# What is principal vanishing point in perspective projection?

I am unable to visualize what is principal vanishing point w.r.t to x axis ,y axis z axis do someone can post some simple diagram to understand what is it? Also I have question in my text book

' Now the question is how to calculate vanishing point and what is the line A' B ?Solution is also given which is (0,0,0) as vanishing point but I am unable to understand how do they get it , figure (k)shows it as (0,0,2) ,also what is d in the solution?

Now one confusion arises that the question doesnt give you a set of parallel lines, But it doesn't matter as long as a single line segment is provided. All you need to do is just picture the line segment extending to infinity. So for example, picture the point $$B$$ at infinity and imagine drawing a line $$BE$$. The point at which this line intersects the image plane will be your vanishing point.
There is a simple pattern that applies to every case like this. Just shoot a ray in the direction of the vector $$\vec{AB}$$ from the point $$E$$. The point at which this ray intersects the image plane will be your vanishing point which as you can see is $$(0,0,0)$$.
Now imagine why this works. Picture the point $$B$$ extending up, and at each instance start drawing segments $$BE$$. You'll see that this line segment is moving towards the $$Z$$ axis. So if you are projecting a point $$B$$ at infinity then the projecting line or segment $$BE$$ gonna become parallel to the line $$AB$$. However, it can't go past the $$Z$$ axis, as this will start going in the opposite direction.