User Interaction in WebGL (updated)

Duncan Murdoch

August 20, 2021

Introduction

This document describes how to embed rgl scenes in HTML documents and use embedded Javascript to control a WebGL display in an HTML document. For more general information about rgl, see rgl Overview.

We assume that the HTML document is produced from R markdown source using knitr or rmarkdown. This format mixes text with Markdown markup with chunks of R code. There is a limited amount of discussion of other methods.

There are two ways to embed an rgl scene in the document. The recommended one is to call rglwidget to produce a “widget” which can be embedded into your document by printing it.

Older methods (e.g. writeWebGL) used before rgl version 0.102.0 are no longer supported.

I have conducted experiments on a third method. This is intended to be similar to the way standard 2D graphics are included by knitr, i.e. it will detect the fact that you’ve drawn something, and just include it automatically. At present it is not recommended, but that may change in the future.

Browser support

Most browsers now support WebGL, but in some browsers it may be disabled by default. See https://get.webgl.org for help on a number of different browsers.

Examples

We start with a simple plot of the iris data. We insert a code chunk and call the rglwidget function with optional argument elementId. This allows later Javascript code to refer to the image. We also save the object ids from the plot, so that they can be manipulated later.

library(rgl)
plotids <- with(iris, plot3d(Sepal.Length, Sepal.Width, Petal.Length, 
                  type="s", col=as.numeric(Species)))
rglwidget(elementId = "plot3drgl")

Next we insert a button to toggle the display of the data.

toggleWidget(sceneId = "plot3drgl", ids = plotids["data"], label = "Data")

The sceneId is the same as the elementId we used in rglwidget(), the ids are the object ids of the objects that we’d like to toggle, and the label is the label shown on the button. To find the names in the plotids variable, apply names() or unclass():

names(plotids)
## [1] "data" "axes" "xlab" "ylab" "zlab"
unclass(plotids)
## data axes xlab ylab zlab 
##   13   14   15   16   17

Using magrittr or base pipes

It can be error-prone to set the elementId in the rglwidget() to match the sceneId in the toggleWidget() (or playwidget(), described below). In the usual case where both are intended to appear together, magrittr-style pipes can be used quite flexibly: the first argument of the control widget accepts the result of rglwidget() (or other control widgets), and the controllers argument of rglwidget() accepts control widgets. In R 4.1.0, the new base pipe operator |> should be usable in the same way.

For example,

rglwidget() %>%
toggleWidget(ids = plotids["data"], label = "Data")

If you have R 4.1.0 or greater, this should do the same:

rglwidget() |>
toggleWidget(ids = plotids["data"], label = "Data")

You can swap the order of button and scene; use the magrittr dot (or the => syntax in base pipes) to pass the toggleWidget to rglwidget in the controllers argument:

toggleWidget(NA, ids = plotids["data"], label = "Data") %>%
rglwidget(controllers = .) 

or using R 4.1.0 or later,

toggleWidget(NA, ids = plotids["data"], label = "Data") |> 
  w => rglwidget(controllers = w) 

Controls

We have seen how to change the contents of the plot using toggleWidget. We can do more elaborate displays. For example, we can redo the previous plot, but with the three species as separate “spheres” objects and buttons to toggle them:

clear3d() # Remove the earlier display

setosa <- with(subset(iris, Species == "setosa"), 
     spheres3d(Sepal.Length, Sepal.Width, Petal.Length, 
                  col=as.numeric(Species),
                  radius = 0.211))
versicolor <- with(subset(iris, Species == "versicolor"), 
     spheres3d(Sepal.Length, Sepal.Width, Petal.Length, 
               col=as.numeric(Species),
               radius = 0.211))
virginica <- with(subset(iris, Species == "virginica"), 
     spheres3d(Sepal.Length, Sepal.Width, Petal.Length, 
               col=as.numeric(Species),
               radius = 0.211))
aspect3d(1,1,1)
axesid <- decorate3d()
rglwidget() %>%
toggleWidget(ids = setosa) %>%
toggleWidget(ids = versicolor) %>%
toggleWidget(ids = virginica) %>%
toggleWidget(ids = axesid) %>% 
asRow(last = 4)

Since we skipped the label argument, the buttons are labelled with the name of the variable passed as ids. The asRow function is discussed below.

toggleWidget() is actually a convenient wrapper for two functions: playwidget and subsetControl. playwidget() adds the button to the web page (and can also add sliders, do animations, etc.), while subsetControl() chooses a subset of objects to display.

subsetControl

For a more general example, we could use a slider to select several subsets of the data in the iris display. For example,

rglwidget() %>%
playwidget(start = 0, stop = 3, interval = 1,
       subsetControl(1, subsets = list(
                 Setosa = setosa,
                 Versicolor = versicolor,
                 Virginica = virginica,
                 All = c(setosa, versicolor, virginica)
                 )))