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

[Duncan Murdoch]

I'm somewhat aware of how tricky it all is.  For now I'm going to do it 
in R (usng textshaping for layout and base graphics on the 
ragg::agg_capture device to draw to the bitmap).  I'll avoid allowing 
changes to happen in the C++ code.

Eventually I'll see if I can translate the code into C++.  I know 
textshaping has a C interface, but for the actual drawing I'll have to 
work something else out.  Or maybe just leave it in R, and only try to 
write a new bitmap when it's safe.

For future reference, will the measurements reported by 
textshaping::shape_text() match the values used by your Cairo package, 
or are equivalent measurements available elsewhere?

Duncan Murdoch

On 24/09/2023 6:55 p.m., Simon Urbanek wrote:
> Duncan,
> 
> drawing text is one of the most complicated things you can do, so it really depends how for you want to go. You can do it badly with a simple cairo show_text API. The steps involved in doing it properly are detecting the direction of the language, finding fonts, finding glyphs (resolving ligatures), applying hints, drawing glyphs etc. Fortunately there are libraries that help with than, but even then it's non-trivial. Probably the most modern pipeline is icu + harfbuzz + freetype + fontconfig + cairo. This is implemented, e.g in https://github.com/s-u/Cairo/blob/master/src/cairotalk.c (the meat is in  L608-) and for all but the drawing part there is an entire R package (in C++) devoted to this: https://github.com/r-lib/textshaping/tree/main/src -- Thomas Lin Pedersen is probably THE expert on this.
> 
> Cheers,
> Simon
> 
> 
>> On 24/09/2023, at 7:44 AM, Duncan Murdoch <murdoch.duncan using gmail.com> wrote:
>>
>> 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
>>
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>