With the general reoganistion introduced to Blender 2.8+ a number of the options formally available in the Tool Shelf have been moved to more context appropriate locations, included the Shading options, Smooth and Flat.
Download: rivets & bolts example *.blend (c.200 KB).
Design note: the Smooth option available in the Toolbar is not the same as Smooth in this context.
The effect that gives the impression that a surface or area seems curved or has an organic appearance is called Smooth Shading in Blender. To assign smoothing, select the object in the 3D View then from the Object menu select Shade Smooth – Object » Shade Smooth.
Important: Shade Smooth (and Shade Flat) can also be applied in Edit Mode but is specifically selection based, only the selected elements – vertices, edges and faces – are affected by the property (note the options to smooth/sharp edges/vertices act as a proxy of sorts for Mark Sharp).
What Smoothing essentially does is treat non-planar surfaces (anything that’s not flat or part of a flat plain) as being contiguous, belonging to the same collection or selection.
Design note: Smooth Shading goes by many different names, Mesh Smoothing, Smooth Groups, Mesh Shading, Smoothing and so on, all relate to the same effect, whether a surface appears continuous or an edge hard or sharp.
In practice when applied to a mesh, Smoothing tends to make surfaces and objects appear ‘soft’ and undefined, edges difficult to discern because they are all treated as though they all belong to the single ‘smooth group‘. To alleviate this and create hard or sharp edges, surface continuity can be broken or split manually or automatically depending on the look required.
Design note: smoothing is particularly useful for games or low(er) polygon environments where the amount of mesh structure available to describe curved or smooth surfaces is limited. It can however, be used in most if not all circumstances to improve appearances.
Meshes are shaded Flat (left) in Blender by default which makes them appear faceted. When Smooth (right) is applied the opposite tends to happen, edges loose all definition and become difficult to determine.
Because Smoothing is applied globally, it affects the entire object, the most precise way to control it is to tag or mark specific edges with a special property that causes surface continuity, and thus smoothing, to break. This is done using Mark Sharp.
Design note: creating hard or sharp edges actually splits vertices along an edge into coincidental pairings without physically separating the mesh as would otherwise be done using Split.
To do this, select the object (or objects) to be smoothed and toggle in to Edit Mode (Tab). For ease of assignment switch to Edge Select mode and select the edges where a sharp or hard break in the surface is to appear. From the Edge menu upper-left of the 3D View select Mark Sharp – Edge » Mark Sharp. The highlighted edges will immediately tint pale-blue confirming property assignment.
Once edges have been marked, access Modifiers properties and click Add Modifier and from the list select Edge Split. In the panel that appears ensure Sharp Edges is selected, Edge Angle deselected. The mesh will appear appropriately in the 3D View with tagged edges appearing hard or sharp, with others appearing soft or part of a contiguous surface.
Edge Split Modifier
As discussed above the Edge Split modifier can be used in conjunction with edges marked Sharp to provide selective (manual) control over the placement of soft/hard edges. As an alternative the same modifier can be used to break surface continuity based on the angle-of-incidence between faces; beyond a certain point (value in degrees or rotation) surfaces split, below and it remain intact and contiguous. This is done enabling Edge Angle and using the Split Angle setting.
Design note: used solely, angle-based smoothing will break surface continuity whether the effect is wanted in a particular area or not.
To do this, select the object then in Modifiers properties click Add Modifiers and select Edge Split. The properties panel will appear. Ensure Edge Angle is active, Sharp Edges deselected (deactivated). The appearance of the selected object will change based on the default value, 30°, likely giving the appearance of faceting. To change this increase the Split Angle: value to include surfaces currently out-of-range, or lower to exclude surfaces currently within range.
Design note: the Edge Split modifier assumes Edge Angle and Sharp Edges are being used for mesh smoothing so both are active by default.
As a further alternative to the Edge Split modifier Smoothing can also be defined using Auto Smooth to similarly manage surface continuity based on an angular value. In addition, if edges are also marked as Sharp they too will also determine smoothing, where edges appear soft or hard.
Design note: Auto Smooth ostensibly functions the same way as the Edge Split modifier except the ability to enable/disable Edge Angle and/or Sharp Edges is not available – if a mesh is marked with Sharp edges they will be used, else they must be removed (Edges » Clear Sharp).
To use Auto Smooth, select the object (or objects) to be smoothed in the 3D View then access Object Data properties. Scroll down to/find the Normals options (click the ► arrow to expand) and click the checkbox marked Auto Smooth to enable. To change the degree to which a surface or edge smooths alter the Angle value, 30º, increase to include more acute faces, decrease to remove those already included.
Strictly speaking Blender 2.8+ (all versions in fact) does not use smooth groups to smooth meshes, it takes the hard/soft edge approach as explained above. Whilst this may have its limitations by comparison, namely groups within groups, these are almost exclusively managerial in nature as the overall effect is the same, a hard edge where the mesh has been/is to be split when rendered. To achieve a similar effect in Blender care should be taken with respect to achieving the end goal (visually) using judicial Sharp edge placement and the techniques described above.
Design note: in all circumstances it’s immaterial whether a given edge belongs to a groups of other edges/faces/elements because all that actually matters is whether the mesh, and thus surface continuity, should be broken at a given point. In other words, smooth groups are a way to manage smoothing and little else.
The colours above represent different ‘smooth groups’ that might be assigned to a mesh, each causing a surface break where it meets a neighbouring group at their respective borders – multiple regions can be associated with a single group so long as they are not contiguous.