Author: Duncan Murdoch
Date: 2025-06-24

# User Interaction in WebGL

#### Duncan Murdoch

#### June 24, 2025

## 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 newest one is recommended: call `setupKnitr` with argument `autoprint = TRUE` early in the document. This will set things up to be quite 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.

If `autoprint = FALSE` is used or no call is made to `setupKnitr()`, an explicit call to `rglwidget` will produce a “widget” which can be embedded into your document by printing it. This document uses that method.

Older methods (e.g. `writeWebGL` or various hooks) that were used before `rgl` version 0.102.0 are no longer supported.

## 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. (The first example in Controls uses tags instead of saving the ids.)

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

elementId example.

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

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

Since we skipped the `label` argument, the buttons are labelled with the values of the tags. 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 = tagged3d("setosa"),
                     Versicolor = tagged3d("versicolor"),
                     Virginica = tagged3d("virginica"),
                     All = tagged3d(c("setosa", "versicolor", "virginica"))
                     )))

There are several other “control” functions.

### `par3dinterpControl`

`par3dinterpControl` approximates the result of `par3dinterp`.

For example, the following code (similar to the `play3d` example) rotates the scene in a complex way.

    M <- r3dDefaults$userMatrix
    fn <- par3dinterp(time = (0:2)*0.75, userMatrix = list(M,
                                          rotate3d(M, pi/2, 1, 0, 0),
                                          rotate3d(M, pi/2, 0, 1, 0)) )
    rglwidget() %>%
    playwidget(par3dinterpControl(fn, 0, 3, steps=15),
           step = 0.01, loop = TRUE, rate = 0.5)

Some things to note: The generated Javascript slider has 300 increments, so that motion appears smooth. However, storing 300 `userMatrix` values would take up a lot of space, so we use interpolation in the Javascript code. However, the Javascript code can only do linear interpolation, not the more complex spline-based SO(3) interpolation done by `par3dinterp`. Because of this, we need to output 15 steps from `par3dinterpControl` so that the distortions of linear interpolation are not visible.

### `propertyControl`

`propertyControl` is a more general function to set the value of properties of the scene. Currently most properties are supported, but use does require knowledge of the internal implementation.

### `clipplaneControl`

`clipplaneControl` allows the user to control the location of a clipping plane by moving a slider.

### `vertexControl`

Less general than `propertyControl` is `vertexControl`. This function sets attributes of individual vertices in a scene. For example, to set the x-coordinate of the closest point in the setosa group, and modify its colour from black to white,

    setosavals <- subset(iris, Species == "setosa")
    which <- which.min(setosavals$Sepal.Width)
    init <- setosavals$Sepal.Length[which]
    rglwidget() %>%
    playwidget(
      vertexControl(values = matrix(c(init,   0,   0,   0,
                                         8,   1,   1,   1),
                                    nrow = 2, byrow = TRUE),
                    attributes = c("x", "red", "green", "blue"),
                    vertices = which, tag = "setosa"),
        step = 0.01)

### `ageControl`

A related function is `ageControl`, though it uses a very different specification of the attributes. It is used when the slider controls the “age” of the scene, and attributes of vertices change with their age.

To illustrate we will show a point moving along a curve. We give two `ageControl` calls in a list; the first one controls the colour of the trail, the second controls the position of the point:

    time <- 0:500
    xyz <- cbind(cos(time/20), sin(time/10), time)
    lineid <- plot3d(xyz, type="l", col = "black")["data"]
    sphereid <- spheres3d(xyz[1, , drop=FALSE], radius = 8, col = "red")
    rglwidget() %>%
    playwidget(list(
      ageControl(births = time, ages = c(0, 0, 50),
            colors = c("gray", "red", "gray"), objids = lineid),
      ageControl(births = 0, ages = time,
            vertices = xyz, objids = sphereid)),
      start = 0, stop = max(time) + 20, rate = 50,
      components = c("Reverse", "Play", "Slower", "Faster",
                     "Reset", "Slider", "Label"),
      loop = TRUE)

### `rglMouse`

While not exactly a control in the sense of the other functions in this section, the `rglMouse` function is used to add an HTML control to a display to allow the user to select the mouse mode.

For example, the display below initially allows selection of particular points, but the mouse mode may be changed to let the user rotate the display for a another view of the scene.

    # This example requires the crosstalk package
    # We skip it if crosstalk is not available.

    ids <- with(iris, plot3d(Sepal.Length, Sepal.Width, Petal.Length,
                      type="s", col=as.numeric(Species)))
    par3d(mouseMode = "selecting")
    rglwidget(shared = rglShared(ids["data"])) %>%
    rglMouse()

The `rglShared()` call used here is described below.

## Layout of the display

Many `rgl` displays will contain several elements: one or more `rgl` scenes and controls. Internally `rgl` uses the `combineWidgets` function from the `manipulateWidget` package.

The `rgl` package provides 3 convenience functions for arranging displays. We have already met the first: the `magrittr` pipe, `%>%`. When the display is constructed as a single object using pipes, the objects in the pipeline will be arranged in a single column.

The second convenience function is `asRow`. This takes as input a list of objects or a `combineWidgets` object (perhaps the result of a pipe), and rearranges (some of) them into a horizontal row. As in the toggleWidget example, the `last` argument can be used to limit the actions of `asRow` to the specified number of components. (If `last = 0`, all objects are stacked: this can be useful if some of them are not from the `rgl` package, so piping doesn’t work for them.)

Finally, `getWidgetId` can be used to extract the HTML element ID from an HTML widget. This is useful when combining widgets that are not all elements of the same pipe, as in the `crosstalk` example below.

If these convenience functions are not sufficient, you can call manipulateWidget::combineWidgets or other functions from `manipulateWidget` for more flexibility in the display arrangements.

## Integration with `crosstalk`

The `crosstalk` package allows widgets to communicate with each other. Currently it supports selection and filtering of observations.

`rgl` can send, receive and display these messages. An `rgl` display may have several subscenes, each displaying different datasets. Each object in the scene is potentially a shared dataset in the `crosstalk` sense.

The linking depends on the `rglShared` function. Calling `rglShared(id)`, where `id` is the `rgl` id value for an object in the current scene, creates a shared data object containing the coordinates of the vertices of the `rgl` object. This object is passed to `rglwidget` in the `shared` argument. It can also be passed to other widgets that accept shared data, linking the two displays.

If a shared data object has been created in some other way, it can be linked to a particular `rgl` `id` value by copying its `key` and `group` properties as shown in the example below.

    # This example requires the crosstalk package.
    # We skip it if crosstalk is not available.

    library(crosstalk)
    sd <- SharedData$new(mtcars)
    ids <- plot3d(sd$origData(), col = mtcars$cyl, type = "s")
    # Copy the key and group from existing shared data
    rglsd <- rglShared(ids["data"], key = sd$key(), group = sd$groupName())
    rglwidget(shared = rglsd) %>%
    asRow("Mouse mode: ", rglMouse(getWidgetId(.)),
          "Subset: ", filter_checkbox("cylinderselector",
                            "Cylinders", sd, ~ cyl, inline = TRUE),
          last = 4, colsize = c(1,2,1,2), height = 60)

If multiple objects in the `rgl` scene need to be considered as shared data, you can pass the results of several `rglShared()` calls in a list, i.e. `rglwidget(shared = <list>)`. The key values will be assumed to be shared across datasets; if this is not wanted, use a prefix or some other means to make sure they differ between objects.

If the same `rgl` id is used in more than one `rglShared()` object, it will respond to messages from all of them. This may lead to undesirable behaviour as one message cancels the previous one.

## Low level controls

We repeat the initial plot from this document:

    plotids <- with(iris, plot3d(Sepal.Length, Sepal.Width, Petal.Length,
                      type="s", col=as.numeric(Species)))
    subid <- currentSubscene3d()
    rglwidget(elementId="plot3drgl2")

plot3d2 example.

We might like a button on the web page to cause a change to the display, e.g. a rotation of the plot. First we add buttons, with the “onclick” event set to a function described below:

    <button type="button" onclick="rotate(10)">Forward</button>
    <button type="button" onclick="rotate(-10)">Backward</button>

which produces these buttons:

We stored the subscene number that is currently active in `subid` in the code chunk above, and use it as ```r subid``` in the script below. `knitr` substitutes the value when it processes the document.

The `rotate()` function uses the Javascript function `document.getElementById` to retrieve the `<div>` component of the web page containing the scene. It will have a component named `rglinstance` which contains information about the scene that we can modify:

    <script type="text/javascript">
    var rotate = function(angle) {
      var rgl = document.getElementById("plot3drgl2").rglinstance;
      rgl.getObj(`r subid`).par3d.userMatrix.rotate(angle, 0,1,0);
      rgl.drawScene();
    };
    </script>

If we had used `webGL=TRUE` in the chunk header, the `knitr` WebGL support would create a global object with a name of the form `<chunkname>rgl`. For example, if the code chunk was named `plot3d2`, the object would be called `plot3d2rgl`, and this code would work:

    <script type="text/javascript">
    var rotate = function(angle) {
      plot3d2rgl.getObj(`r subid`).par3d.userMatrix.rotate(angle, 0,1,0);
      plot3d2rgl.drawScene();
    };
    </script>

## Index

The following functions are described in this document:

ageControl | getWidgetId | propertyControl | subsetControl |
asRow | par3dinterpControl | rglMouse | toggleWidget |
clipplaneControl | playwidget | rglShared | vertexControl |