Win 8.1, GTX1070
Broken: 2.79RC1, 2.78c
Short description of error
The smooth brush in sculpting mode breaks edit mode symmetry. Which means it isn't 100% accurate. Of course this isn't really a problem when doing any pure dyntopo style sculpting and all you're after is a high poly mesh for baking/retopo and so on. It is a problem however when using sculpting as a poly modeling tool in tandem with edit mode proportional editing.
In reality it means that when working on characters you have to keep in mind to keep in mind whether your character really is symmetrical or not when switching between sculpt mode and edit mode. If you have broken symmetry by using the smooth brush in sculpting mode (or the smoothing "mode" of any brush) and continue working on your character you can quickly run into problems, especially if you use shapekeys and happen to mess up the basis key. You would then have to use Symmetrize or similar workarounds and hope that the symmetry is restored in the way you expect.
Maybe the Smooth brush works in this way because it's more performant (which would be good for dyntopo sculpting) but performance isn't really an issue most times when using sculpting as a poly modeling tool. For example the characters I make are at most 100k polys. I can sculpt at realtime speeds at at least 2 million tris. This means that a perfect symmetry version of the Smooth brush could be ten times slower without me even noticing.
Without sculpting working reliably I have resorted to using a mirror modifier for the whole workflow instead of using the sculpting symmetry at all. This method works but is kind of a mess because you rely on addons for applying mirror modifiers on meshes with shapekeys, working with UVs which shouldn't be mirrored and other issues. Still it's more reliable and harder to mess up your characters which is regrettable because the ability to use sculpting for poly modeling is a very cool feature in Blender. If this issue was fixed and you could use sculpting and edit mode proportional editing without worrying about symmetry it would make character creation workflows a whole lot smoother.
Exact steps for others to reproduce the error
- Create a new file and add a cube (or use a cube if it already exists).
- Add and apply subsurface modifiers with the Simple algorithm until you've got a reasonable amount of tris, maybe 50k.
- Enter sculpting mode and select the Smooth brush, enabling X mirror (symmetry).
- Sculpt the edges of the cube until it has slightly rounded edges.
- Enter edit mode, enable X-mirror and proportional editing (increasing the radius until it's big enough).
- Drag the edge of the cube. The symmetry has been broken and some vertices are not affected at all. Operators such as Select Mirror confirm this.
Layer 1: cube before sculpting. Layer 2: cube after sculpting with broken symmetry.