Perspective Projection

Hi,
Can somebody plz help me to understand perspective projection equation.
The equations are:

The perspective equations are:
x' = x - xu
y' = y - yu
z' = z - (z- \(Z_{prp}\))u

Click to expand...

I cant understand how we got these equations. According to book

Parameter u takes values from 0 to 1 and coordinate position (x', y', z') represents any point along the projection line. When u=0, we are at position P = (x, y, z). At the other end of the line u=1, and we have the projection reference point coordinates (0, 0, \(Z_{prp}\))

Click to expand...

I cant understand how we get the above eq and how (0,0,\(Z_{prp}\)).


I got the following image from a web site:

Zulfi.