[Rd] Help requested: writing text to a raster in memory

[Duncan Murdoch]

I am in the process of updating the rgl package.  One thing I'd like to 
do is to change text support in it when using OpenGL to display to be 
more like the way text is drawn in WebGL displays (i.e. the ones 
rglwidget() produces).

Currently in R, rgl uses the FTGL library to draw text.  That library is 
unsupported these days, and uses the old fixed pipeline in OpenGL.

In WebGL, text is displayed by "shaders", programs that run on the GPU. 
Javascript code prepares bitmap images of the text to display, then the 
shader transfers parts of that bitmap to the output display.

I'd like to duplicate the WebGL process in the C++ code running the 
OpenGL display in R.  The first step in this is to render a character 
vector full of text into an in-memory raster, taking account of font, 
cex, etc.  (I want one raster for the whole vector, with a recording of 
locations from which the shader should get each component of it.)

It looks to me as though I could do this using the ragg::agg_capture 
device in R code, but I'd prefer to do it all in C++ code because I may 
need to make changes to the raster at times when it's not safe to call 
back to R, e.g. if some user interaction requires the axis labels to be 
recomputed and redrawn.

Does anyone with experience doing this kind of thing know of examples I 
can follow, or have advice on how to proceed?  Or want to volunteer to 
help with this?

Duncan Murdoch