Two Views of a Graphical World

Although Quartz 2D and OpenGL ES overlap a lot in terms of what you can accomplish with them, there are distinct differences between them.

Quartz 2D is a set of functions, datatypes, and objects designed to let you draw directly into a view or an image in memory. Quartz 2D treats the view or image that is being drawn into as a virtual canvas. It follows what's called a painter's model, which is just a fancy way of saying that the drawing commands are applied in much the same way as paint is applied to a canvas.

If a painter paints an entire canvas red, and then paints the bottom half of the canvas blue, the canvas will be half red and half either blue or purple (blue if the paint is opaque; purple if the paint is semitransparent). Quartz 2D's virtual canvas works the same way. If you paint the whole view red, and then paint the bottom half of the view blue, you'll have a view that's half red and half either blue or purple, depending on whether the second drawing action was fully opaque or partially transparent. Each drawing action is applied to the canvas on top of any previous drawing actions.