Almost working shadows

shader/lighting/lineShadow.geom

@@ -10,18 +10,31 @@ void main()
     vec4 p0 = gl_in[0].gl_Position;
     vec3 n0 = gl_in[2].gl_Position.xyz;
     vec3 n1 = gl_in[3].gl_Position.xyz;
+    vec3 n2 = n0 + n1; // assumes the summands are normalized
     vec3 lightDir = p0.xyz - lightPos.xyz;
-    if ( dot(n0 , lightDir) * dot(n1 , lightDir) < 0 );
+    // first test if the edge is part of the silhouette
+    // that is, if the normals of the faces connected by the edge point in
+    // "different directions" wrt the light direction
+    if ( dot(n0 , lightDir) * dot(n1 , lightDir) < 0 )
     {
-        gl_Position = f(p0); EmitVertex();
         vec4 p1 = gl_in[1].gl_Position;
-        gl_Position = f(p1); EmitVertex();
         vec4 p2 = shift(p0);
-        gl_Position = f(p2); EmitVertex();
         vec4 p3 = shift(p1);
-        gl_Position = f(p3); EmitVertex();
+        if ( dot(n0 , lightDir) > 0)
+        {
+            gl_Position = f(p0); EmitVertex();
+            gl_Position = f(p1); EmitVertex();
+            gl_Position = f(p2); EmitVertex();
+            gl_Position = f(p3); EmitVertex();
+        }
+        else
+        {
+            gl_Position = f(p1); EmitVertex();
+            gl_Position = f(p0); EmitVertex();
+            gl_Position = f(p3); EmitVertex();
+            gl_Position = f(p2); EmitVertex();
+        }
         EndPrimitive();
     }
-    {
-    }
+    else { }
 }