>>1693126Here's how it's done.
Extend parallel lines (top and bottom of rail) until they meet. CYAN
Draw a vertical line from the vanishing point. ORANGE
Now a RED line from the bottom of the rail at the end of post 1 to the top of the rail at the end of post 2.
ALL diagonals from the bottom of the rail at post N to the top of the rail at post N+1 have the same vanishing point.
I've added a few in dark blue. Each one you draw sets you up for the next one.
The widths of the posts are proportional to their distance from the VP.
One way of doing this is to repeat the above procedure, but working with the left sides of the posts instead of the rights.
Alternately, select post 1 AND the upper VP as a single object. Scale this down (keeping the VP in the same spot) until the copy of post 1 covers post 2. Note the scaling factor. Say it's 0.8. Just keep making copies, each one 0.8 the size of the previous one. All the copies should be positioned so they share the VP.
This technique ensures they all have the right position, width, and increasingly small detail.
