Many RGL shapes contain lists of vertices with various
attributes (available via `rgl.attrib`

). This function
modifies the data for those attributes in a WebGL scene.

```
ageSetter(births, ages,
colors = NULL, alpha = NULL, radii = NULL,
vertices = NULL, normals = NULL, origins = NULL,
texcoords = NULL,
objids, prefixes = "", digits = 7,
param = seq(floor(min(births)), ceiling(max(births))))
```

## Arguments

- births
Numeric vector with one value per vertex, used to determine the
“age” of the vertex when displaying it.

- ages
A non-decreasing sequence of “ages”.

- colors, alpha, radii, vertices, normals, origins, texcoords
Attributes of the vertices. Non-`NULL`

attributes will be
interpolated from these values. See the Details section below.

- objids, prefixes
The object ids and scene prefixes to modify. These are recycled to
the same length.

- digits
How many digits to output in the generated Javascript code.

- param
Default values to be used by a slider control calling the generated
function.

## Details

The vertex attributes
are specified as follows:

- colors
A vector of colors in a format suitable for input to `col2rgb`

- alpha
A numeric vector of alpha values between 0 and 1.

- radii
A numeric vector of sphere radii.

- vertices
A 3-column matrix of vertex coordinates.

- normals
A 3-column matrix of vertex normals.

- origins
A 2-column matrix of origins for text or sprites.

- texcoords
A 2-column matrix of texture coordinates.

All attributes must have the same number of entries (rows for the matrices)
as the `ages`

vector. The `births`

vector must have the
same number of entries as the number of vertices in the object.

Not all objects contain all attributes listed here; if one is
chosen that is not a property of the corresponding object, a
Javascript `alert()`

will be generated.

## Value

A character vector of class `c("ageSetter", "propertySetter")`

containing Javascript
code defining a function suitable for use in a `propertySlider`

.
The function takes a single argument, `time`

, and uses it to compute the
“age” of vertex `i`

as `time - births[i]`

.
Those are then used with the `ages`

argument to linearly interpolate settings of the specified attributes.
Extrapolation is constant. Repeated values in `ages`

can be used
to obtain discontinuities in the settings.