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fancy way to generate scale bars with Tikz

The way to display scale bars shown here is nice, but can be a bit cumbersome if the images are not that easy, e.g. the image shows a skewed sample in 3D. Everything shown below could also be done completely by hand, but it's quite a boring task to calculate multiple rules of three for every image you want to add a scale bar. I've written a small MATLAB-script to facilitate the calculation and addition to LaTeX.

You'll need:

  • an image you know the scale of and want to draw the scale bar on, e.g. the image shown on the right1)
  • MATLAB with this m-file: scalebar.m (or any other graphics-capable calculation program like Octave)
  • this minimal tex-file: scalebar.tex (I suppose you have a running installation of LaTeX to work with this example…)

Both files are explained in a bit more detail at the end of this page, for now I'll just walk you through:

WorkFlow

  1. Start MATLAB, load scalebar.m and start it.
  2. The script prompts you to load an image, choose “scalebarimage.jpg” you've downloaded from above.
  3. The script prompts you to define the length of the scale bar you're gonna draw in the next step, choose the default “21 cm” for now
  4. The script displays the image of my head and asks you to define a start-point of the scale-bar, choose the tip of the nose
  5. After that, choose the back of my head for the end of the scale-bar
  6. Note the “Positions” shown in the help-dialog, these are to make sure you've clicked correctly and that the positions are correct.
  7. copy the output in the command-line-section of MATLAB
  8. paste this output into your .tex-file
  9. if the .tex file and the image are in the same folder, just proceed, if not, change the path to the image!
  10. compile the file
  11. Figure 1 of the document should show
    • the exact same scale-bar you've drawn in MATLAB overlayed over the image.
    • a second scale bar which shows a reasonable length (5cm in our case)
  12. change \def\x{100} and \def\y{100} on lines 24 and 25 of the copied text to position the scale bar where you'd like to have it and uncomment line 29 to turn off the original scale bar.
  13. enjoy typographically correct scale bars in the resulting pdf (47 kB)

LaTeX-File

The preamble is nothing too fancy

\documentclass{article}
\usepackage{tikz}                 % used for actually drawing the scale bar
\usepackage[ansinew]{inputenc}    % is used so I can write my name without too much hassle 
\usepackage{siunitx}              % excellent package to correctly typeset units
\usepackage{hyperref}             % used for URLs

The block below is used to start the document and to set up the image

\begin{document}
\author{David Haberthür}
\title{David Haberthür's Head - Scalebar}
\date{\today}
\maketitle
\begin{figure}[h]
	\centering

Inside here you'll copy the result from the MATLAB-skript, the rest ist just to finish off the document.

\caption{Three dimensional visualization of David Haberthürs Head, released under a \href{http://creativecommons.org/licenses/by/2.0/deed.en_GB}{creative commons Attribution 2.0 Generic license} by the owner of the head.}
	\label{fig:head}
\end{figure}
\end{document}

MATLAB-File

The block below asks the user to select an image-file that is then subsequently used for calculations

[ filename, pathname] = ...
     uigetfile({'*.jpg;*.tif;*.png;*.gif','All Image Files';...
          '*.*','All Files' },'Choose Input Image');
 
image=imread([pathname filesep filename]);

The scale bar is defined by asking the user to click on the image for a start- and end-point.

h = helpdlg('choose start-point of scale bar','ScaleBar');
uiwait(h);
[ x1,y1 ] = ginput(1);

the rest is just simple, but boring calculations and semi-nice output, which takes care of setting up the necessary lengths for TikZ.

References

1) it's an MRI of my head…