I downloaded some sample from internet. but the sharp always disappear when i changed any value of Position, or Look Direction, or Up Direction. I can't understand these properties, can anyone help me?
what's relationship between these properties? is there any sample to show these relationship?
Check out the 3-D Graphics Overview on MSDN, specifically the section on cameras.
Position is a vector from the scene root(origin) to the camera.
Look direction is a unary vector positioned at the camera that points in the direction the camera is looking.
Up direction is a unary vector that points to "the sky", I mean, points in the direction the camera considers as "up". Normally you use a (0,1,0) vector here.
Look here for more info.
Hope that helps.