Detecting a symbol's final location in the PDF

Question

We're using LaTeX to render a PDF with a form that will be filled in by humans, using a pen, and then fed to an OpenCV application for recognition. The form consists of questions with small squares (\fbox symbols) that people fill to answer yes or no.

The thing is, our recognition algorithm works better when we can provide to it some hint as to where are the \fboxes located. So we arrange them in a pattern, with some fixed distance from the borders and in between. However, if we want to create more complex layouts, we need to first render the document, and then looking at the PDF figure out the margins, separations, etc.

We would like to automate this process as much as possible, so the general question is: what is the best way to "mark" some symbols in LaTeX so that I can later somehow recover their location in the rendered PDF (probably using a second tool to look inside the PDF, I guess)?

Answer 1

Use

\fbox{%
\pdfsavepos\write-1{hello is at \the\pdflastxpos,\the\pdflastypos}%
hello%
}

and the log file will have the coordinates in scaled points

The above produces a line such as the following in the log file:

hello is at 10005380,43282099

Answer 2

Here is a possible solution via TikZ:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc}

\newcommand\mybox{%
  \begin{tikzpicture}[remember picture]
    \draw (0,0) rectangle (.6em,.6em);
    \path let \p1=($(.3em,.3em) - (current page.south west)$)
    in \pgfextra{
      \pgfmathsetmacro\xpos{\x1/1cm}
      \pgfmathsetmacro\ypos{\y1/1cm}
      \typeout{checkboxpos:(\xpos cm,\ypos cm)}
    };
  \end{tikzpicture}%
}
\begin{document}

This is a test \mybox{}.

\begin{itemize}
\item[\mybox{}] First item.
  \begin{itemize}
  \item[\mybox{}] First subitem.
  \item[\mybox{}] Second subitem.
  \item[\mybox{}] Third subitem.
  \end{itemize}
\item[\mybox{}] Second item.
\item[\mybox{}] Third item.
\end{itemize}

\end{document}

After two compilations, you got this log:

checkboxpos:(7.49284cm,23.29828cm)
checkboxpos:(5.30951cm,22.52507cm)
checkboxpos:(6.08272cm,21.82214cm)
checkboxpos:(6.08272cm,21.25981cm)
checkboxpos:(6.08272cm,20.69748cm)
checkboxpos:(5.30951cm,19.99455cm)
checkboxpos:(5.30951cm,19.29164cm)

Answer 3

The main point of this task is usage of pdfTeX primitives \pdfsavepos, \pdflastxpos, \pdflastypos. You can create a macro \saveposition which saves the actual position to the file \jobname.pos in the form:

1: 67.65254mm, 267.37009mm
2: 101.77405mm, 267.37009mm
3: 67.65254mm, 261.73753mm
4: 101.77405mm, 261.73753mm

The number of the checkbox is here followed by x and y coordinate. The (0,0) of coordinate system is the left-bottom corner of the paper. You need not two pass of TeX processing. This file is created immediately after first TeX run.

The implemetation of \saveposition macro can be as following:

\newcount\posnum
\newwrite\posout

\immediate\openout\posout=\jobname.pos

\def\saveposition{\global\advance\posnum by1 \pdfsavepos
   \write\posout\expandafter{\the\posnum: \tomm\pdflastxpos, \tomm\pdflastypos}}

\def\tomm#1{\expandafter\ignorept\the\dimexpr.35146\dimexpr#1sp\relax\relax mm}
{\lccode`\?=`\p \lccode`\!=`\t  \lowercase{\gdef\ignorept#1?!{#1}}}

The conversion from sp to mm unit is done by \tomm macro using eTeX primitive \dimexpr.

Now the test. You need to put the \saveposition to your \fbox macro. Because I don't use LaTeX, I have to create the whole \fbox:

\def\fbox{\leavevmode \saveposition
  \lower3pt \hbox{\vrule \vbox{\hrule width1cm \kern.5cm \hrule}\vrule}%
}

aghd sajhas hds ddjds \fbox\ dhjdka ak das \fbox

aghd sajhas hds ddjds \fbox\ dhjdka ak das \fbox

\end

Note that you needn't use any special package (like tikz mentioned here in an another answer).