title: Ray-tracing a sphere with displacement mapping
body: <p>I'm interested in building a simple "Google Earth" type app (for overlaying my own information, not the huge quantity of data that Google has).  I'd like it to just be a simple X11 app that ray-traces a sphere with displacement (topographic) information.  Ray-sphere intersection is pretty simple, but when the displayment mapping is throw in there it starts to get muddy in my head.  </p>
  
  <p>I was wondering if there's a simple technique to extend basic ray-sphere intersection to include displacement data...</p>