Implement correctly shadows on geometry, with arbitrary near plane cap

shader/lighting/cap.geom

@@ -8,9 +8,9 @@ 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 ;
+    float a = (140 - pos.z) / dir.z ;
     vec2 xy = (pos.xyz + a * dir).xy ;
-    return vec4 ( xy, 150 , 1) ;
+    return vec4 ( xy, 140 , 1) ;
 }
 void main()
 {

shader/lighting/lineShadow.geom

@@ -4,6 +4,24 @@ layout (triangle_strip, max_vertices = 4) out;
 layout (std140, binding = 0) uniform TheMat { mat4 theMat; } ;
 uniform vec3 lightPos;
 vec4 shift (vec4 p) { return (vec4 (p.xyz + (100000*(p.xyz-lightPos)), 1));}
+// copied from lighting/cap.geom
+vec4 projNear (vec4 pos)
+{
+    // note we project to a specific height
+    // this is quite brittle, not ideal
+    vec3 dir = pos.xyz - lightPos ;
+    float a = (140 - pos.z) / dir.z ;
+    vec2 xy = (pos.xyz + a * dir).xy ;
+    return vec4 ( xy, 140 , 1) ;
+}
+vec4 shiftNear (vec4 pos)
+{ vec4 sp = shift(pos);
+    if (sp.z > 140)
+    { return projNear(pos) ; }
+    else
+    { return sp ; }
+}
+
 vec4 f (vec4 p) {return (theMat * p);}
 void main()
 {
@@ -18,8 +36,8 @@ void main()
     if ( dot(n0 , lightDir) * dot(n1 , lightDir) < 0 )
     {
         vec4 p1 = gl_in[1].gl_Position;
-        vec4 p2 = shift(p0);
-        vec4 p3 = shift(p1);
+        vec4 p2 = shiftNear(p0);
+        vec4 p3 = shiftNear(p1);
         if ( dot(n0 , lightDir) > 0)
         {
             gl_Position = f(p0); EmitVertex();