Hi,

I have problem understand following sentences in my textbook Computer Graphics with OpenGL.

"To make viewing process independent of the requirements of any output device,graphic system convert object descriptions to normalized coordinates and apply the clipping routines."

Why normalized coordinates could make viewing process independent of the requirements of any output devices?
Isn't the projection coordinates already independent of output device?We only need to first scale and then translate the projection coordinate then we will get device coordinate.
So why do we need first convert the projection coordinate to normalized coordinate first?

"Clipping is usually performed in normlized coordinates.This allows us to reduce computations by first concatenating the various transformation matrices"

Why clipping is usually performed in normlized coordinates? What kind of transformation concatenated?

thanks. jcyang.