How to create integer labels on TikZ grid when we don't know the grid size at compile time?

Question

I want to put integer labels on the grid. The grid size depends on the image loaded.

The real scenario is to vectorize a JPG image but I need a grid label for navigation as follows

\documentclass{article}
\usepackage{tikz}
\usepackage{graphicx}


\newsavebox\TempBox
\savebox\TempBox{\includegraphics{batman.jpg}}

\begin{document}
\begin{tikzpicture} 
    \node[inner sep=0] at (0,0) {\usebox\TempBox};
    \draw[white](-0.5\wd\TempBox,-0.5\ht\TempBox) grid (0.5\wd\TempBox,0.5\ht\TempBox);

    % I want to put integer labels for navigation purpose. The following code cannot be compiled!
    %\foreach \x in {-0.5\wd\TempBox,1,...,0.5\wd\TempBox} { \node [anchor=north] at (\x,0) {\x}; }
    %\foreach \y in {-0.5\ht\TempBox,1,...,0.5\ht\TempBox} { \node [anchor=east] at (0,\y) {\y}; }

    \draw[line width=2pt,blue] (0,0) ellipse (6.3 and 3.4);
    \draw[line width=2pt,blue] (0,0) ellipse (6.9 and 4.0);
\end{tikzpicture}
\end{document}

The following figure is the original image to complete the MWE.

How to create integer labels on TikZ grid?

I might need something like floor or ceiling functions here.

Answer 1

You need to do the loop with integer values, and compute the positions based on these integer values. Below I have used \pgfmathsetmacro to compute the step size in each direction, and \pgfmathtruncatemacro to compute the integer start and end points for the loops:

Code:

\documentclass{article}
\usepackage{graphicx}
\usepackage{tikz}

\newsavebox\TempBox
\savebox\TempBox{\includegraphics{batman.jpg}}

\begin{document}
\begin{tikzpicture} 
    % Compute step size and number of grid lines in each direction:
    \pgfmathsetmacro{\NumberOfXGrid}{\wd\TempBox/1.0cm}%
    \pgfmathtruncatemacro{\XGridStart}{-0.5*\NumberOfXGrid}%
    \pgfmathtruncatemacro{\XGridEnd}{0.5*\NumberOfXGrid}%

    \pgfmathsetmacro{\NumberOfYGrid}{\ht\TempBox/1.0cm}%
    \pgfmathtruncatemacro{\YGridStart}{-0.5*\NumberOfYGrid}%
    \pgfmathtruncatemacro{\YGridEnd}{0.5*\NumberOfYGrid}%

    \foreach \x in {\XGridStart,...,\XGridEnd} { 
        \node [anchor=north] at (\x,-0.5\ht\TempBox-1ex) {$\x$}; 
    }
    \foreach \y in {\YGridStart,...,\YGridEnd} { 
        \node [anchor=east] at (-0.5\wd\TempBox-0.5em,\y) {$\y$};
    }

    \node[inner sep=0] at (0,0) {\usebox\TempBox};
    \draw[white](-0.5\wd\TempBox,-0.5\ht\TempBox) grid (0.5\wd\TempBox,0.5\ht\TempBox);

    \draw[line width=2pt,blue] (0,0) ellipse (6.3 and 3.4);
    \draw[line width=2pt,blue] (0,0) ellipse (6.9 and 4.0);
\end{tikzpicture}
\end{document}

Answer 2

I updated the onimage package form the "From Answers to Packages" project to support absolutely spaced help lines. You can download the .dtx from Lauchpad. Then run (pdf)latex on it to obtain the package (.sty) file and documentation.

To get grid lines spaced 1cm apart, you can use it as follows:

\documentclass{article}
\usepackage{onimage}

\begin{document}

\begin{tikzonimage}[width=0.8\textwidth]{some_image.jpg}
    [tsx/show help lines=1cm,tsx/scale cs=none]
    % Some optional TikZ drawing commands here.
\end{tikzonimage}

\end{document}

The first optional argument of tikzonimage is passed to \includegraphics, then comes the file name and then TikZ options.

Answer 3

Here is another version with numbers are on the picture rather than axes with slightly different code than Peter's.

\documentclass{article}
\usepackage{tikz}
\usepackage{graphicx}
\newsavebox\TempBox
\savebox\TempBox{\includegraphics{batman.jpg}}
\begin{document}
\begin{tikzpicture} 
    \node[inner sep=0] at (0,0) {\usebox\TempBox};
    \draw[white](-0.5\wd\TempBox,-0.5\ht\TempBox) grid (0.5\wd\TempBox,0.5\ht\TempBox);
        \pgfmathparse{int(-0.5\wd\TempBox/28.4)}%
        \edef\ww{\pgfmathresult}
        \pgfmathparse{int(-0.5\ht\TempBox/28.4)}%
        \edef\hh{\pgfmathresult}
    % I want to put integer labels for navigation purpose. The following code cannot be compiled!
    \foreach \x in {\ww,...,-\ww} { \node [white,anchor=north] at (\x,0) {\x}; }
    \foreach \y in {\hh,...,-\hh} { \node [white,anchor=east] at (0,\y) {\y}; }

    \draw[line width=2pt,blue] (0,0) ellipse (6.3 and 3.4);
    \draw[line width=2pt,blue] (0,0) ellipse (6.9 and 4.0);
\end{tikzpicture}
\end{document}