l3backend: dvips driver for draw does not properly handle scoped boxes/coffins

[jasperhabicht]

It seems that the PS code generated by the l3draw driver/backend for dvips is not correct so that a stack underflow error occurs when boxes or coffins are scoped inside a l3draw environment. No error occurs if the box/coffin is not scoped.

Consider the following MWE (also see this issue on TeX.SX where some ideas regarding the reason for this issue have been mentioned already):

\documentclass[border=10pt]{standalone}
\usepackage{l3draw}

\begin{document}
    \ExplSyntaxOn
    \draw_begin:
    
        \draw_path_circle:nn { 0cm , 0cm } { 1cm }
        \draw_path_use_clear:n { stroke }
        
        \draw_scope_begin:
        \hbox_set:Nn \l_tmpa_box {foo}
        \draw_box_use:N \l_tmpa_box
        \draw_scope_end:
    
    \draw_end:
    \ExplSyntaxOff    
\end{document}

Steps

1. run latex to produce a DVI file;
2. run dvips to produce PS file;
3. run Acrobat Distiller or some other software parsing the PS file.

[josephwright]

Drat: I had hoped to avoid having to manually save/restore stuff as this more-or-less relies on internals of how dvips sets things up. But I can't think of another way around this other than using the approach pgf does - I guess I'll need to adjust.

[davidcarlisle]

Not sure how safe it is but adding this to the example...

\cs_set_protected:Npn \__draw_backend_scope_begin:
{ \__draw_backend_literal:n {
    @endspecial ~ save ~ @beginspecial } }
\cs_set_protected:Npn \__draw_backend_scope_end:
  { \__draw_backend_literal:n { @endspecial ~ restore ~ @beginspecial} }

produces

dvisvgm also manages to understand this dvips syntax enough and the resulting svg looks like

[josephwright]

@davidcarlisle Your idea and my original approach to the box insertion share a common issue. By issuing @endspecial, a scope is closed. That means that if a user has a setting before the box, e.g. a transformation or colour change, it would not apply after the box. For example, before the change I've commited,

\documentclass[border=10pt]{standalone}
\usepackage{l3draw}
\begin{document}
\ExplSyntaxOn
  \draw_begin:
    \color_select:n { red }
    \draw_path_circle:nn { 0cm , 0cm } { 1cm }
    \draw_path_use_clear:n { stroke }
    \hbox_set:Nn \l_tmpa_box {foo}
    \draw_box_use:N \l_tmpa_box
    \draw_path_circle:nn { 0cm , 0cm } { 2cm }
    \draw_path_use_clear:n { stroke }
  \draw_end:
\ExplSyntaxOff    
\end{document}

gives the wrong result with dvips. So I think a change is needed: I'll make some more notes in the sources and add a test or two.

[josephwright]

I think things are OK now - testing suggests so.

[davidcarlisle]

I saw as I posted that you'd just done a checkin saving and restoring explicitly (which looks cleaner anyway than relying on dvips output quite so much). but I'm surprised it closes a scope if used as suggested. The idea was you'd end up with

@beginspecial % from dvips
@endspecial,   % from the \special content
save                % from the \special content
@beginspecial % from the \special content
@endspecial,  % from dvips

so the save is in the outer context and the two empty scopes do nothing, then you do the same wtih restore so following text should be in the original scope.

[josephwright]

I meant that if you end up with for example

@beginspecial % from start-of-picture
1.0 0.0 0.0 TeXcolorrgb % from a color
@endspecial,   % from the \special content
save                % from the \special content
@beginspecial % from the \special content
...                      % Content here
@endspecial,  % from dvips

then the content will come out black not red. As a concrete example, using dvips try

\documentclass{article}
\usepackage{l3draw}
\begin{document}
\ExplSyntaxOn
\cs_set_protected:Npn \__draw_backend_scope_begin:
{ \__draw_backend_literal:n {
    @endspecial ~ save ~ @beginspecial } }
\cs_set_protected:Npn \__draw_backend_scope_end:
  { \__draw_backend_literal:n { @endspecial ~ restore ~ @beginspecial} }
\draw_begin:
  \color_select:n { red }
  \draw_scope_begin:
  \draw_scope_end:
  \draw_path_moveto:n { 0cm, 0cm }
  \draw_path_lineto:n { 10cm, 0cm }
  \draw_path_moveto:n { 5cm, 0cm }
  \draw_path_lineto:n { 5cm , 5cm }
  \draw_path_use_clear:n { stroke }
\draw_end:
\ExplSyntaxOff    
\end{document}

with and without the lines commented. (You also see the line thickness revert to the one not set by l3draw.)

[josephwright]

@davidcarlisle BTW, dvips doesn't add @beginspecial automatically, that comes from the \draw_begin: implementation.

[davidcarlisle]

ah if you get stuf between the begin and end that would be bad:, agreed:-)