WGS84 uses would have been 6378137 m. But to ensure that the graphical elements we will plot on the globe later on are visible, we use a smaller globe. NOTE that the graphical elements are all plotted on a sphere with the WGS84 radius of 6378137 m, only the underlying globe is a bit smaller for visualization reasons.

Stack graphical elements

If one 3D-plot is active, all following graphical elements will be added to it directly.

# open first device for 3D plotting by calling graphical element directly
spheres3d(x=0, y=0, z=0, radius = 6350000, color = "royalblue3", specular = "royalblue3", shininess = 25, alpha = 0.7)
# create geocentric coordinate for greenwich
g_coord <- matrix(data=c(11169.39, 3.918492e-10, 0, 0, 6356743, -6356752), nrow=2, ncol=3)
# add an arc to the sphere
arc3d(from=g_coord[1,], to=g_coord[2,], center = c(0,0,0), color = "black")

However if you do not want add graphical elements to the current globe make sure you open another empty one with open3d(). Closing the previous plot is optional but keeps your windows tidy.

3. Multi-figure layout

Additionally you could also plot those two plots next to eachother using multi-figure layout functions of mfrow3d and next3d. You can find more advanced settings for multi-layout figures in this rgl documentation chapter

# set the multi-figure layout
mfrow3d(nr=1, nc=2, sharedMouse = TRUE)

# open first device for 3D plotting by calling graphical element directly
spheres3d(x=0, y=0, z=0, radius = 6350000, color = "royalblue3", specular = "royalblue3", shininess = 25, alpha = 0.7)
# create geocentric coordinate for greenwich
g_coord <- matrix(data=c(11169.39, 3.918492e-10, 0, 0, 6356743, -6356752), nrow=2, ncol=3)
# add an arc to the sphere
arc3d(from=g_coord[1,], to=g_coord[2,], center = c(0,0,0), color = "black")

# new empty globe plot at the right next to the previous one
next3d()
spheres3d(x=0, y=0, z=0, radius = 6350000, color = "royalblue3", specular = "royalblue3", shininess = 25, alpha = 0.7)