I try modify the first tutorial example, but had problem with that. Could anybody explain what is a geometrical meaning of a ray direction in the RTCRay structure?
The ray origin is where the ray starts, the ray direction is the direction the ray travels. Have a look at some introduction to ray tracing, e.g. see section 4.1 here: http://viola.informatik.uni-bremen.de/typo/html/qingteng/index.html