getting things in perspective - anamorphic art (WIP)

I'm a little bit lottle bit fascinated by anamorphic art at the moment -  Street art and optical illusions both tick the right boxes with me, so rolling them into one can't miss really!
Julian Beaver (see vid below) is probably one of the best, and best known, artists doing this sort of thing...

...then the other day I saw this video...

...and got quite excited by the distorted images that they were working from and I'm starting to understand how it works a bit better now.

There is an instructable here that shows you how to manipulate an image using a computer, but I'm a bit old school when it comes to art and I'm much happier with a pen, pencil or brush in my hand than a mouse, so I drew out some grids myself and made a start...

 ...needs a lot more shading yet, but hopefully you can see how it works...