First attempt at using exclusively depth pass with cap on near plane

shader/lighting/cap.geom

@@ -3,17 +3,38 @@ layout (triangles) in;
 layout (triangle_strip, max_vertices = 3) out;
 layout (std140, binding = 0) uniform TheMat { mat4 theMat; } ;
 uniform vec3 lightPos;
+vec4 projNear (vec4 pos)
+{
+    // note we project to a specific height
+    // this is quite brittle, not ideal
+    vec3 dir = pos.xyz - lightPos ;
+    float a = (150 - pos.z) / dir.z ;
+    vec2 xy = (pos.xyz + a * dir).xy ;
+    return vec4 ( xy, 150 , 1) ;
+}
 void main()
 {
     vec4 p0 = gl_in[0].gl_Position ;
     vec4 p1 = gl_in[1].gl_Position ;
     vec4 p2 = gl_in[2].gl_Position ;
-    if ( dot( p0.xyz - lightPos, cross( p0.xyz - p1.xyz, p1.xyz - p2.xyz)) < 0)
+    if ( //dot( p0.xyz - vec3 (0,0,1), cross( p0.xyz - p1.xyz, p1.xyz - p2.xyz)) < 0
+           // && 
+            ( p0.z - lightPos.z > 0 )
+            && ( p1.z - lightPos.z > 0 )
+            && ( p2.z - lightPos.z > 0 )
+            )
     {
-        gl_Position = theMat * p0; EmitVertex();
-        gl_Position = theMat * p1; EmitVertex();
-        gl_Position = theMat * p2; EmitVertex();
+        // front cap
+        vec4 v1 = vec4 (0,0,1,0) ;
+        gl_Position = theMat * projNear(p0); EmitVertex();
+        gl_Position = theMat * projNear(p1); EmitVertex();
+        gl_Position = theMat * projNear(p2); EmitVertex();
         EndPrimitive();
+        // if light points downwards onto surface, draw back cap too
+        if (dot ( p0.xyz - lightPos, vec3 ( 0,0,1) ) > 0)
+        {
+        }
+        else {}
     }
     else {}
 }