This is a discussion bug. We need automatic tests to verify changes in rendering (e.g. in the imageVtkMapper2D).
Currentyl, a testprotocol (http://www.mitk.org/wiki/internal/RenderingTest) is applied manually to verify the functions of the mappers and other involved classes. This is very time consuming.
VTK offers some methods to test rendering automatically. Markus Engel already used them to test the levelwindow. See the attached file for an example.