Processing uses a Cartesian coordinate system. It is important to note that the zero point is in the upper left corner in processing. The X axis is from left to right and the Y axis is from top to bottom. While this might seem odd, it comes from the tradition that bitmaps were always read in that order. In computer graphics the cartesian coordinate system is almost always used. This provides some convenience, so you can also move, scale, or even rotate coordinate systems. I can simplify the drawing of objects in many cases. Imagine an animation of a car. The wheels should rotate naturally. If you draw this per processing you must always add the current position of the body to the wheels and then turn them. If you do this with the change of the coordinate system, this is a lot simpler. Here you just have to move the coordinate system, draw the car body and then move the coordinate system again for the respective wheels. Such coordinate system changes are called transformations. There are three basic types:
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