This technique is known as antialiasing and produces a shape with a smoother outline. The rasterizer takes a shape and produces alpha values to represent how much of the pixel is covered by the shape. Values of 0.0 represent no coverage and 1.0 represent full coverage. The rasterizer will produce values within the range of 0.0-1.0 when antialiasing is used. These mid-range values are then used to determine the varying shades of gray surrounding the image, making it appear smoother and less jagged.
User space is the plane defined by the Cartesian coordinates.
When objects are drawn on the screen or printer the user space coordinates are transformed into device space coordinates. Device space corrsponds to a monitor or printer. User space and device space are aligned at the x and y axes. One unit of device space corresponds to one pixel. Scaling may have to be done to make sure the objects will be drawn the same size, regardless of the output device.