--- vtkCamera.cxx.orig	2009-11-03 11:43:07.612776025 +0800
+++ vtkCamera.cxx	2009-11-03 11:44:25.227781381 +0800
@@ -606,8 +606,11 @@
     farz = temp;
     }
 
-  // front should be greater than 1e-20
-  if (nearz < 1e-20)
+   // front should be greater than 1e-20
+   // nearz should be greater than 1e-10 for perspective projections.
+   // For parallel projections, negative values of nearz are ok.
+   // Otherwize, the actors would get clipped by the near plane.
+  if (!this->ParallelProjection && nearz < 1e-20)
     {
     farz += 1e-20 - nearz;
     nearz = 1e-20;