### What about diagonals? Let's learn the Sliding Grid method (with a video tutorial).

In the previous post we went through the usual ad-hoc method for drawing equirectangular perspectives. It is a pretty simple method as far as it goes; you just print out an equirectangular grid and draw over the horizontal and vertical lines, like this: You can go pretty far with that, but you are also very limited. First of all, you don't really know where that grid comes for. Following a grid made by others is not knowing perspective, it is avoiding knowing it. But well, perhaps you're not the sort of fellow that worries about such things, and that's fine too. But there is a more practical problem. Horizontals and verticals are only a small subset of all lines. What about all the diagonals? For instance, what is the shape of the diagonals lines that join opposing vertices of a wall? Those are pretty important as crossing them allow you to find the center of the wall. At the moment we have no way of finding these, as they are not one of our printed lines. But we ne