[Algorithms] Cyrus-beck line clipping algorithm

[Phil Y. <ph...@cl...>]

I have just been looking through the definition of the Cyrus-Beck line
clipping algorithm, as defined in 'Procedural elements for computer
graphics' by David Rogers.

On talking about deriving the normals to the top, bottom and side planes of
the viewing frustrum, he talks about using the 'cross products of the
vectors from the centre of projection to the corners at z=0, the plane of
projection'.

However, he previously defines the view frustrum as a 'perspective volume
with (xl, xr, yb, yt, zh, zy ) = (-1, 1, -1, 1, 1, -1), with a centre of
projection at zcp = 5'.

He lists the vectors as:
	v1 = [ 1, 1, -5 ]
	v2 = [ -1, 1, -5 ]
	v3 = [ -1, -1, -5 ]
	v4 = [ 1, -1, -5 ]

Surely the plane of projection should be z = -1, and therefore the z
component in the above vectors should be -6?

Or am I (more likely!) missing something obvious here?

_____________________________________
Phil Yard, Lead Programmer, Climax (Brighton) Ltd
01273 764109  mob: 0771 258 1164  web: www.climax.co.uk