l3backend: adding new DVI specials?

[Joseph Wright]

On 13/02/2020 04:26, Norbert Preining wrote:
> Hi all,
> 
> the recent update of the l3 packages changed something with the DVI
> specials. pyxplot suddently gives warnings about unhandled DVI specials:
> 	header=l3backend-dvips.pro
> is this a known change and to be expected?
> 
> Best
> 
> Norbert

Hello Norbert,

A while ago now we added some code to l3backend to start to address 'out 
of the box' PDF feature handling. That's for stuff like hyperlinks, 
color, etc. In DVI mode, one needs stuff adding to the PostScript 
header. (At present not all of this is 'live', in particular link breaking.)

Initially, we simply added everything as \special lines directly 
containing the PostScript, but that leads to warnings with dvipdfmx if 
the right backend isn't selected. It also meant there was lots of 
material in the \showbox output. Moving it to a .pro file meant that 
inclusion used only one line

     \special{header=l3backend-dvips.pro}

and this line is simply ignored by dvipdfmx. We realised that other 
systems might not know this one line, but felt that it was hopefully 
possible that a similar '"skip header=" lines' might be added to any 
cases. The one we had in mind was dvisvgm: I'm not familiar with pyxplot.

Joseph