Hi, this is my first time posting! I'm using a macbook pro with xcode and trying to do lesson 21, but all I seem to be getting is a black screen (although I do have text showing up). I'm using text code from a previous tutorial and that seems to be working, but nothing else does. Using printf's I've been able to definitively know that all the code is being run - there doesn't seem to be a place where an infinite loop is being run for example. I'm not sure if the grid lines are being translated off the screen or what is going on really, but they are just not there. Any ideas?