>
<span class="r-in"><span><span class="co">#### compute scale (dots per unit of known distance) ####</span></span></span>
<span class="r-in"><span><span class="co"># only works in an interactive section</span></span></span>
<span class="r-in"><span><span class="co"># objects_300dpi.jpg has a known resolution of 300 dpi</span></span></span>
<span class="r-in"><span><span class="va">img</span> <span class="op">&lt;-</span> <span class="fu"><a href="utils_image.html">image_pliman</a></span><span class="op">(<