How a Subdivision Surfaces object works
Contrary to other Generator objects, such as the Extrude object or Sweep object, the Subdivision Surfaces relies on polygons and not on spline curves.
In the images above, a linear (black) spline is shown. The points of the spline have been highlighted in red. This linear depiction can be compared directly to the edges of a polygon, which also flow in a linear fashion between a surface’s corner points. The blue curve in the image above depicts a B-Spline that makes use of exactly the same points as the linear spline. You can clearly see how all corners of the linear spline have been rounded (the intermediate points must be selected in a mode other than None). The closer two points lie together, the sharper the curve of a B-Spline (blue curve) will be.
B-Splines created in a 3D environment will appear as shown in the images below:
The image at the left shows how useful Subdivision Surfaces can be when creating organic shapes. A flowing, organic shape was created using simple geometry. However, the Subdivision Surfaces doesn’t free us completely from the use of polygons. As you can see in the image at the right, the organic shape that was created also consists of polygons - which cannot be directly modified. Therefore the roughly shaped model to which the Subdivision Surfaces was assigned must be modified.
Subdividing a Subdivision Surfaces object
As already mentioned, a Subdivision Surfaces automatically subdivides the object to which it was assigned. A Subdivision Surfaces offers parameters for fine-tuning this subdivision. Since increasing the subdivision can quickly lead to an exponential increase in the total number of polygons, two separate parameters are available that helps optimize workflow. The Subdivision Editor parameter defines the subdivision for display in the Viewport, the Subdivision Renderer parameter defines the subdivision that should be used for the final rendering. A subdivision value of 2 or 3 may not sound like much - but don’t let this fool you. Each numeric increase represents an additional subdivision for each of an object’s edges! Let’s say you have a simple, four-sided polygon that consists of four points and a single surface. Assigning it a Subdivision Surfaces with a Subdivision value of 1 would increase the number of polygons to four and the number of points to nine - each one of the object’s edges would be subdivided once. Setting the Subdivision value to 2 would result in the creation of sixteen quads and twenty-five points!
Since most polygon objects used for modeling contain hundreds or even thousands of polygons, keeping the Subdivision value as low as possible will prevent you from using unnecessary amounts of memory or hard disk space.
In the image below you can see what effect increasing the Subdivision value had on our object’s surface. This is evident at the object’s ends - especially when compared to the previous image.