A question about derivation in oblique view frustum Problem Description:


Reference:
Oblique View Frustum Depth Projection and Clipping

"Mathematics for 3D Game Programming and Computer Graphics" 3rd edition, chapter 5.6.
M is the original projection matrix. 
Plane C is the modified near plane, and C'=transpose(inverse(M))*C is the projected plane by M.
Why in NDC the corner Q' of the view frustum lying opposite the plane C' is given by Q'=(Cx, Cy, 1, 1)?
check this link

quotations ：

“The points (+/-1, +/-1, 1, 1) are the four corners of the view frustum on the far plane in clip space.”

this should be the point Q' which equals (sgn(Cx), sgn(Cy), 1, 1)