Coloured text in plain tex

Question

Recently I became intrigued by plain TeX because it gives even more flexibility than regular LaTeX.

In my document I wish to use grayscales of text. In LaTeX, producing this result is achieved through the color package. texdoc color gives great documentation on the graphics bundle by David Carlisle, but extensive documentation on the implementation is missing.

Alternatively I could try working my way through xcolor which is substantially more complex when it comes to its implementation, but behold, a complete chapter on the implementation is missing in that package too.

I've found some workarounds on the comp.text.tex forums (http://compgroups.net/comp.text.tex/problem-using-color-and-graphicx-in-plain-tex/1918061), which is of course very easy and (if you will) convenient, but these solutions conflict at its heart with the primary reason as outlined in Reasons to use plain TeX, namely that the plain TeX format is not affected by the sands of time. Using a LaTeX package in a plain TeX document could possibly be argued to touch on a gray area, e.g. "the color package is unlikely to change in the near future". However, from my vantage point, the purpose of this question could stretch even further, giving an outline of implementation of an interlocking between a graphical driver (which I assume is at play in the color package and related packages) and TeX.

So I would like to ask about the implementation of the color package in plain TeX, or an alternative implementation with similar functionality.

Answer 1

As the question is focussed on learning how these things may be done just using the primitives (I'd agree with David's answer that loading the color package in plain is an easier route).

What I'll do here is implement much the same approach as is taken by the color package, with appropriate tests for classical TeX (dvips or dvipdfm(x) drivers), pdfTeX/LuaTeX in PDF mode and XeTeX. As there is a bit going on, I'll intersperse the code with comments.

First, set up a conditional to test for direct PDF output

\newif\ifpdfmode

\begingroup\expandafter\expandafter\expandafter\endgroup
\expandafter\ifx\csname pdfoutput\endcsname\relax
\else
  \ifnum\pdfoutput>0 %
    \expandafter\expandafter\expandafter\pdfmodetrue
  \fi
\fi

Define the current colour as black, using an \edef so that once defined there are no conditionals about (the same idea applies to the rest of the code)

\edef\currentcolor{%
  \ifpdfmode
    0 g 0 G%
  \else
    gray 0%
  \fi
}

Set up a pre-defined colour: I've just done one (red) as a demo:

\edef\colorred{%
  \ifpdfmode
    1 0 0 rg 1 0 0 RG%
  \else
    rgb 1 0 0%
  \fi
}

For direct PDF output, there may be a colour stack available (since pdfTeX 1.40.0). A one-off test will tell us this: if there is no stack, just restore the colour manually. See the pdfTeX manual for the details here.

\begingroup\expandafter\expandafter\expandafter\endgroup
\expandafter\ifx\csname pdfcolorstack\endcsname\relax
  \ifpdfmode
    \def\pdfcolorstackpush{\pdfliteral{\currentcolor}}%
    \let\pdfcolorstackpop\pdfcolorstackpush
  \fi
\else
  \chardef\colorstack=0 %
  \def\pdfcolorstackpush{%
    \pdfcolorstack\colorstackcnt push{\currentcolor}%
  }%
  \def\pdfcolorstackpop{%
    \pdfcolorstack\colorstackcnt pop\relax%
  }%
\fi

The main macro to set colour starts with a test: if the argument is the name of a pre-defined colour use that, otherwise assume a hard-coded engine-specific value. (A more sophisticated approach is to convert the colour to the correct format: as that is not asked for in the question I'll leave as an exercise). Once the colour is set up, insert the appropriate special (noting the \edef will again mean at point of use there are no conditionals):

\edef\color#1{%
  \begingroup\noexpand\expandafter\noexpand\expandafter\noexpand\expandafter\endgroup
  \noexpand\expandafter\noexpand\ifx\noexpand\csname color#1\noexpand\endcsname\relax
    \def\noexpand\currentcolor{#1}%
  \noexpand\else
    \noexpand\expandafter\let\noexpand\expandafter\noexpand\currentcolor
      \noexpand\csname color#1\noexpand\endcsname
  \noexpand\fi
  \ifpdfmode
    \noexpand\pdfcolorstackpush
  \else
    \special{color push \noexpand\currentcolor}%
  \fi
  \aftergroup\noexpand\resetcolor
}

Following the color approach, a reset macro is also created using the appropriate special.

\edef\resetcolor{%
  \ifpdfmode
    \noexpand\pdfcolorstackpop
  \else
    \special{color pop}%
  \fi
}

The demo itself. The implementation above relies on a level of grouping inside boxes, which in LaTeX would be done by the \savebox 'wrapper' for \hbox (and so on). In plain that's not the case, so a colour-safe box needs a group. This could of course be put inside an appropriate set of wrapper macros:

\newbox\mybox
\setbox\mybox=\hbox{\begingroup\color{red}Red text\endgroup}
Surrounding text \box\mybox \space and more of it.
\bye

(Note: I've constructed the above in much the same way as I'd do using DocStrip for creating separate files. As DocStrip is not involved, this costs of some conditional/edef work.)

Answer 2

The latex graphics and color packages may be used in plain tex. http://www.ctan.org/pkg/miniltx

The maintainers of texdoc have chosen to show grfguide.pdf for texdoc color If you go texdoc color.pdf then you get the color package documentation, including fully indexed source listings.

plain tex has some virtues in being a simple format useful for teaching how to build a format, but I can't really imagine any way in which it could be said to be more flexible than latex. LaTeX doesn't remove any functionality.

Answer 3

I've implemented full color stuff in my OPmac package for plainTeX. So I know, where are problems.

You can put the color selector to the typesetting material simply using \pdfliteral or \special but you have to keep in mind that the color is changed from this point to the end of page independent on grouping / paragraphs etc. from TeX point of view. For example \pdfliteral{1 0 0 r} means that all the text from this point to the end of the page (including footline, page number etc.) is red and the next page is reinitialized black. If you print something in the box which cannot break to more pages, then the

\hbox{\pdfliteral{1 0 0 r}Something in red\pdfliteral{0 g}}

is sufficient. But if the color can break into more pages or if you need the color stack (i.e the possibility to return to the previous color -- no explicitly Black) then you have to correct the headline/footline in \plainoutput and you have to implement color managing system over pages, managing colors stack etc. Moreover, there are two types of color selectors in PDF: for strokes and for fills. You can manage them independently or set both to the same color everywhere.

I've described all these features from implementation point of view in my technical documantation for OPmac: opmac-d.pdf, but it is only in Czech, sorry.

Answer 4

I tried to rewrite macros written by Joseph Wright in order they are more readable. His macros are less legible because of 1) redundant expandafters, begingroups csnames etc. when testing of the existence of the primitives and 2) usage of \edef for two variants of macros instead of usage of \ifpdfmode only once.

The comments are the same as in the Josephs answer.

\newif\ifpdfmode

\ifx\pdfoutput\undefined
\else
  \ifnum\pdfoutput>0 \pdfmodetrue
  \fi
\fi

\ifpdfmode

\def\colorblack   {0 g 0 G}
\def\colorred     {1 0 0 rg 1 0 0 RG}
\def\colorgreen   {0 1 0 rg 0 1 0 RG}
\def\colorblue    {0 0 1 rg 0 0 1 RG}

\ifx\pdfcolorstack\undefined
  \def\pdfcolorstackpush{\pdfliteral{\currentcolor}}
  \let\pdfcolorstackpop\pdfcolorstackpush
\else
  \mathchardef\colorstackcnt=\pdfcolorstackinit page {\colorblack}
  \def\pdfcolorstackpush{\pdfcolorstack\colorstackcnt push{\currentcolor}}
  \def\pdfcolorstackpop{\pdfcolorstack\colorstackcnt pop}
\fi

\else % of \ifpdfmode

\def\colorblack   {gray 0}
\def\colorred     {rgb 1 0 0}
\def\colorgreen   {rgb 0 1 0}
\def\colorblue    {rgb 0 0 1}

\def\pdfcolorstackpush{\special{color push \currentcolor}}
\def\pdfcolorstackpop{\special{color pop}}

\fi  % of \ifpdfmode

\def\color#1{\expandafter
  \ifx\csname color#1\endcsname \relax \def\currentcolor{#1}%
  \else \edef\currentcolor{\csname color#1\endcsname}%
  \fi
  \pdfcolorstackpush  
  \aftergroup\pdfcolorstackpop
}
\let\currentcolor=\colorblack

We need to set the pagenumber to black, if the color breaks to more pages:

\footline={\color{black}\hss\tenrm\folio\hss}

And the test:

Surrounding text {\color{red}Red text} and more of it.

Text {\color{blue}text
     {\color{green}gree gr ee g re eg re
     \vfil\break 
     egrg ree g reeg ree greeg grr reeg ree gre eee
     green text} textik} end.

\bye

Edit My first advice about making pagenumber black was only a simple introduction to the new problem. Now, we can remove the line \footline=... from our code and we will do things more conceptual.

We need to redefine \plainoutput in order to the \headline, \footline \topins and \footins are initialized by black color at every page. To do this, we set \currentcolor to \colorblack locally in output routine and we need to surround the \makeheadline, \makefooline, \unvbox\topins and \footnoterule\unvbox\footins by \pdfcolorstackpush, \pdfcolorstackpop pair. It means the following redefinition:

\def\plainoutput{%
  \let\currentcolor=\colorblack
  \shipout\vbox{\pdfcolorstackpush\makeheadline\pdfcolorstackpop
                \pagebody
                \pdfcolorstackpush\makefootline\pdfcolorstackpop}%
  \advancepageno
  \ifnum\outputpenalty>-20000 \else\dosupereject\fi
}    
\def\pagecontents{%
  \ifvoid\topins\else\pdfcolorstackpush\unvbox\topins\pdfcolorstackpop\fi
  \dimen0=\dp255 \unvbox255
  \ifvoid\footins\else 
    \vskip\skip\footins
    \pdfcolorstackpush\footnoterule
    \unvbox\footins\pdfcolorstackpop\fi
  \csname ifr@ggedbottom\endcsname \kern-\dimen0 \vfil \fi
}

Note The synchronization of color stack with TeX grouping mechanism is implemented via \aftergroup and this is sometimes fragile. Joseph mentioned that the problem is in \setbox=\hbox{...\aftergroup\token...} because the \aftergroup token is processed after the box is set thus the \pdfcolorstackpop is inserted into the material where \setbox is used and not inside the \hbox. Thus we need to write

\setbox0=\hbox{{\color{red}text}}

The second example, where it is fragile, is \footnote*{\color{green}text} but you can re-define \footnote to make things more comfortable for user.

Note2 PDFTeX offers more than one stack and PDF specification gives two types of color selectors: 1 0 0 r for fills and 1 0 0 R for strokes. The example above uses both selectors in parallel (like in color.sty) but we can initialize second independent color stack for the second color selector and give to the user the possibility to select the right color selector (for text or for lines) like OPmac does it. But the problem is here: there are no more independent color stacks when xdvipdfmx is used. And the implementation of two independent color stacks at macro level is slightly more complicated (OPmac does this because its macros are older than \pdfcolorstack primitive).