robustwireframe.geom Example File
wireframe/shaders/robustwireframe.geom#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(); } }
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