% \iffalse meta-comment
%
%% File: l3backend-pdf.dtx
%
% Copyright (C) 2019-2024 The LaTeX Project
%
% It may be distributed and/or modified under the conditions of the
% LaTeX Project Public License (LPPL), either version 1.3c of this
% license or (at your option) any later version.  The latest version
% of this license is in the file
%
%    https://www.latex-project.org/lppl.txt
%
% This file is part of the "l3backend bundle" (The Work in LPPL)
% and all files in that bundle must be distributed together.
%
% -----------------------------------------------------------------------
%
% The development version of the bundle can be found at
%
%    https://github.com/latex3/latex3
%
% for those people who are interested.
%
%<*driver>
\documentclass[full,kernel]{l3doc}
\begin{document}
  \DocInput{\jobname.dtx}
\end{document}
%</driver>
% \fi
%
% \title{^^A
%   The \pkg{l3backend-pdf} module\\ Backend PDF features^^A
% }
%
% \author{^^A
%  The \LaTeX{} Project\thanks
%    {^^A
%      E-mail:
%        \href{mailto:latex-team@latex-project.org}
%          {latex-team@latex-project.org}^^A
%    }^^A
% }
%
% \date{Released 2024-03-14}
%
% \maketitle
%
% \begin{documentation}
%
% \end{documentation}
%
% \begin{implementation}
%
% \section{\pkg{l3backend-pdf} implementation}
%
%    \begin{macrocode}
%<*package>
%<@@=pdf>
%    \end{macrocode}
%
% Setting up PDF resources is a complex area with only limited documentation
% in the engine manuals. The following code builds heavily on existing ideas
% from \pkg{hyperref} work by Sebastian Rahtz and Heiko Oberdiek, and
% significant contributions by Alexander Grahn, in addition to the specific
% code referenced a various points.
%
% \subsection{Shared code}
%
% A very small number of items that belong at the backend level but which
% are common to most backends.
%
%    \begin{macrocode}
%<*!dvisvgm>
%    \end{macrocode}
%
% \begin{variable}{\l_@@_internal_box}
%    \begin{macrocode}
\box_new:N \l_@@_internal_box
%    \end{macrocode}
% \end{variable}
%
%    \begin{macrocode}
%</!dvisvgm>
%    \end{macrocode}
%
% \subsection{\texttt{dvips} backend}
%
%    \begin{macrocode}
%<*dvips>
%    \end{macrocode}
%
% \begin{macro}{\@@_backend_pdfmark:n, \@@_backend_pdfmark:e}
%   Used often enough it should be a separate function.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_pdfmark:n #1
  { \__kernel_backend_postscript:n { mark #1 ~ pdfmark } }
\cs_generate_variant:Nn \@@_backend_pdfmark:n { e }
%    \end{macrocode}
% \end{macro}
%
% \subsubsection{Catalogue entries}
%
% \begin{macro}{\@@_backend_catalog_gput:nn, \@@_backend_info_gput:nn}
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_catalog_gput:nn #1#2
  { \@@_backend_pdfmark:n { { Catalog } << /#1 ~ #2 >> /PUT } }
\cs_new_protected:Npn \@@_backend_info_gput:nn #1#2
  { \@@_backend_pdfmark:n { /#1 ~ #2 /DOCINFO } }
%    \end{macrocode}
% \end{macro}
%
% \subsubsection{Objects}
%
% \begin{variable}{\g_@@_backend_object_int}
%   For tracking objects.
%    \begin{macrocode}
\int_new:N \g_@@_backend_object_int
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}{\@@_backend_object_new:n}
% \begin{macro}[EXP]{\@@_backend_object_ref:n}
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_object_new:n #1
  {
    \int_gincr:N \g_@@_backend_object_int
    \int_const:cn
      { c_@@_object_ \tl_to_str:n {#1} _int }
      { \g_@@_backend_object_int }
  }
\cs_new:Npn \@@_backend_object_ref:n #1
  { { pdf.obj \int_use:c { c_@@_object_ \tl_to_str:n {#1} _int } } }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {
%     \@@_backend_object_write:nnn, \@@_backend_object_write:nne,
%     \@@_backend_object_write_aux:nnn
%   }
% \begin{macro}
%   {
%     \@@_backend_object_write_array:nn   ,
%     \@@_backend_object_write_dict:nn    ,
%     \@@_backend_object_write_fstream:nn ,
%     \@@_backend_object_write_stream:nn
%   }
% \begin{macro}{\@@_backend_object_write_stream:nnn}
%   This is where we choose the actual type: some work to get things
%   right. To allow code sharing with the anonymous version, we use an
%   auxiliary.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_object_write:nnn #1#2#3
  {
    \@@_backend_object_write_aux:nnn
      { \@@_backend_object_ref:n {#1} }
      {#2} {#3}
  }
\cs_generate_variant:Nn \@@_backend_object_write:nnn { nne }
\cs_new_protected:Npn \@@_backend_object_write_aux:nnn #1#2#3
  {
    \@@_backend_pdfmark:e
      {
        /_objdef ~ #1
        /type
        \str_case:nn {#2}
          {
            { array }   { /array }
            { dict }    { /dict }
            { fstream } { /stream }
            { stream }  { /stream }
          }
        /OBJ
      }
    \use:c { @@_backend_object_write_ #2 :nn } {#1} {#3}
  }
\cs_new_protected:Npn \@@_backend_object_write_array:nn #1#2
  {
    \@@_backend_pdfmark:e
      { #1 ~0~ [ ~ \exp_not:n {#2} ~ ] ~ /PUTINTERVAL }
  }
\cs_new_protected:Npn \@@_backend_object_write_dict:nn #1#2
  {
    \@@_backend_pdfmark:e
      { #1 << \exp_not:n {#2} >> /PUT }
  }
\cs_new_protected:Npn \@@_backend_object_write_fstream:nn #1#2
  {
    \exp_args:Ne
      \@@_backend_object_write_fstream:nnn {#1} #2
  }
\cs_new_protected:Npn \@@_backend_object_write_fstream:nnn #1#2#3
  {
    \__kernel_backend_postscript:n
      {
        SDict ~ begin ~
        mark ~ #1 ~ << #2 >> /PUT ~ pdfmark ~
        mark ~ #1 ~ ( #3 )~ ( r )~ file ~ /PUT ~ pdfmark ~
        end
      }
  }
\cs_new_protected:Npn \@@_backend_object_write_stream:nn #1#2
  {
    \exp_args:Ne
      \@@_backend_object_write_stream:nnn {#1} #2
  }
\cs_new_protected:Npn \@@_backend_object_write_stream:nnn #1#2#3
  {
    \__kernel_backend_postscript:n
      {
        mark ~ #1 ~ ( #3 ) /PUT ~ pdfmark ~
        mark ~ #1 ~ << #2 >> /PUT ~ pdfmark
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_backend_object_now:nn, \@@_backend_object_now:ne}
%   No anonymous objects, so things are done manually.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_object_now:nn #1#2
  {
    \int_gincr:N \g_@@_backend_object_int
    \@@_backend_object_write_aux:nnn
      { { pdf.obj \int_use:N \g_@@_backend_object_int } }
      {#1} {#2}
  }
\cs_generate_variant:Nn \@@_backend_object_now:nn { ne }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_backend_object_last:}
%   Much like the annotation version.
%    \begin{macrocode}
\cs_new:Npn \@@_backend_object_last:
  { { pdf.obj \int_use:N \g_@@_backend_object_int } }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_backend_pageobject_ref:n}
%   Page references are easy in \texttt{dvips}.
%    \begin{macrocode}
\cs_new:Npn \@@_backend_pageobject_ref:n #1
  { { Page #1 } }
%    \end{macrocode}
% \end{macro}
%
% \subsubsection{Annotations}
%
% In \texttt{dvips}, annotations have to be constructed manually. As such,
% we need the object code above for some definitions.
%
% \begin{variable}{\l_@@_backend_content_box}
%   The content of an annotation.
%    \begin{macrocode}
\box_new:N \l_@@_backend_content_box
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_backend_model_box}
%   For creating model sizing for links.
%    \begin{macrocode}
\box_new:N \l_@@_backend_model_box
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\g_@@_backend_annotation_int}
%   Needed as objects which are not annotations could be created.
%    \begin{macrocode}
\int_new:N \g_@@_backend_annotation_int
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}{\@@_backend_annotation:nnnn}
%   Annotations are objects, but we track them separately. Notably, they are
%   not in the object data lists. Here, to get the co-ordinates of the
%   annotation, we need to have the data collected at the PostScript level.
%   That requires a bit of box trickery (effectively a \LaTeXe{} |picture|
%   of zero size). Once the data is collected, use it to set up the annotation
%   border.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_annotation:nnnn #1#2#3#4
  {
    \exp_args:Nf \@@_backend_annotation_aux:nnnn
      { \dim_eval:n {#1} } {#2} {#3} {#4}
  }
\cs_new_protected:Npn \@@_backend_annotation_aux:nnnn #1#2#3#4
  {
    \box_move_down:nn {#3}
      { \hbox:n { \__kernel_backend_postscript:n { pdf.save.ll } } }
    \box_move_up:nn {#2}
      {
        \hbox:n
          {
            \__kernel_kern:n {#1}
            \__kernel_backend_postscript:n { pdf.save.ur }
            \__kernel_kern:n { -#1 }
          }
      }
    \int_gincr:N \g_@@_backend_object_int
    \int_gset_eq:NN \g_@@_backend_annotation_int \g_@@_backend_object_int
    \@@_backend_pdfmark:e
      {
        /_objdef { pdf.obj \int_use:N \g_@@_backend_object_int }
        pdf.rect
        #4 ~
        /ANN
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_backend_annotation_last:}
%   Provide the last annotation we created: could get tricky of course if
%   other packages are loaded.
%    \begin{macrocode}
\cs_new:Npn \@@_backend_annotation_last:
  { { pdf.obj \int_use:N \g_@@_backend_annotation_int } }
%    \end{macrocode}
% \end{macro}
%
% \begin{variable}{\g_@@_backend_link_int}
%   To track annotations which are links.
%    \begin{macrocode}
\int_new:N \g_@@_backend_link_int
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\g_@@_backend_link_dict_tl}
%   To pass information to the end-of-link function.
%    \begin{macrocode}
\tl_new:N \g_@@_backend_link_dict_tl
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\g_@@_backend_link_sf_int}
%   Needed to save/restore space factor, which is needed to deal with the face
%   we need a box.
%    \begin{macrocode}
\int_new:N \g_@@_backend_link_sf_int
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\g_@@_backend_link_math_bool}
%   Needed to save/restore math mode.
%    \begin{macrocode}
\bool_new:N \g_@@_backend_link_math_bool
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\g_@@_backend_link_bool}
%   Track link formation: we cannot nest at all.
%    \begin{macrocode}
\bool_new:N \g_@@_backend_link_bool
%    \end{macrocode}
% \end{variable}
%
% \begin{variable}{\l_@@_breaklink_pdfmark_tl}
%   Swappable content for link breaking.
%    \begin{macrocode}
\tl_new:N \l_@@_breaklink_pdfmark_tl
\tl_set:Nn \l_@@_breaklink_pdfmark_tl { pdfmark }
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}{\@@_breaklink_postscript:n}
%   To allow dropping material unless link breaking is active.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_breaklink_postscript:n #1 { }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_breaklink_usebox:N}
%   Swappable box unpacking or use.
%    \begin{macrocode}
\cs_new_eq:NN \@@_breaklink_usebox:N \box_use:N
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_backend_link_begin_goto:nnw, \@@_backend_link_begin_user:nnw}
% \begin{macro}{\@@_backend_link:nw, \@@_backend_link_aux:nw}
% \begin{macro}{\@@_backend_link_end:, \@@_backend_link_end_aux:}
% \begin{macro}{\@@_backend_link_minima:}
% \begin{macro}{\@@_backend_link_outerbox:n}
% \begin{macro}{\@@_backend_link_sf_save:, \@@_backend_link_sf_restore:}
%   Links are crated like annotations but with dedicated code to allow for
%   adjusting the size of the rectangle. In contrast to \pkg{hyperref}, we
%   grab the link content as a box which can then unbox: this allows the same
%   interface as for \pdfTeX{}.
%
%   Notice that the link setup here uses |/Action| not |/A|. That is because
%   Distiller \emph{requires} this trigger word, rather than a \enquote{raw}
%   PDF dictionary key (Ghostscript can handle either form).
%
%   Taking the idea of |evenboxes| from \pkg{hypdvips}, we implement a minimum
%   box height and depth for link placement. This means that \enquote{underlining}
%   with a hyperlink will generally give an even appearance. However, to ensure
%   that the full content is always above the link border, we do not allow
%   this to be negative (contrast \pkg{hypdvips} approach). The result should
%   be similar to \pdfTeX{} in the vast majority of foreseeable cases.
%
%   The object number for a link is saved separately from the rest of the
%   dictionary as this allows us to insert it just once, at either an
%   unbroken link or only in the first line of a broken one. That makes the
%   code clearer but also avoids a low-level PostScript error with the code
%   as taken from \pkg{hypdvips}.
%
%   Getting the outer dimensions of the text area may be better using a two-pass
%   approach and |\tex_savepos:D|. That plus generic mode are still to re-examine.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_link_begin_goto:nnw #1#2
  {
    \@@_backend_link_begin:nw
      { #1 /Subtype /Link /Action << /S /GoTo /D ( #2 ) >> }
  }
\cs_new_protected:Npn \@@_backend_link_begin_user:nnw #1#2
  { \@@_backend_link_begin:nw {#1#2} }
\cs_new_protected:Npn \@@_backend_link_begin:nw #1
  {
    \bool_if:NF \g_@@_backend_link_bool
      { \@@_backend_link_begin_aux:nw {#1} }
  }
%    \end{macrocode}
%   The definition of |pdf.link.dict| here is needed as there is code in the
%   PostScript headers for breaking links, and that can only work with this
%   available.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_link_begin_aux:nw #1
  {
    \bool_gset_true:N \g_@@_backend_link_bool
    \__kernel_backend_postscript:n
      { /pdf.link.dict ( #1 ) def }
    \tl_gset:Nn \g_@@_backend_link_dict_tl {#1}
    \@@_backend_link_sf_save:
    \mode_if_math:TF
      { \bool_gset_true:N \g_@@_backend_link_math_bool }
      { \bool_gset_false:N \g_@@_backend_link_math_bool }
    \hbox_set:Nw \l_@@_backend_content_box
      \@@_backend_link_sf_restore:
      \bool_if:NT \g_@@_backend_link_math_bool
        { \c_math_toggle_token }
  }
\cs_new_protected:Npn \@@_backend_link_end:
  {
    \bool_if:NT \g_@@_backend_link_bool
      { \@@_backend_link_end_aux: }
  }
\cs_new_protected:Npn \@@_backend_link_end_aux:
  {
      \bool_if:NT \g_@@_backend_link_math_bool
        { \c_math_toggle_token }
      \@@_backend_link_sf_save:
    \hbox_set_end:
    \@@_backend_link_minima:
    \hbox_set:Nn \l_@@_backend_model_box { Gg }
    \exp_args:Ne \@@_backend_link_outerbox:n
      {
        \int_if_odd:nTF { \value { page } }
          { \oddsidemargin }
          { \evensidemargin }
      }
    \box_move_down:nn { \box_dp:N \l_@@_backend_content_box }
      { \hbox:n { \__kernel_backend_postscript:n { pdf.save.linkll } } }
    \@@_breaklink_postscript:n { pdf.bordertracking.begin }
    \@@_breaklink_usebox:N \l_@@_backend_content_box
    \@@_breaklink_postscript:n { pdf.bordertracking.end }
    \box_move_up:nn { \box_ht:N \l_@@_backend_content_box }
      {
        \hbox:n
          { \__kernel_backend_postscript:n { pdf.save.linkur } }
      }
    \int_gincr:N \g_@@_backend_object_int
    \int_gset_eq:NN \g_@@_backend_link_int \g_@@_backend_object_int
    \__kernel_backend_postscript:e
      {
        mark
        /_objdef { pdf.obj \int_use:N \g_@@_backend_link_int }
        \g_@@_backend_link_dict_tl \c_space_tl
        pdf.rect
        /ANN ~ \l_@@_breaklink_pdfmark_tl
      }
    \@@_backend_link_sf_restore:
    \bool_gset_false:N \g_@@_backend_link_bool
  }
\cs_new_protected:Npn \@@_backend_link_minima:
  {
    \hbox_set:Nn \l_@@_backend_model_box { Gg }
    \__kernel_backend_postscript:e
      {
        /pdf.linkdp.pad ~
          \dim_to_decimal:n
            {
              \dim_max:nn
                {
                    \box_dp:N \l_@@_backend_model_box
                  - \box_dp:N \l_@@_backend_content_box
                }
                { 0pt }
            } ~
              pdf.pt.dvi ~ def
        /pdf.linkht.pad ~
          \dim_to_decimal:n
            {
              \dim_max:nn
                {
                    \box_ht:N \l_@@_backend_model_box
                  - \box_ht:N \l_@@_backend_content_box
                }
                { 0pt }
            } ~
              pdf.pt.dvi ~ def
      }
  }
\cs_new_protected:Npn \@@_backend_link_outerbox:n #1
  {
    \__kernel_backend_postscript:e
      {
        /pdf.outerbox
          [
            \dim_to_decimal:n {#1} ~
            \dim_to_decimal:n { -\box_dp:N \l_@@_backend_model_box } ~
            \dim_to_decimal:n { #1 + \textwidth } ~
            \dim_to_decimal:n { \box_ht:N \l_@@_backend_model_box }
          ]
          [ exch { pdf.pt.dvi } forall ] def
        /pdf.baselineskip ~
          \dim_to_decimal:n { \tex_baselineskip:D } ~ dup ~ 0 ~ gt
            { pdf.pt.dvi ~ def }
            { pop ~ pop }
          ifelse
      }
  }
\cs_new_protected:Npn \@@_backend_link_sf_save:
  {
    \int_gset:Nn \g_@@_backend_link_sf_int
      {
        \mode_if_horizontal:TF
          { \tex_spacefactor:D }
          { 0 }
      }
  }
\cs_new_protected:Npn \@@_backend_link_sf_restore:
  {
    \mode_if_horizontal:T
      {
        \int_compare:nNnT \g_@@_backend_link_sf_int > { 0 }
          { \int_set_eq:NN \tex_spacefactor:D \g_@@_backend_link_sf_int }
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
%   Hooks to allow link breaking: something will be needed in format mode
%   at some stage. At present this code is disabled as there is an open
%   question about the name of the hook: to be resolved at the \LaTeXe{}
%   end.
%    \begin{macrocode}
\use_none:n
  {
    \cs_if_exist:NT \@makecol@hook
      {
        \tl_put_right:Nn \@makecol@hook
          {
            \box_if_empty:NF \l_shipout_box
              {
                \vbox_set:Nn \l_shipout_box
                  {
                    \__kernel_backend_postscript:n
                      {
                        pdf.globaldict /pdf.brokenlink.rect ~ known
                          { pdf.bordertracking.continue }
                        if
                      }
                    \vbox_unpack_drop:N \l_shipout_box
                    \__kernel_backend_postscript:n
                      { pdf.bordertracking.endpage }
                  }
              }
          }
        \tl_set:Nn \l_@@_breaklink_pdfmark_tl { pdf.pdfmark }
        \cs_set_eq:NN \@@_breaklink_postscript:n \__kernel_backend_postscript:n
        \cs_set_eq:NN \@@_breaklink_usebox:N \hbox_unpack:N
      }
  }
%    \end{macrocode}
%
% \begin{macro}{\@@_backend_link_last:}
%   The same as annotations, but with a custom integer.
%    \begin{macrocode}
\cs_new:Npn \@@_backend_link_last:
  { { pdf.obj \int_use:N \g_@@_backend_link_int } }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_backend_link_margin:n}
%   Convert to big points and pass to PostScript.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_link_margin:n #1
  {
    \__kernel_backend_postscript:e
      {
        /pdf.linkmargin { \dim_to_decimal:n {#1} ~ pdf.pt.dvi } def
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_backend_destination:nn}
% \begin{macro}{\@@_backend_destination:nnnn, \@@_backend_destination_aux:nnnn}
%   Here, we need to turn the zoom into a scale. We also need to know where
%   the current anchor point actually is: worked out in PostScript. For the
%   rectangle version, we have a bit more PostScript: we need two points.
%   fitr without rule spec doesn't work, so it falls back to \texttt{/Fit} here.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_destination:nn #1#2
  {
    \__kernel_backend_postscript:n { pdf.dest.anchor }
    \@@_backend_pdfmark:e
      {
        /View
        [
          \str_case:nnF {#2}
            {
              { xyz }   { /XYZ ~ pdf.dest.point ~ null }
              { fit }   { /Fit }
              { fitb }  { /FitB }
              { fitbh } { /FitBH ~ pdf.dest.y }
              { fitbv } { /FitBV ~ pdf.dest.x }
              { fith }  { /FitH ~ pdf.dest.y }
              { fitv }  { /FitV ~ pdf.dest.x }
              { fitr }  { /Fit }
            }
            {
              /XYZ ~ pdf.dest.point ~ \fp_eval:n { (#2) / 100 }
            }
        ]
        /Dest ( \exp_not:n {#1} ) cvn
        /DEST
      }
  }
\cs_new_protected:Npn \@@_backend_destination:nnnn #1#2#3#4
  {
    \exp_args:Ne \@@_backend_destination_aux:nnnn
      { \dim_eval:n {#2} } {#1} {#3} {#4}
  }
\cs_new_protected:Npn \@@_backend_destination_aux:nnnn #1#2#3#4
  {
    \vbox_to_zero:n
      {
        \__kernel_kern:n {#4}
        \hbox:n { \__kernel_backend_postscript:n { pdf.save.ll } }
        \tex_vss:D
      }
    \__kernel_kern:n {#1}
    \vbox_to_zero:n
      {
        \__kernel_kern:n { -#3 }
        \hbox:n { \__kernel_backend_postscript:n { pdf.save.ur } }
        \tex_vss:D
      }
    \__kernel_kern:n { -#1 }
    \@@_backend_pdfmark:n
      {
        /View
        [
          /FitR ~
            pdf.llx ~ pdf.lly ~ pdf.dest2device ~
            pdf.urx ~ pdf.ury ~ pdf.dest2device
        ]
        /Dest ( #2 ) cvn
        /DEST
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsubsection{Structure}
%
% \begin{macro}{\@@_backend_compresslevel:n}
% \begin{macro}{\@@_backend_compress_objects:n}
%   Doable for the usual \texttt{ps2pdf} method.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_compresslevel:n #1
  {
    \int_compare:nNnT {#1} = 0
      {
        \__kernel_backend_literal_postscript:n
          {
            /setdistillerparams ~ where
              { pop << /CompressPages ~ false >> setdistillerparams }
            if
          }
      }
  }
\cs_new_protected:Npn \@@_backend_compress_objects:n #1
  {
    \bool_if:nF {#1}
      {
        \__kernel_backend_literal_postscript:n
          {
            /setdistillerparams ~ where
              { pop << /CompressStreams ~ false >> setdistillerparams }
            if
          }
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {\@@_backend_version_major_gset:n, \@@_backend_version_minor_gset:n}
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_version_major_gset:n #1
  {
    \cs_gset:Npe \@@_backend_version_major: { \int_eval:n {#1} }
  }
\cs_new_protected:Npn \@@_backend_version_minor_gset:n #1
  {
    \cs_gset:Npe \@@_backend_version_minor: { \int_eval:n {#1} }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_backend_version_major:, \@@_backend_version_minor:}
%   Data not available!
%    \begin{macrocode}
\cs_new:Npn \@@_backend_version_major: { -1 }
\cs_new:Npn \@@_backend_version_minor: { -1 }
%    \end{macrocode}
% \end{macro}
%
% \subsubsection{Marked content}
%
% \begin{macro}{\@@_backend_bdc:nn}
% \begin{macro}{\@@_backend_emc:}
%   Simple wrappers.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_bdc:nn #1#2
  { \@@_backend_pdfmark:n { /#1 ~ #2 /BDC } }
\cs_new_protected:Npn \@@_backend_emc:
  { \@@_backend_pdfmark:n { /EMC } }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
%    \begin{macrocode}
%</dvips>
%    \end{macrocode}
%
% \subsection{\LuaTeX{} and \pdfTeX{} backend}
%
%    \begin{macrocode}
%<*luatex|pdftex>
%    \end{macrocode}
%
% \subsubsection{Annotations}
%
% \begin{macro}{\@@_backend_annotation:nnnn}
%   Simply pass the raw data through, just dealing with evaluation of dimensions.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_annotation:nnnn #1#2#3#4
  {
%<*luatex>
    \tex_pdfextension:D annot ~
%</luatex>
%<*pdftex>
    \tex_pdfannot:D
%</pdftex>
      width  ~ \dim_eval:n {#1} ~
      height ~ \dim_eval:n {#2} ~
      depth  ~ \dim_eval:n {#3} ~
      {#4}
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_backend_annotation_last:}
%   A tiny amount of extra data gets added here; we use \texttt{x}-type
%   expansion to get the space in the right place and form. The \enquote{extra}
%   space in the \LuaTeX{} version is \emph{required} as it is consumed in
%   finding the end of the keyword.
%    \begin{macrocode}
\cs_new:Npe \@@_backend_annotation_last:
  {
    \exp_not:N \int_value:w
%<*luatex>
      \exp_not:N \tex_pdffeedback:D lastannot ~
%</luatex>
%<*pdftex>
      \exp_not:N \tex_pdflastannot:D
%</pdftex>
      \c_space_tl 0 ~ R
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}
%   {\@@_backend_link_begin_goto:nnw, \@@_backend_link_begin_user:nnw}
% \begin{macro}{\@@_backend_link_begin:nnnw}
% \begin{macro}{\@@_backend_link_end:}
%   Links are all created using the same internals.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_link_begin_goto:nnw #1#2
  { \@@_backend_link_begin:nnnw {#1} { goto~name } {#2} }
\cs_new_protected:Npn \@@_backend_link_begin_user:nnw #1#2
  { \@@_backend_link_begin:nnnw {#1} { user } {#2} }
\cs_new_protected:Npn \@@_backend_link_begin:nnnw #1#2#3
  {
%<*luatex>
    \tex_pdfextension:D startlink ~
%</luatex>
%<*pdftex>
    \tex_pdfstartlink:D
%</pdftex>
      attr {#1}
      #2 {#3}
  }
\cs_new_protected:Npn \@@_backend_link_end:
  {
%<*luatex>
    \tex_pdfextension:D endlink \scan_stop:
%</luatex>
%<*pdftex>
    \tex_pdfendlink:D
%</pdftex>
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_backend_link_last:}
%   Formatted for direct use.
%    \begin{macrocode}
\cs_new:Npe \@@_backend_link_last:
  {
    \exp_not:N \int_value:w
%<*luatex>
      \exp_not:N \tex_pdffeedback:D lastlink ~
%</luatex>
%<*pdftex>
      \exp_not:N \tex_pdflastlink:D
%</pdftex>
      \c_space_tl 0 ~ R
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_backend_link_margin:n}
%   A simple task: pass the data to the primitive.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_link_margin:n #1
  {
%<*luatex>
    \tex_pdfvariable:D linkmargin
%</luatex>
%<*pdftex>
    \tex_pdflinkmargin:D
%</pdftex>
      \dim_eval:n {#1} \scan_stop:
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_backend_destination:nn}
% \begin{macro}{\@@_backend_destination:nnnn}
%   A simple task: pass the data to the primitive. The |\scan_stop:| deals
%   with the danger of an unterminated keyword. The zoom given here is a
%   percentage, but we need to pass it as \emph{per mille}. The rectangle
%   version is also easy as everything is build in.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_destination:nn #1#2
  {
%<*luatex>
    \tex_pdfextension:D dest ~
%</luatex>
%<*pdftex>
    \tex_pdfdest:D
%</pdftex>
        name {#1}
        \str_case:nnF {#2}
          {
            { xyz }   { xyz }
            { fit }   { fit }
            { fitb }  { fitb }
            { fitbh } { fitbh }
            { fitbv } { fitbv }
            { fith }  { fith }
            { fitv }  { fitv }
            { fitr }  { fitr }
          }
          { xyz ~ zoom \fp_eval:n { #2 * 10 } }
        \scan_stop:
  }
\cs_new_protected:Npn \@@_backend_destination:nnnn #1#2#3#4
  {
%<*luatex>
    \tex_pdfextension:D dest ~
%</luatex>
%<*pdftex>
    \tex_pdfdest:D
%</pdftex>
    name {#1}
    fitr ~
      width  \dim_eval:n {#2} ~
      height \dim_eval:n {#3} ~
      depth  \dim_eval:n {#4} \scan_stop:
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsubsection{Catalogue entries}
%
% \begin{macro}{\@@_backend_catalog_gput:nn, \@@_backend_info_gput:nn}
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_catalog_gput:nn #1#2
  {
%<*luatex>
    \tex_pdfextension:D catalog
%</luatex>
%<*pdftex>
    \tex_pdfcatalog:D
%</pdftex>
      { / #1 ~ #2 }
  }
\cs_new_protected:Npn \@@_backend_info_gput:nn #1#2
  {
%<*luatex>
    \tex_pdfextension:D info
%</luatex>
%<*pdftex>
    \tex_pdfinfo:D
%</pdftex>
      { / #1 ~ #2 }
  }
%    \end{macrocode}
% \end{macro}
%
% \subsubsection{Objects}
%
% \begin{variable}{\g_@@_backend_object_prop}
%   For tracking objects to allow finalisation.
%    \begin{macrocode}
\prop_new:N \g_@@_backend_object_prop
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}{\@@_backend_object_new:n}
% \begin{macro}[EXP]{\@@_backend_object_ref:n}
%   Declaring objects means reserving at the PDF level plus starting
%   tracking.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_object_new:n #1
  {
%<*luatex>
    \tex_pdfextension:D obj ~
%</luatex>
%<*pdftex>
    \tex_pdfobj:D
%</pdftex>
      reserveobjnum ~
      \int_const:cn
        { c_@@_object_ \tl_to_str:n {#1} _int }
%<*luatex>
        { \tex_pdffeedback:D lastobj }
%</luatex>
%<*pdftex>
        { \tex_pdflastobj:D }
%</pdftex>
  }
\cs_new:Npn \@@_backend_object_ref:n #1
  { \int_use:c { c_@@_object_ \tl_to_str:n {#1} _int } ~ 0 ~ R }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_backend_object_write:nnn, \@@_backend_object_write:nne}
% \begin{macro}[EXP]{\@@_backend_object_write:nn}
% \begin{macro}[EXP]{\@@_exp_not_i:nn, \@@_exp_not_ii:nn}
%   Writing the data needs a little information about the structure of the
%   object.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_object_write:nnn #1#2#3
  {
%<*luatex>
    \tex_immediate:D \tex_pdfextension:D obj ~
%</luatex>
%<*pdftex>
    \tex_immediate:D \tex_pdfobj:D
%</pdftex>
      useobjnum ~
      \int_use:c
        { c_@@_object_ \tl_to_str:n {#1} _int }
      \@@_backend_object_write:nn {#2} {#3}
  }
\cs_new:Npn \@@_backend_object_write:nn #1#2
  {
      \str_case:nn {#1}
        {
          { array } { { [ ~ \exp_not:n {#2} ~ ] } }
          { dict }  { { << ~ \exp_not:n {#2} ~ >> } }
          { fstream }
            {
              stream ~ attr ~ { \@@_exp_not_i:nn #2 } ~
                file ~ { \@@_exp_not_ii:nn #2 }
            }
          { stream }
            {
              stream ~ attr ~ { \@@_exp_not_i:nn #2 } ~
                { \@@_exp_not_ii:nn #2 }
            }
        }
  }
\cs_generate_variant:Nn \@@_backend_object_write:nnn { nne }
\cs_new:Npn \@@_exp_not_i:nn #1#2 { \exp_not:n {#1} }
\cs_new:Npn \@@_exp_not_ii:nn #1#2 { \exp_not:n {#2} }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_backend_object_now:nn, \@@_backend_object_now:ne}
%   Much like writing, but direct creation.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_object_now:nn #1#2
  {
%<*luatex>
    \tex_immediate:D \tex_pdfextension:D obj ~
%</luatex>
%<*pdftex>
    \tex_immediate:D \tex_pdfobj:D
%</pdftex>
      \@@_backend_object_write:nn {#1} {#2}
  }
\cs_generate_variant:Nn \@@_backend_object_now:nn { ne }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_backend_object_last:}
%   Much like annotation.
%    \begin{macrocode}
\cs_new:Npe \@@_backend_object_last:
  {
    \exp_not:N \int_value:w
%<*luatex>
      \exp_not:N \tex_pdffeedback:D lastobj ~
%</luatex>
%<*pdftex>
      \exp_not:N \tex_pdflastobj:D
%</pdftex>
      \c_space_tl 0 ~ R
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_backend_pageobject_ref:n}
%   The usual wrapper situation; the three spaces here are essential.
%    \begin{macrocode}
\cs_new:Npe \@@_backend_pageobject_ref:n #1
  {
    \exp_not:N \int_value:w
%<*luatex>
      \exp_not:N \tex_pdffeedback:D pageref
%</luatex>
%<*pdftex>
      \exp_not:N \tex_pdfpageref:D
%</pdftex>
          \c_space_tl #1 \c_space_tl \c_space_tl \c_space_tl 0 ~ R
  }
%    \end{macrocode}
% \end{macro}
%
% \subsubsection{Structure}
%
% \begin{macro}{\@@_backend_compresslevel:n}
% \begin{macro}{\@@_backend_compress_objects:n}
% \begin{macro}{\@@_backend_objcompresslevel:n}
%   Simply pass data to the engine.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_compresslevel:n #1
  {
    \tex_global:D
%<*luatex>
      \tex_pdfvariable:D compresslevel
%</luatex>
%<*pdftex>
      \tex_pdfcompresslevel:D
%</pdftex>
        \int_value:w \int_eval:n {#1} \scan_stop:
  }
\cs_new_protected:Npn \@@_backend_compress_objects:n #1
  {
    \bool_if:nTF {#1}
      { \@@_backend_objcompresslevel:n { 2 } }
      { \@@_backend_objcompresslevel:n { 0 } }
  }
\cs_new_protected:Npn \@@_backend_objcompresslevel:n #1
  {
    \tex_global:D
%<*luatex>
      \tex_pdfvariable:D objcompresslevel
%</luatex>
%<*pdftex>
      \tex_pdfobjcompresslevel:D
%</pdftex>
        #1 \scan_stop:
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {\@@_backend_version_major_gset:n, \@@_backend_version_minor_gset:n}
%   The availability of the primitive is not universal, so we have to test
%   at load time.
%    \begin{macrocode}
\cs_new_protected:Npe \@@_backend_version_major_gset:n #1
  {
%<*luatex>
    \int_compare:nNnT \tex_luatexversion:D > { 106 }
      {
        \exp_not:N \tex_global:D \tex_pdfvariable:D majorversion
          \exp_not:N \int_eval:n {#1} \scan_stop:
      }
%</luatex>
%<*pdftex>
    \cs_if_exist:NT \tex_pdfmajorversion:D
      {
        \exp_not:N \tex_global:D \tex_pdfmajorversion:D
          \exp_not:N \int_eval:n {#1} \scan_stop:
      }
%</pdftex>
  }
\cs_new_protected:Npn \@@_backend_version_minor_gset:n #1
  {
    \tex_global:D
%<*luatex>
      \tex_pdfvariable:D minorversion
%</luatex>
%<*pdftex>
      \tex_pdfminorversion:D
%</pdftex>
        \int_eval:n {#1} \scan_stop:
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_backend_version_major:, \@@_backend_version_minor:}
%   As above.
%    \begin{macrocode}
\cs_new:Npe \@@_backend_version_major:
  {
%<*luatex>
    \int_compare:nNnTF \tex_luatexversion:D > { 106 }
      { \exp_not:N \tex_the:D \tex_pdfvariable:D majorversion }
      { 1 }
%</luatex>
%<*pdftex>
    \cs_if_exist:NTF \tex_pdfmajorversion:D
      { \exp_not:N \tex_the:D \tex_pdfmajorversion:D }
      { 1 }
%</pdftex>
  }
\cs_new:Npn \@@_backend_version_minor:
  {
    \tex_the:D
%<*luatex>
      \tex_pdfvariable:D minorversion
%</luatex>
%<*pdftex>
      \tex_pdfminorversion:D
%</pdftex>
  }
%    \end{macrocode}
% \end{macro}
%
% \subsubsection{Marked content}
%
% \begin{macro}{\@@_backend_bdc:nn}
% \begin{macro}{\@@_backend_emc:}
%   Simple wrappers. May need refinement: see
%   \url{https://chat.stackexchange.com/transcript/message/49970158#49970158}.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_bdc:nn #1#2
  { \__kernel_backend_literal_page:n { /#1 ~ #2 ~ BDC } }
\cs_new_protected:Npn \@@_backend_emc:
  { \__kernel_backend_literal_page:n { EMC } }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
%    \begin{macrocode}
%</luatex|pdftex>
%    \end{macrocode}
%
% \subsection{\texttt{dvipdfmx} backend}
%
%    \begin{macrocode}
%<*dvipdfmx|xetex>
%    \end{macrocode}
%
% \begin{macro}{\@@_backend:n, \@@_backend:e}
%   A generic function for the backend PDF specials: used where we can.
%    \begin{macrocode}
\cs_new_protected:Npe \@@_backend:n #1
  { \__kernel_backend_literal:n { pdf: #1 } }
\cs_generate_variant:Nn \@@_backend:n { e }
%    \end{macrocode}
% \end{macro}
%
% \subsubsection{Catalogue entries}
%
% \begin{macro}{\@@_backend_catalog_gput:nn, \@@_backend_info_gput:nn}
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_catalog_gput:nn #1#2
  { \@@_backend:n { put ~ @catalog << /#1 ~ #2 >> } }
\cs_new_protected:Npn \@@_backend_info_gput:nn #1#2
  { \@@_backend:n { docinfo << /#1 ~ #2 >> } }
%    \end{macrocode}
% \end{macro}
%
% \subsubsection{Objects}
%
% \begin{variable}{\g_@@_backend_object_int, \g_@@_backend_object_prop}
%   For tracking objects to allow finalisation.
%    \begin{macrocode}
\int_new:N \g_@@_backend_object_int
\prop_new:N \g_@@_backend_object_prop
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}{\@@_backend_object_new:n}
% \begin{macro}[EXP]{\@@_backend_object_ref:n}
%   Objects are tracked at the macro level, but we don't have to do anything
%   at this stage.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_object_new:n #1
  {
    \int_gincr:N \g_@@_backend_object_int
    \int_const:cn
      { c_@@_object_ \tl_to_str:n {#1} _int }
      { \g_@@_backend_object_int }
  }
\cs_new:Npn \@@_backend_object_ref:n #1
  { @pdf.obj \int_use:c { c_@@_object_ \tl_to_str:n {#1} _int } }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_backend_object_write:nnn, \@@_backend_object_write:nne}
% \begin{macro}
%   {
%     \@@_backend_object_write_array:nn   ,
%     \@@_backend_object_write_dict:nn    ,
%     \@@_backend_object_write_fstream:nn ,
%     \@@_backend_object_write_stream:nn
%   }
% \begin{macro}{\@@_backend_object_write_stream:nnnn}
%   This is where we choose the actual type.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_object_write:nnn #1#2#3
  {
    \use:c { @@_backend_object_write_ #2 :nn }
      { \@@_backend_object_ref:n {#1} } {#3}
  }
\cs_generate_variant:Nn \@@_backend_object_write:nnn { nne }
\cs_new_protected:Npn \@@_backend_object_write_array:nn #1#2
  {
    \@@_backend:e
      { obj ~ #1 ~ [ ~ \exp_not:n {#2} ~ ] }
  }
\cs_new_protected:Npn \@@_backend_object_write_dict:nn #1#2
  {
    \@@_backend:e
      { obj ~ #1 ~ << ~ \exp_not:n {#2} ~ >> }
  }
\cs_new_protected:Npn \@@_backend_object_write_fstream:nn #1#2
  { \@@_backend_object_write_stream:nnnn { f } {#1} #2 }
\cs_new_protected:Npn \@@_backend_object_write_stream:nn #1#2
  { \@@_backend_object_write_stream:nnnn { } {#1} #2 }
\cs_new_protected:Npn \@@_backend_object_write_stream:nnnn #1#2#3#4
  {
    \@@_backend:e
      {
        #1 stream ~ #2 ~
          ( \exp_not:n {#4} ) ~ << \exp_not:n {#3} >>
      }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_backend_object_now:nn, \@@_backend_object_now:ne}
%   No anonymous objects with \texttt{dvipdfmx} so we have to give an
%   object name.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_object_now:nn #1#2
  {
    \int_gincr:N \g_@@_backend_object_int
    \exp_args:Nne \use:c { @@_backend_object_write_ #1 :nn }
      { @pdf.obj \int_use:N \g_@@_backend_object_int }
      {#2}
  }
\cs_generate_variant:Nn \@@_backend_object_now:nn { ne }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_backend_object_last:}
%    \begin{macrocode}
\cs_new:Npn \@@_backend_object_last:
  { @pdf.obj \int_use:N \g_@@_backend_object_int }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_backend_pageobject_ref:n}
%   Page references are easy in \texttt{dvipdfmx}/\XeTeX{}.
%    \begin{macrocode}
\cs_new:Npn \@@_backend_pageobject_ref:n #1
  { @page #1 }
%    \end{macrocode}
% \end{macro}
%
% \subsubsection{Annotations}
%
% \begin{variable}{\g_@@_backend_annotation_int}
%   Needed as objects which are not annotations could be created.
%    \begin{macrocode}
\int_new:N \g_@@_backend_annotation_int
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}{\@@_backend_annotation:nnnn}
%   Simply pass the raw data through, just dealing with evaluation of dimensions.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_annotation:nnnn #1#2#3#4
  {
    \int_gincr:N \g_@@_backend_object_int
    \int_gset_eq:NN \g_@@_backend_annotation_int \g_@@_backend_object_int
    \@@_backend:e
      {
        ann ~ @pdf.obj \int_use:N \g_@@_backend_object_int \c_space_tl
        width  ~ \dim_eval:n {#1} ~
        height ~ \dim_eval:n {#2} ~
        depth  ~ \dim_eval:n {#3} ~
        << /Type /Annot #4 >>
      }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_backend_annotation_last:}
%    \begin{macrocode}
\cs_new:Npn \@@_backend_annotation_last:
  { @pdf.obj \int_use:N \g_@@_backend_annotation_int }
%    \end{macrocode}
% \end{macro}
%
% \begin{variable}{\g_@@_backend_link_int}
%   To track annotations which are links.
%    \begin{macrocode}
\int_new:N \g_@@_backend_link_int
%    \end{macrocode}
% \end{variable}
%
% \begin{macro}
%   {\@@_backend_link_begin_goto:nnw, \@@_backend_link_begin_user:nnw}
% \begin{macro}{\@@_backend_link_begin:n}
% \begin{macro}{\@@_backend_link_end:}
%   All created using the same internals.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_link_begin_goto:nnw #1#2
  { \@@_backend_link_begin:n { #1 /Subtype /Link /A << /S /GoTo /D ( #2 ) >> } }
\cs_new_protected:Npn \@@_backend_link_begin_user:nnw #1#2
  { \@@_backend_link_begin:n {#1#2} }
\cs_new_protected:Npe \@@_backend_link_begin:n #1
  {
    \exp_not:N \int_gincr:N \exp_not:N  \g_@@_backend_link_int
    \@@_backend:e
      {
        bann ~
        @pdf.lnk
        \exp_not:N \int_use:N \exp_not:N  \g_@@_backend_link_int
        \c_space_tl
        <<
          /Type /Annot
          #1
        >>
      }
  }
\cs_new_protected:Npn \@@_backend_link_end:
  { \@@_backend:n { eann } }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_backend_link_last:}
%   Available using the backend mechanism with a suitably-recent
%   version.
%    \begin{macrocode}
\cs_new:Npn \@@_backend_link_last:
  { @pdf.lnk \int_use:N \g_@@_backend_link_int }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_backend_link_margin:n}
%   Pass to \texttt{dvipdfmx}.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_link_margin:n #1
  { \__kernel_backend_literal:e { dvipdfmx:config~g~ \dim_eval:n {#1} } }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_backend_destination:nn}
% \begin{macro}{\@@_backend_destination:nnnn,\@@_backend_destination_aux:nnnn}
%   Here, we need to turn the zoom into a scale. The method for \texttt{FitR}
%   is from Alexander Grahn: the idea is to avoid needing to do any calculations
%   in \TeX{} by using the backend data for \texttt{@xpos} and \texttt{@ypos}.
%   \texttt{/FitR} without rule spec doesn't work, so it falls back to
%   \texttt{/Fit} here.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_destination:nn #1#2
  {
    \@@_backend:e
      {
        dest ~ ( \exp_not:n {#1} )
        [
          @thispage
          \str_case:nnF {#2}
            {
              { xyz }   { /XYZ ~ @xpos ~ @ypos ~ null }
              { fit }   { /Fit }
              { fitb }  { /FitB }
              { fitbh } { /FitBH }
              { fitbv } { /FitBV ~ @xpos }
              { fith }  { /FitH ~ @ypos }
              { fitv }  { /FitV ~ @xpos }
              { fitr }  { /Fit }
            }
            { /XYZ ~ @xpos ~ @ypos ~ \fp_eval:n { (#2) / 100 } }
        ]
      }
  }
\cs_new_protected:Npn \@@_backend_destination:nnnn #1#2#3#4
  {
    \exp_args:Ne \@@_backend_destination_aux:nnnn
      { \dim_eval:n {#2} } {#1} {#3} {#4}
  }
\cs_new_protected:Npn \@@_backend_destination_aux:nnnn #1#2#3#4
  {
    \vbox_to_zero:n
      {
        \__kernel_kern:n {#4}
        \hbox:n
          {
            \@@_backend:n { obj ~ @pdf_ #2 _llx ~ @xpos }
            \@@_backend:n { obj ~ @pdf_ #2 _lly ~ @ypos }
          }
        \tex_vss:D
      }
    \__kernel_kern:n {#1}
    \vbox_to_zero:n
      {
        \__kernel_kern:n { -#3 }
        \hbox:n
          {
            \@@_backend:n
              {
                dest ~ (#2)
                [
                  @thispage
                  /FitR ~
                    @pdf_ #2 _llx ~ @pdf_ #2 _lly ~
                    @xpos ~ @ypos
                ]
              }
          }
        \tex_vss:D
      }
    \__kernel_kern:n { -#1 }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsubsection{Structure}
%
% \begin{macro}{\@@_backend_compresslevel:n}
% \begin{macro}{\@@_backend_compress_objects:n}
%   Pass data to the backend: these are a one-shot.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_compresslevel:n #1
  { \__kernel_backend_literal:e { dvipdfmx:config~z~ \int_eval:n {#1} } }
\cs_new_protected:Npn \@@_backend_compress_objects:n #1
  {
    \bool_if:nF {#1}
      { \__kernel_backend_literal:n { dvipdfmx:config~C~0x40 } }
  }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {\@@_backend_version_major_gset:n, \@@_backend_version_minor_gset:n}
%   We start with the assumption that the default is active.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_version_major_gset:n #1
  {
    \cs_gset:Npe \@@_backend_version_major: { \int_eval:n {#1} }
    \__kernel_backend_literal:e { pdf:majorversion~ \@@_backend_version_major: }
  }
\cs_new_protected:Npn \@@_backend_version_minor_gset:n #1
  {
    \cs_gset:Npe \@@_backend_version_minor: { \int_eval:n {#1} }
    \__kernel_backend_literal:e { pdf:minorversion~ \@@_backend_version_minor: }
  }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_backend_version_major:, \@@_backend_version_minor:}
%   We start with the assumption that the default is active.
%    \begin{macrocode}
\cs_new:Npn \@@_backend_version_major: { 1 }
\cs_new:Npn \@@_backend_version_minor: { 5 }
%    \end{macrocode}
% \end{macro}
%
% \subsubsection{Marked content}
%
% \begin{macro}{\@@_backend_bdc:nn}
% \begin{macro}{\@@_backend_emc:}
%   Simple wrappers. May need refinement: see
%   \url{https://chat.stackexchange.com/transcript/message/49970158#49970158}.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_bdc:nn #1#2
  { \__kernel_backend_literal_page:n { /#1 ~ #2 ~ BDC } }
\cs_new_protected:Npn \@@_backend_emc:
  { \__kernel_backend_literal_page:n { EMC } }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
%    \begin{macrocode}
%</dvipdfmx|xetex>
%    \end{macrocode}
%
% \subsection{\texttt{dvisvgm} backend}
%
%    \begin{macrocode}
%<*dvisvgm>
%    \end{macrocode}
%
% \subsubsection{Annotations}
%
% \begin{macro}{\@@_backend_annotation:nnnn}
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_annotation:nnnn #1#2#3#4 { }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_backend_annotation_last:}
%    \begin{macrocode}
\cs_new:Npn \@@_backend_annotation_last: { }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}
%   {\@@_backend_link_begin_goto:nnw, \@@_backend_link_begin_user:nnw}
% \begin{macro}{\@@_backend_link_begin:nnnw}
% \begin{macro}{\@@_backend_link_end:}
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_link_begin_goto:nnw #1#2 { }
\cs_new_protected:Npn \@@_backend_link_begin_user:nnw #1#2 { }
\cs_new_protected:Npn \@@_backend_link_begin:nnnw #1#2#3 { }
\cs_new_protected:Npn \@@_backend_link_end: { }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \begin{macro}{\@@_backend_link_last:}
%    \begin{macrocode}
\cs_new:Npe \@@_backend_link_last: { }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_backend_link_margin:n}
%   A simple task: pass the data to the primitive.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_link_margin:n #1 { }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_backend_destination:nn}
% \begin{macro}{\@@_backend_destination:nnnn}
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_destination:nn #1#2 { }
\cs_new_protected:Npn \@@_backend_destination:nnnn #1#2#3#4 { }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \subsubsection{Catalogue entries}
%
% \begin{macro}{\@@_backend_catalog_gput:nn, \@@_backend_info_gput:nn}
%   No-op.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_catalog_gput:nn #1#2 { }
\cs_new_protected:Npn \@@_backend_info_gput:nn #1#2 { }
%    \end{macrocode}
% \end{macro}
%
% \subsubsection{Objects}
%
% \begin{macro}{\@@_backend_object_new:n}
% \begin{macro}[EXP]{\@@_backend_object_ref:n}
% \begin{macro}{\@@_backend_object_write:nnn, \@@_backend_object_write:ne}
% \begin{macro}{\@@_backend_object_now:nn, , \@@_backend_object_now:ne}
% \begin{macro}{\@@_backend_object_last:}
% \begin{macro}[EXP]{\@@_backend_pageobject_ref:n}
%   All no-ops here.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_object_new:nn #1 { }
\cs_new:Npn \@@_backend_object_ref:n #1 { }
\cs_new_protected:Npn \@@_backend_object_write:nnn #1#2#3 { }
\cs_new_protected:Npn \@@_backend_object_write:nne #1#2#3 { }
\cs_new_protected:Npn \@@_backend_object_now:nn #1#2 { }
\cs_new_protected:Npn \@@_backend_object_now:ne #1#2 { }
\cs_new:Npn \@@_backend_object_last: { }
\cs_new:Npn \@@_backend_pageobject_ref:n #1 { }
%    \end{macrocode}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
% \end{macro}
%
% \subsubsection{Structure}
%
% \begin{macro}{\@@_backend_compresslevel:n}
% \begin{macro}{\@@_backend_compress_objects:n}
%   These are all no-ops.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_compresslevel:n #1 { }
\cs_new_protected:Npn \@@_backend_compress_objects:n #1 { }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
% \begin{macro}
%   {\@@_backend_version_major_gset:n, \@@_backend_version_minor_gset:n}
%   Data not available!
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_version_major_gset:n #1 { }
\cs_new_protected:Npn \@@_backend_version_minor_gset:n #1 { }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}[EXP]{\@@_backend_version_major:, \@@_backend_version_minor:}
%   Data not available!
%    \begin{macrocode}
\cs_new:Npn \@@_backend_version_major: { -1 }
\cs_new:Npn \@@_backend_version_minor: { -1 }
%    \end{macrocode}
% \end{macro}
%
% \begin{macro}{\@@_backend_bdc:nn}
% \begin{macro}{\@@_backend_emc:}
%   More no-ops.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_bdc:nn #1#2 { }
\cs_new_protected:Npn \@@_backend_emc: { }
%    \end{macrocode}
% \end{macro}
% \end{macro}
%
%    \begin{macrocode}
%</dvisvgm>
%    \end{macrocode}
%
% \subsection{PDF Page size (media box)}
%
% For setting the media box, the split between backends is somewhat different
% to other areas, thus we approach this separately. The code here assumes a
% recent \LaTeXe{}: that is ensured at the level above.
%
%    \begin{macrocode}
%<*dvipdfmx|dvips>
%    \end{macrocode}
%
% \begin{macro}{\@@_backend_pagesize_gset:nn}
%   This is done as a backend literal, so we deal with it using the shipout
%   hook.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_pagesize_gset:nn #1#2
  {
    \__kernel_backend_first_shipout:n
      {
        \__kernel_backend_literal:e
          {
%<*dvipdfmx>
            pdf:pagesize ~
              width  ~ \dim_eval:n {#1} ~
              height ~ \dim_eval:n {#2}
%</dvipdfmx>
%<*dvips>
            papersize = \dim_eval:n {#1} , \dim_eval:n {#2}
%</dvips>
          }
      }
  }
%    \end{macrocode}
% \end{macro}
%
%    \begin{macrocode}
%</dvipdfmx|dvips>
%    \end{macrocode}
%
%    \begin{macrocode}
%<*luatex|pdftex|xetex>
%    \end{macrocode}
%
% \begin{macro}{\@@_backend_pagesize_gset:nn}
%   Pass to the primitives.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_pagesize_gset:nn #1#2
  {
    \dim_gset:Nn \tex_pagewidth:D  {#1}
    \dim_gset:Nn \tex_pageheight:D {#2}
  }
%    \end{macrocode}
% \end{macro}
%
%    \begin{macrocode}
%</luatex|pdftex|xetex>
%    \end{macrocode}
%
%    \begin{macrocode}
%<*dvisvgm>
%    \end{macrocode}
%
% \begin{macro}{\@@_backend_pagesize_gset:nn}
%   A no-op.
%    \begin{macrocode}
\cs_new_protected:Npn \@@_backend_pagesize_gset:nn #1#2 { }
%    \end{macrocode}
% \end{macro}
%
%    \begin{macrocode}
%</dvisvgm>
%    \end{macrocode}
%
%    \begin{macrocode}
%</package>
%    \end{macrocode}
%
% \end{implementation}
%
% \PrintIndex