How can I use the subdivision surface modifier to create more high poly meshes without ruining sharp edges?
For example this mesh
The sphere should be subdivided but the rim of the cylinder should stay the same because it is a sharp edge.
This is the result of the subdevision
How can I get sharp edges to not be affected by the sub surf mod?
You have two options:
Add supporting geometry (Loopcuts CtrlR and beveling CtrlB both work well):
The Subsurf modifier demonstrates why good, clean topology is so important. As you can see in the figure, the Subsurf modifier has a drastic effect on a default Cube. Until you add in additional Loops (with CtrlR), the shape is almost unrecognizable. A mesh with a deliberate topology has good placement of Edge Loops, which allow the placement of more Loops (or removal of Loops, with X » Edge Loop) to control the sharpness/smoothness of the resultant mesh.
Use Creases (ShiftE):
Weighted edge creases for subdivision surfaces allows you to change the way Subsurf subdivides the geometry to give the edges a smooth or sharp appearance.
The crease weight of selected edges can be changed using Transform Properties (N) and change the Median Transform slider. A higher value makes the edge “stronger” and more resistant to subsurf. Another way to remember it is that the weight refers to the edge’s sharpness. Edges with a higher weight will be deformed less by subsurf. Recall that the subsurfed shape is a product of all intersecting edges, so to make the edges of an area sharper, you have to increase the weight of all the surrounding edges.
Creases tend to produce a sharp, hard edge which tends to look less realistic (and also tends to require an Edge Split modifier if used with smooth shading), while supporting geometry tends to give a subtly rounded edge which looks more realistic and does not require an edge split modifier. However, using supporting edge loops also tends to add a lot of extra geometry.
Another option that keeps edges sharp is to use an Edge Split modifier.
The sharp Edges option allows you to specify which edges are split.
A trick that I like to use, is the add a bevel modifier before a subserf in the modifier stack.
I set the segments to 2, and the profile to one, adjust the amount by eye, and then choose between the option of using the the angle of the edges to decide what will be hard edged (similar to using an edge split modifier), or using a bevel weight to have complete control over which parts are hard edged.
To add the bevel weights, press CtrlE and select Edge Bevel Weight. Then type in 1.
And the result
This is really good for a non destructive workflow, allowing the mesh to be changed considerably without having to remove supporting loops and re-add them.
There are some issues though.The bevel modifier can produce some sloppy topology, especially when dealing with only beveling some parts of a mesh. This can cause shading issues on depending on the original model.
