Always have the path you use for find_path
match the one in your #include
statements.
If you want to #include <ni/XnOS.h>
you should write
find_path(OPENNI_INCLUDE_PATH ni/XnOS.h)
If instead you want to #include <XnOS.h>
, use
find_path(OPENNI_INCLUDE_PATH XnOS.h)
Just be sure to make up your mind beforehand which one you want to use and stick to it. Mixing several include paths for the same library is a sure way to unnecessarily overcomplicate the build environment.