This is a sort of continuation to this question. I would like to know how does LaTeX generate pictures with its 'picture' environment. Since TeX deals with typesetting and not with graphics, I suppose LaTeX uses METAPOST. So I guess once you ask it to compile, LaTeX translates the 'picture' code into METAPOST. Am I correct?
The standard picture mode places lines and small arcs and circles by using fonts with the relevant pieces. That is the reason for the slightly weird line syntax where you specify a fixed slope and a horizontal length, the slopes are constrained to the slopes of the available line segments in the fonts.
There are re-implementations for picture mode using pdf or postscript specials that allow a wider range of slopes and circles of larger sizes.