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();
     }
 }