title: Drawing a mix of quads and triangles using the geometry shader and lines_adjacency
body: <p>My current rendering implementation is as follows:</p>
  
  <ul>
  <li>Store all vertex information as quads rather than triangles</li>
  <li>For triangles, simply repeat the last vertex (i.e. <code>v0 v1 v2 v2</code>)</li>
  <li>Pass vertex information as <code>lines_adjacency</code> to geometry shader</li>
  <li>Check if quad or triangle, output as <code>triangle_strip</code></li>
  </ul>
  
  <p>The reason I went this route was because I was implementing a wireframe shader, and I wanted to draw the quads without a diagonal line through them. But, I've since discarded the feature.</p>
  
  <p>I'm now wondering if I should go back to simply drawing <code>GL_TRIANGLES</code>, and leave the geometry shader out of the equation. But that got me thinking... what's actually more efficient from a performance point of view?</p>
  
  <ul>
  <li>In average, my scenes are composed of quads and triangles in equal amounts.</li>
  <li>Drawing with all triangles would mean: 6 vertices per quad, 3 per triangle.</li>
  <li>Drawing with lines_adjacency would mean: 4 vertices per quad, 4 per triangle.</li>
  <li>(This is with indexed drawing, so the vertex buffer is the same size for both of them)</li>
  </ul>
  
  <p>So the vertex ratio is 9:8 (triangles : lines_adjacency). </p>
  
  <p>Would I be correct in assuming that with indexed drawing, each vertex is only getting processed once by the vertex shader (as opposed to once per index)? In which case drawing triangles is going to be more efficient (since there isn't an extra geometry-shader step to perform), with the only negative being the slight amount of extra memory the indices take up.</p>
  
  <p>Then again, if the vertices <em>do</em> get processed once per index, I could see the edge being with the lines_adjacency method, considering the geometry conversion is very simple, whilst the vertex shader might be running more intensive lighting calculations.</p>
  
  <p>So that pretty much sums up my question: how do vertices get treated with indexed drawing, and what sort of performance impact could be expected if including a simple geometry shader?</p>