title: cylinder impostor in GLSL
body: <p>I am developing a small tool for 3D visualization of molecules.
  For my project i choose to make a thing in the way of what Mr "Brad Larson" did with his Apple software "<strong>Molecules</strong>". A link where you can find a small presentation of the technique used : <a href="http://www.sunsetlakesoftware.com/2011/05/08/enhancing-molecules-using-opengl-es-20" rel="nofollow">Brad Larsson software presentation</a></p>
  
  <p>For doing my job i must compute <strong>sphere impostor</strong> and <strong>cylinder impostor</strong>. </p>
  
  <p>For the moment I have succeed to do the "Sphere Impostor" with the help of another tutorial <a href="https://paroj.github.io/gltut/Illumination/Tutorial%2013.html" rel="nofollow">Lies and Impostors</a></p>
  
  <p>for summarize the computing of the sphere impostor : first we send a "sphere position" and the "sphere radius" to the "vertex shader" which will create in the camera-space an square which always face the camera, after that we send our square to the fragment shader where we use a simple ray tracing to find which fragment of the square is included in the sphere, and finally we compute the normal and the position of the fragment to compute lighting. (another thing we also write the gl_fragdepth for giving a good depth to our impostor sphere !)    </p>
  
  <p>But now i am blocked in the computing of the cylinder impostor, i try to do a parallel between the sphere impostor and the cylinder impostor but i don't find anything, my problem is that for the sphere it was some easy because the sphere is always the same no matter how we see it, we will always see the same thing : "a circle" and another thing is that the sphere was perfectly defined by Math then we can find easily the position and the normal for computing lighting and create our impostor.</p>
  
  <p>For the cylinder it's not the same thing, and i failed to find a hint to modeling a form which can be used as "cylinder impostor", because the cylinder shows many different forms depending on the angle we see it ! </p>
  
  <p>so my request is to ask you about a solution or an indication for my problem of "cylinder impostor".</p>