new-transparent.sty

%%
%% This is file `new-transparent.sty',
%% base on transparent sty from Heiko Oberdiek
%%
\NeedsTeXFormat{LaTeX2e}[2020/10/01]
\ProvidesExplPackage{new-transparent}%
  {2021-02-10}{v1.43}{Transparency with color stacks (based on transparent from Heiko Oberdiek)}%

\cs_if_free:NT \color_set:nn{\RequirePackage{l3color}}

\RequirePackage{l3opacity}
\cs_set_eq:NN \__opacity_backend_stack_push:nn\__color_backend_stack_push:nn
\cs_set_eq:NN \__opacity_backend_stack_pop:n  \__color_backend_stack_pop:n

\NewDocumentCommand{\transparent} { m }
      {
       \opacity_select:n{\fp_eval:n{ min(max(0,#1),1) } }
      }

\NewDocumentCommand{\texttransparent}{m m}
  {
   \protect\leavevmode
   \begingroup
     \transparent{#1}
     #2
   \endgroup
  }


\endinput

%%% old code


%pdftex + luatex in pdf mode
\sys_if_output_pdf:T
  {
    \bool_lazy_and:nnF
      {\cs_if_exist_p:N \pdfmanagement_add:nnn }
      { \pdfmanagement_if_active_p: }
      {
        \PackageWarningNoLine{new-transparent}
         {
           Loading~aborted,~because~pdf~resource~management~code~not~found~%
           or~not~active
         }
        \endinput
      }
   }
\sys_if_engine_pdftex:T
  {
    \let\TRP@pdfcolorstackinit\tex_pdfcolorstackinit:D
    \let\TRP@pdfcolorstack\tex_pdfcolorstack:D
  }

\sys_if_engine_luatex:T
  {
    \def\TRP@pdfcolorstackinit           {\pdffeedback~colorstackinit}
    \protected\def\TRP@pdfcolorstack     {\pdfextension~colorstack}
  }

\sys_if_output_pdf:T
  {
    %initialize opacity 1
    \pdfmanagement_add:nnn{Page/Resources/ExtGState}{TRP1}{<</ca~1/CA~1>>}
    %
    \cs_new_protected:Npn \__transparent_use:n #1
      {
        \tl_if_exist:cF { c__transparent_TRP#1_tl }
          {
            \pdfmanagement_add:nnn{Page/Resources/ExtGState}{TRP#1}{<</ca~#1/CA~#1>>}
            \tl_const:cn { c__transparent_TRP#1_tl }{ /TRP#1~gs }
          }
      }
    \xdef\TRP@colorstack{%
         \TRP@pdfcolorstackinit~page~direct{/TRP1~gs}}
    \NewDocumentCommand{\transparent} { m }
      {
        \exp_args:Nx\__transparent_use:n { \fp_eval:n{ min(max(0,#1),1)}}
        \tl_set:Nx  \transparent@current { \fp_eval:n{ min(max(0,#1),1)}}
        \transparent@set
      }

    \def\transparent@current{/TRP1~gs}
    \def\transparent@set
      {
        \TRP@pdfcolorstack\TRP@colorstack~push~
          {
            \tl_use:c { c__transparent_TRP\transparent@current _tl }
          }
            \aftergroup\transparent@reset
      }

    \def\transparent@reset{\TRP@pdfcolorstack\TRP@colorstack~pop\relax}

  }

%xetex, dvipdfmx
\str_case_e:nnT {\g__sys_backend_tl}
 {
   {xetex}{}
   {dvipdfmx}{}
 }
 {
   \PackageInfo{new-transparent}{xetex~and~dvipdfmx~need~dvipdfmx~newer~than~20200901}{}
   \NewDocumentCommand{\transparent} { m }
      {
       \transparent@set{#1}
      }
   \def\transparent@set #1
      {
        \special{pdf:bxgstate~<</CA~#1/ca~#1>>}
        \aftergroup\transparent@reset
      }

    \def\transparent@reset{\special{pdf:exgstate}}
 }

%warning for dvips currently missing
% but it should at least not error
\ProvideDocumentCommand\transparent { m }{}

\NewDocumentCommand{\texttransparent}{m m}
  {
   \protect\leavevmode
   \begingroup
     \transparent{#1}
     #2
   \endgroup
  }


\endinput
%%
%% End of file `new-transparent.sty'.


dvips could at least on one page along this lines
\ExplSyntaxOn
\let\backendpostscript\__kernel_backend_postscript:n
\ExplSyntaxOff
xxxx

\backendpostscript
 {
   /STACK 50 array def % the stack data structure
   /PTR 0 def % the stack pointer
   /stackpush % any stackpush
     { %def
       STACK exch PTR exch put
       /PTR PTR 1 add def
     } bind def
   /stackget % stackget any
    { %def
      /PTR PTR 1 sub def
      STACK PTR get
    } bind def
 }
\ExplSyntaxOn
\__kernel_backend_postscript:n {.currentstrokeconstantalpha~stackpush}
\__kernel_backend_postscript:n {.currentfillconstantalpha~stackpush}
\__kernel_backend_postscript:n {0.1~.setstrokeconstantalpha}
\__kernel_backend_postscript:n {0.1~.setfillconstantalpha}
zzzz
\__kernel_backend_postscript:n {.currentstrokeconstantalpha~stackpush}
\__kernel_backend_postscript:n {.currentfillconstantalpha~stackpush}
\__kernel_backend_postscript:n {0.4~.setstrokeconstantalpha}
\__kernel_backend_postscript:n {0.4~.setfillconstantalpha}
blub
\__kernel_backend_postscript:n {stackget~.setstrokeconstantalpha}
\__kernel_backend_postscript:n {stackget~.setfillconstantalpha}
aaaa
\__kernel_backend_postscript:n {stackget~.setstrokeconstantalpha}
\__kernel_backend_postscript:n {stackget~.setfillconstantalpha}

xxxx
\ExplSyntaxOff

%% colorstack abstraction
\documentclass[12pt]{article}
\usepackage{kantlipsum,iftex}

\begin{document}
\ifxetex
\special{pdfcolorstackinit 1 page direct (0 g)}
\special{pdfcolorstackinit 2 direct (0 g)}
\special{pdfcolorstack 2 push (0 1 0 rg)}
\special{pdfcolorstack 1 push (1 0 0 rg)}
1: red abc
\special{pdfcolorstack 1 push (0 0 1 rg)}
1: blue abc
\special{pdfcolorstack 1 pop}
1: red again
\special{pdfcolorstack 2 current}
2: green
\special{pdfcolorstack 1 current}
1: still red
\special{pdfcolorstack 2 current}
2: green
\newpage

should be red
\else
\edef\stackA {\pdfcolorstackinit page direct {0 g}}
\edef\stackB {\pdfcolorstackinit direct {0 g}}

\pdfcolorstack\stackB push {0 1 0 rg}
\pdfcolorstack\stackA push {1 0 0 rg}
1: red abc
\pdfcolorstack\stackA push {0 0 1 rg}
1: blue abc
\pdfcolorstack\stackA pop
1: red again
\pdfcolorstack\stackB current
2: green
\pdfcolorstack\stackA current
1: still red
\pdfcolorstack\stackB current
2: green
\newpage

should be red

\pdfcolorstack\stackB pop
aa


\fi
\end{document}