robustwireframe.geom Example File

  #version 330 core

  layout( triangles ) in;
  layout( triangle_strip, max_vertices = 3 ) out;

  in EyeSpaceVertex {
      vec3 position;
      vec3 normal;
  } gs_in[];

  out WireframeVertex {
      vec3 position;
      vec3 normal;
      noperspective vec4 edgeA;
      noperspective vec4 edgeB;
      flat int configuration;
  } gs_out;

  uniform mat4 viewportMatrix;

  const int infoA[]  = int[]( 0, 0, 0, 0, 1, 1, 2 );
  const int infoB[]  = int[]( 1, 1, 2, 0, 2, 1, 2 );
  const int infoAd[] = int[]( 2, 2, 1, 1, 0, 0, 0 );
  const int infoBd[] = int[]( 2, 2, 1, 2, 0, 2, 1 );

  vec2 transformToViewport( const in vec4 p )
  {
      return vec2( viewportMatrix * ( p / p.w ) );
  }

  void main()
  {
      gs_out.configuration = int(gl_in[0].gl_Position.z < 0) * int(4)
             + int(gl_in[1].gl_Position.z < 0) * int(2)
             + int(gl_in[2].gl_Position.z < 0);

      // If all vertices are behind us, cull the primitive
      if (gs_out.configuration == 7)
          return;

      // Transform each vertex into viewport space
      vec2 p[3];
      p[0] = transformToViewport( gl_in[0].gl_Position );
      p[1] = transformToViewport( gl_in[1].gl_Position );
      p[2] = transformToViewport( gl_in[2].gl_Position );

      if (gs_out.configuration == 0)
      {
          // Common configuration where all vertices are within the viewport
          gs_out.edgeA = vec4(0.0);
          gs_out.edgeB = vec4(0.0);

          // Calculate lengths of 3 edges of triangle
          float a = length( p[1] - p[2] );
          float b = length( p[2] - p[0] );
          float c = length( p[1] - p[0] );

          // Calculate internal angles using the cosine rule
          float alpha = acos( ( b * b + c * c - a * a ) / ( 2.0 * b * c ) );
          float beta = acos( ( a * a + c * c - b * b ) / ( 2.0 * a * c ) );

          // Calculate the perpendicular distance of each vertex from the opposing edge
          float ha = abs( c * sin( beta ) );
          float hb = abs( c * sin( alpha ) );
          float hc = abs( b * sin( alpha ) );

          // Now add this perpendicular distance as a per-vertex property in addition to
          // the position and normal calculated in the vertex shader.

          // Vertex 0 (a)
          gs_out.edgeA = vec4( ha, 0.0, 0.0, 0.0 );
          gs_out.normal = gs_in[0].normal;
          gs_out.position = gs_in[0].position;
          gl_Position = gl_in[0].gl_Position;
          EmitVertex();

          // Vertex 1 (b)
          gs_out.edgeA = vec4( 0.0, hb, 0.0, 0.0 );
          gs_out.normal = gs_in[1].normal;
          gs_out.position = gs_in[1].position;
          gl_Position = gl_in[1].gl_Position;
          EmitVertex();

          // Vertex 2 (c)
          gs_out.edgeA = vec4( 0.0, 0.0, hc, 0.0 );
          gs_out.normal = gs_in[2].normal;
          gs_out.position = gs_in[2].position;
          gl_Position = gl_in[2].gl_Position;
          EmitVertex();

          // Finish the primitive off
          EndPrimitive();
      }
      else
      {
          // Viewport projection breaks down for one or two vertices.
          // Caclulate what we can here and defer rest to fragment shader.
          // Since this is coherent for the entire primitive the conditional
          // in the fragment shader is still cheap as all concurrent
          // fragment shader invocations will take the same code path.

          // Copy across the viewport-space points for the (up to) two vertices
          // in the viewport
          gs_out.edgeA.xy = p[infoA[gs_out.configuration]];
          gs_out.edgeB.xy = p[infoB[gs_out.configuration]];

          // Copy across the viewport-space edge vectors for the (up to) two vertices
          // in the viewport
          gs_out.edgeA.zw = normalize( gs_out.edgeA.xy - p[ infoAd[gs_out.configuration] ] );
          gs_out.edgeB.zw = normalize( gs_out.edgeB.xy - p[ infoBd[gs_out.configuration] ] );

          // Pass through the other vertex attributes
          gs_out.normal = gs_in[0].normal;
          gs_out.position = gs_in[0].position;
          gl_Position = gl_in[0].gl_Position;
          EmitVertex();

          gs_out.normal = gs_in[1].normal;
          gs_out.position = gs_in[1].position;
          gl_Position = gl_in[1].gl_Position;
          EmitVertex();

          gs_out.normal = gs_in[2].normal;
          gs_out.position = gs_in[2].position;
          gl_Position = gl_in[2].gl_Position;
          EmitVertex();

          // Finish the primitive off
          EndPrimitive();
      }
  }