Eye Coordinate System
(OpenGL/GLU library)
• gluLookAt (eyex , eyey , eyez , lookatx , lookaty , lookatz , upx , upy , upz );
• In our terminology:
• eye = VRP
•        lookat = VRP + VPN
• up  = VUP
• gluLookAt also works even if:
–lookat is any point along the VPN
–VUP is not perpendicular to VPN