title: How to set fill alpha in PDF
body: <p>This is a red box:</p>
  
  <pre><code>162 86 m 162 286 l 362 286 l 362 86 l h
  1 0 0 rg f
  </code></pre>
  
  <p>How can I add partial transparency to it?</p>
  
  <p>I've read the transparency section of the PDF spec, but I can only seem to find models and formulas, not how to actually add alpha to a fill.</p>