Legacy WebGL Methods

Duncan Murdoch

March 08, 2017

Introduction

This document describes the original ways to embed rgl scenes in HTML documents and use embedded Javascript to control a WebGL display in an HTML document. For more general information, see rgl Overview. For more recent recommended methods, see User Interaction in WebGL.

We mainly assume that the HTML document is produced from R markdown source using knitr or rmarkdown, though some methods don’t require this. This format mixes text with Markdown markup with chunks of R code.

There are two ways to embed an rgl scene in the document. The older one is to use the chunk option webgl = TRUE. With that option, whatever rgl scene is active at the end of the chunk will be embedded. See the setupKnitr help page.

The second way is to use a call to rglwidget. Each call to this function will insert a scene into the document. Do not set webgl = TRUE. That is the recommended method, and is described in User Interaction in WebGL.

The second method is easier for me to maintain, and will receive more support in the future, but for now both methods are supported, and there are examples of both in this document.

Browser support

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

If you are using the internal browser in RStudio, support varies by version. I believe it is enabled by default in Windows versions, but until recently was not enabled in Mac OSX versions. You can run this command in Terminal:

defaults write org.rstudio.RStudio WebKitWebGLEnabled -bool YES

to enable it. I do not have much experience with RStudio in Linux, but it does seem that WebGL is enabled there.

Examples

We start with two simple examples. The next section gives reference information.

Consider the 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.

library(rgl)
with(iris, plot3d(Sepal.Length, Sepal.Width, Petal.Length, 
                  type="s", col=as.numeric(Species)))
subid <- currentSubscene3d()
rglwidget(elementId="plot3drgl")

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 1 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("plot3drgl").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 plot3d, the object would be called plot3drgl, and this code would work:

<script type="text/javascript">
var rotate = function(angle) {
  plot3drgl.getObj(`r subid`).par3d.userMatrix.rotate(angle, 0,1,0);
  plot3drgl.drawScene();
};
</script>

Autogenerated controls

We can also change the contents of the plot using toggleButton (or the more recent toggleWidget). For example, we can redo the previous plot, but with the three species as separate “spheres” objects and buttons to toggle them:

sphereid <- with(subset(iris, Species == "setosa"), 
     spheres3d(Sepal.Length, Sepal.Width, Petal.Length, 
                  col=as.numeric(Species),
                  radius = 0.211))
with(subset(iris, Species == "versicolor"), 
     spheres3d(Sepal.Length, Sepal.Width, Petal.Length, 
               col=as.numeric(Species),
               radius = 0.211))
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()
subid <- currentSubscene3d()

You must enable Javascript to view this page properly.

toggleButton(sphereid, label = "setosa", prefix = "toggle", subscene = subid)
toggleButton(sphereid+1, label = "versicolor", prefix = "toggle", subscene = subid)
toggleButton(sphereid+2, label = "virginica", prefix = "toggle", subscene = subid)

Note that we need to use results="asis" for the button code.
Normally we would also use echo=FALSE, though I didn’t do so above; then the buttons will end up side-by-side. We also add another button to toggle the axes:

An alternate control to achieve the same thing is subsetSlider. Here we also illustrate the elementId2Prefix bridge to allow an rglwidget to be controlled by the old-style slider.

rglwidget(elementId = "slider")
elementId2Prefix("slider")
subsetSlider(subsets = list(setosa = sphereid, 
                  versicolor = sphereid + 1, 
                  virginica = sphereid + 2, 
                  all = sphereid + 0:2),
             prefixes = "slider", subscenes = subid,
             init = 3)
all

There are several other functions to generate the Javascript code for controls. par3dinterpSetter generates a function that approximates the result of par3dinterp. propertySetter is a more general function to set the value of properties of the scene. Both generate Javascript functions, but not the controls to use them; for that, use propertySlider or your own custom code.

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

You must enable Javascript to view this page properly.

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) ) )
propertySlider(setter = par3dinterpSetter(fn, 0, 1.5, steps=15, 
                       prefix = "userMatrix", 
                       subscene = subid),
           step = 0.01)
0

Some things to note: The generated Javascript slider has 150 increments, so that motion appears smooth. However, storing 150 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 par3dinterpSetter so that the distortions of linear interpolation are not visible.

Another function that auto-generates Javascript code is clipplaneSlider. This function allows the user to control the location of a clipping plane by moving a slider. Both it and par3dinterpSetter are implemented using the more general propertySlider, which allows control of multiple objects in multiple scenes, but which does require knowledge of the internal representation of the scene in its Javascript implementation.

Less general than propertySetter is vertexSetter. 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,

You must enable Javascript to view this page properly.

setosa <- subset(iris, Species == "setosa")
which <- which.min(setosa$Sepal.Width)
init <- setosa$Sepal.Length[which]
propertySlider(
    vertexSetter(values = matrix(c(init,8,0,1,0,1,0,1), nrow=2),
                 attributes=c("x", "r", "g", "b"), 
             vertices = which, objid = sphereid, 
             prefix = "vertex"), 
    step=0.01)
1

A related function is ageSetter, 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.

Rather than giving an example, we will illustrate the very similar function ageControl, embedded in a playwidget. We will show a point moving along a curve. In the original scene we need to specify multiple colours so that the colour is not fixed, and can be controlled by the slider. We also give two ageControl calls in a list;

time <- 0:500
xyz <- cbind(cos(time/20), sin(time/10), time)
lineid <- plot3d(xyz, type="l", col = c("black", "black"))["data"]
sphereid <- spheres3d(xyz[1, , drop=FALSE], radius = 8, col = "red")
rglwidget(elementId = "ageExample")
playwidget("ageExample", 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)

The final function of this type is matrixSetter, for setting up multiple controls to modify a matrix, typically userMatrix. This is used when complex manipulation of a matrix requires several controls.

User defined mouse controls

rgl allows user defined mouse controls. For these to work within WebGL, you will need to write a Javascript version as well as the R version. This isn’t easy: R provides a lot of support functions which are not easily available in Javascript.

TODO: give an example here.

Reference for rglClass

NB: This section has not been updated recently, and is not current.

In writing the writeWebGL() function, I haven’t tried to prevent access to anything. On the other hand, I haven’t provided access to everything. The parts documented here should remain relatively stable (unless indicated otherwise). Users may also consult the source to writeWebGL, but should be aware that anything that isn’t documented here is subject to change without notice.

rglClass

As documented in writeWebGL, the call

writeWebGL(..., prefix = "<prefix>")

will create a global object on the output page with name <prefix>rgl and Javascript class rglClass. This class has a large number of properties and methods, some of which are designed to be available for use by other code on the web page.

Most of the properties are stored as Javascript Array objects, indexed by the rgl id of the subscene to which they apply. There are also Javascript methods attached to the rglClass class.

Methods

drawScene()

After any change that will affect the display, code should call <prefix>rgl.drawScene() to redraw the scene.

inSubscene(), addToSubscene(), delFromSubscene()

These methods each take two arguments: id and subscene, which should be the rgl ids of an object and a subscene. inSubscene tests whether id is already included in the subscene, and the others add it or delete it from the subscene.

getSubsceneEntries()

This function takes a subscene id as argument, and returns an Array containing all of the ids displayed in that subscene.

setSubsceneEntries()

This takes an Array of ids and a subscene id as arguments, and sets the contents of the subscene to the ids.

Properties

FOV, listeners, userMatrix, zoom

These correspond to the par3d properties with the same names.

viewport

This property also corresponds to the par3d property, but should be considered to be read-only.

drawFns, clipFns

These two arrays contain the code to display each object in the scene. The functions in the drawFns array are called for each object each time it is displayed. The clipFns functions are called when objects being clipped are drawn.

values, offsets

Most of the data about each object in a scene is contained in the values property. This is an array, indexed by object id. The individual entries are numeric arrays. Though they are singly-indexed, the entries are meant to be interpreted as matrices stored by row. The first 3 columns are generally the coordinates of a vertex, and remaining columns correspond to other values from rgl.attrib.

The offsets property gives the (0-based) offset of the first column for a particular attribute in a named object. Not all columns will be present in every object; if not present, the corresponding offsets entry will be -1. The entries are

Name Meaning
vofs Offset to 3 columns of vertex data in XYZ order
cofs Offset to 4 columns of colour data in RGBA order
nofs Offset to 3 columns of normal data
radofs Offset to 1 column of sphere radius data
oofs Offset to 2 columns of text or sprite origin data
tofs Offset to 2 columns of texture coordinates
stride Total number of columns in values

For example, to find the blue colour entry for vertex i in an object, one would first check if offsets["cofs"] was -1, indicating that no colour information was present. If not, the entry could be found using

values[offsets["stride"]*(i-1) + offsets["cofs"] + 2]

This assumes i is specified using 1-based vertex counting as in R, and writes values and offsets instead of the fully specified <prefix>rgl.values and <prefix>rgl.offsets for clarity.

Changes to values need to be pushed to the graphics system to be reflected in the scene; see the calls to gl.bindBuffer and gl.bufferData in the source to propertySlider for details.

Index

The following functions and rglClass properties and methods are described in this document:

FOV   drawFns   offsets   rglwidget   vertexSetter  
addToSubscene   drawScene   par3dinterpSetter   setSubsceneEntries   viewport  
ageControl   elementId2Prefix   playwidget   subsetSlider   zoom  
ageSetter   getSubsceneEntries   propertySetter   toggleButton  
clipFns   inSubscene   propertySlider   toggleWidget  
clipplaneSlider   listeners   propertySlider   userMatrix  
delFromSubscene   matrixSetter   rglClass   values