   Clipping in 2D
  In a 3D world you would use occulsion testing to see if an object should be rendered or not.  I was wondering if you would do something similar in like a side scroller?I'm talking about enemies/bullets/etc.   
  DEFINATELY!  The logic is nearly the same as it is in the 3d sense, only with one fewer d :P.  Quad trees, object buckets, segmented grids are great for a 4 direction scroller, and can support large worlds, and expand very easily, even to the scale as is seen in many large online worlds [though the exact solution you want comes with what you are doing, and a good solution is tailored to the specific problems to take advantage to the unique set of restrictions that can be taken for granted].  For 2 direction scrollers, a sequenced list sorted by the position along the major axis is often sufficient for any sort of hit detection/object culling you need.  For example, if you're programming a mario clone, a quad tree would be WAY overkill because of the tight restriction placed on vertical movement, but if you sort all the entities by their x-axis position in a list, then you need only check for collision those objects that neighbor eachother in the list.   ;  Basically you should have some variables that hold where the screen is in relation to the world and only render if objects are within that space?   ;  If you're going to be adding clipping later on, you should implement a sort of grid-cell system for sorting objects. That way, you can just render the objects in grid-cells that are onscreen, and also make your clipping algorithm much more efficient.If you're not going to add clipping, just calculate the rectangle which is your screen, increase each side by the radius of your largest objects (so that you don't get objects vanishing when they're only half off the screen), and then only draw objects which fall inside that rectangle. With a bit of tweaking, the second algorithm can be very fast.   