Hide text from displaying but retain it selectable and searchable

Question

Continuing https://tex.stackexchange.com/a/463516, is there a way to create an invisible PDF text associated with a drawn symbol? Assume that we have some command (say, \sqp) which draws some symbol but produces no searchable/selectable text in the PDF. We want to add this textual representation (say, Π, which be neither displayed nor printed) of the drawn symbol to the PDF file such that it would be selectable and searchable. An example of such text is, e.g., Included PDF image has invisible, but selectable text . Reusing @egreg's code from https://tex.stackexchange.com/a/463516/165772 (Thx!), I tried

\documentclass{article}
\usepackage{amsmath}
\usepackage{xparse}
\usepackage{accsupp}

\ExplSyntaxOn
\NewDocumentCommand{\sqp}{s}
 {
  \IfBooleanTF{#1}
   {
     \mathord { \mathpalette \egreg_sqp:Nn { \mathrm{o} } }
   }
   {
     \mathord {
     \mathpalette \egreg_sqp:Nn { }
     }
   }
 }

\box_new:N \l__egreg_sqp_temp_box
\dim_new:N \l__egreg_sqp_wd_dim % width
\dim_new:N \l__egreg_sqp_ht_dim % height
\dim_new:N \l__egreg_sqp_th_dim % thickness

\cs_new_protected:Nn \egreg_sqp:Nn
 {% #1 = style declaration, #2 = maybe o
  \group_begin:
  \dim_zero:N \mathsurround
  \hbox_set:Nn \l__egreg_sqp_temp_box { $#1\mathrm{o}$ }
  \dim_set:Nn \l__egreg_sqp_wd_dim { \box_wd:N \l__egreg_sqp_temp_box }
  \dim_set:Nn \l__egreg_sqp_th_dim { \box_wd:N \l__egreg_sqp_temp_box/4 }
  \hbox_set:Nn \l__egreg_sqp_temp_box { $#1\Pi$ }
  \dim_set:Nn \l__egreg_sqp_ht_dim { \box_ht:N \l__egreg_sqp_temp_box }

  \mspace{1mu}
  \tl_if_empty:nF { #2 }
   {
    \hbox_to_zero:n
     {
      \hbox_to_wd:nn { \l__egreg_sqp_wd_dim + 2\l__egreg_sqp_th_dim } { \hss $#1#2$ \hss }
      \hss
     }
   }
  \hbox_to_wd:nn { \l__egreg_sqp_wd_dim + 2\l__egreg_sqp_th_dim } { \__egreg_sqp_draw:N #1 \hss }
  \mspace{1mu}
  \group_end:
 }

\cs_new_protected:Nn \__egreg_sqp_draw:N
 {
  \driver_draw_begin:
  \driver_draw_moveto:nn { 0pt } { 0pt }
  \driver_draw_lineto:nn { 0pt } { \l__egreg_sqp_ht_dim }
  \driver_draw_lineto:nn { \l__egreg_sqp_wd_dim + 2\l__egreg_sqp_th_dim } { \l__egreg_sqp_ht_dim }
  \driver_draw_lineto:nn { \l__egreg_sqp_wd_dim + 2\l__egreg_sqp_th_dim } { 0pt }
  \driver_draw_lineto:nn { \l__egreg_sqp_wd_dim + \l__egreg_sqp_th_dim } { 0pt }
  \driver_draw_lineto:nn { \l__egreg_sqp_wd_dim + \l__egreg_sqp_th_dim } { \l__egreg_sqp_ht_dim - 0.7\l__egreg_sqp_th_dim }
  \driver_draw_lineto:nn { \l__egreg_sqp_th_dim } { \l__egreg_sqp_ht_dim - 0.7\l__egreg_sqp_th_dim }
  \driver_draw_lineto:nn { \l__egreg_sqp_th_dim } { 0pt }
  \driver_draw_closepath:
  \driver_draw_fill:
  \driver_draw_end:
 }

\ExplSyntaxOff

\begin{document}
\BeginAccSupp{method=hex,unicode,ActualText=03A0}$\sqp$\EndAccSupp{}
\end{document}

In the text layer, this does not produce Π (as I would expect), but only 1 for the end of document. (An ocg-based solution, as opposed to the accsup-based solution, would probably require a fresh id on each use of the symbol; I wouldn't know how to generate it automatically, and it doesn't seem to produce selectable text. Anyway, I have not tried it.)

I'd prefer a solution that works with all three engines {pdf|xe|lua}latex, even if one would have to use the conditionals \if.... As noticed by @Raven, making the background white could work (unless you'd overlay the symbol again with something else like an image) -- that's what I would do when I run out of options.

you need a real glyph, eg a period, in the symbol, then it works (at least with the adobe reader, but not with sumatra which ignores the actualtext setting.) — Ulrike Fischer, Dec 09 '18 at 16:36
then you are probably out of luck and your viewer doesn't support it. — Ulrike Fischer, Dec 09 '18 at 17:10
Give me a (sane) PDF containing text and I only need a minute to make the text "invisible", but still selectable and copy-able.... I could also do the contrary: let the text be still visible, but make it un-selectable and un-copy-able. — Kurt Pfeifle, Dec 20 '18 at 23:54
Some of it is covered by the different "text rendering modes" which are possible in PDF: filling, stroking, filling+stroking, neither-filling-nor-stroking texts/glyhps. -- See also this answer of mine over at StackOverflow. Another option would be to print white text on white background (or any other color where text and background match: this keeps the text still selectable, but disables its visibility... — Kurt Pfeifle, Dec 21 '18 at 00:01

score 5 · Accepted Answer · answered Dec 22 '18 at 02:42

In my comments to the OP/question, I proposed three different methods to write text into a PDF which is invisible, but still selectable and searchable:

Use "text rendering mode 3" (don't strike and don't fill) for the glyph shapes.
Use white color for all characters on standard background (or colorize the text with the same color as its background).
Use the standard text color (black), but print it onto a black background.

Here is an MWE for my first proposal. For now I'll not make one for the other two. The following shows how to use "text rendering mode 3" in order to write invisible (but searchable) text into a PDF and "do it by (more-or-less pure) LaTeX commands".

It makes use of...

...the \pdfliteral command which in general allows to insert raw PDF code into the LaTeX sources which will then be passed into the PDF output generated by pdflatex (not xelatex or lualatex);
...the PDF code snippet 3 Tr which is PDF syntax in order to set the text rendering mode to 3 (meaning "no stroke and no fill"). -- *(The q is PDF syntax to save the current graphics state, and the Q is for restoring it again to the previous text rendering mode).**

Here is the MWE:

\documentclass{article}
\begin{document}
This is my normal text.\\
\makebox[50pt][c]{\pdfliteral page{q 3 Tr}This is hidden (but searchable) text.\pdfliteral page{Q}}
\\This is more normal text.\\
\end{document}

Compile this into PDF: pdflatex my.tex. Open the PDF in any viewer and select all (try to hit ctrl+a (Windows, Linux) or cmd+a (macOS)):

As you can see, there is something highlighted but not visible in between the two readable sentences. To find out what it is,...

...either run pdftotext -layout my.pdf -,
...or search for the string 'This is hidden (but searchable) text.'

@user49915: Have you considered to find this out by yourself? — Kurt Pfeifle, Dec 23 '18 at 18:50
@user49915: I am sorry that my answer does nothing to help you solve the issue you stated in your OP. For the "second" issue: either extend your OP to clearly state it, or please ask a new question as a separate posting. — Kurt Pfeifle, Dec 23 '18 at 20:22

Guest · Answer 2 · 2023-03-21T19:00:41.493

The package pdfrender makes this easy to implement. The code works with PdfLaTeX and LuaLatex but not with XeLaTeX. It works the same like the other answer but is more compatible and less low level.

\documentclass{article}
\usepackage{pdfrender}
\begin{document}
\noindent
This is my normal text.\\
\textpdfrender{TextRenderingMode=Invisible}{This text is invisible}\\
This is more normal text.\\
\end{document}

A solution for XeLaTeX could be to use TikZ with fadings library and set transparency of the input text and also of the rectangle to 0%, see here for a first direction.

Hide text from displaying but retain it selectable and searchable

2 Answers2

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