How Can I Create a Sculpt Mask Based Purely on Face/Vertex Selection, Instead of By Manually Painting It?

Question

This question gets asked from time to time, and doesn't seem to get any attention. Here's one of the more recent instances: Converting a vertex group into a sculpt mask

I find it frustrating that the only way to create a sculpt mask in Blender is to manually paint it. This is needlessly imprecise in some situations, such as when there are already skeletal deformations in play which put various mesh parts too close together to get the brush between them to create the mask. Sure, you could return the armature to its rest pose, but some sculpt edits will require the new pose to be preserved, for the sake of reference.

Given that it is much easier to select multiple sub-elements (face/vertex/edge) precisely than to paint surfaces precisely, I would think there is a way to choose the former approach over the latter.

Is there?

Answer 1

I'm not great at sculpting, but what you can do is:

1. Select faces in Edit Mode.
2. Go to Sculpt Mode and from the Face Sets menu choose Face Set from Edit Mode Selection.
3. Enable Options > Auto-Masking > Face Sets.

Now the Face Set is masked and your sculpt only affects the mesh outside of the mask. Maybe there is even a better way, I don't know if vertex groups with weight paint can be used for masking in Sculpt Mode, perhaps someone else can step in here. If so, you can set the weight according to the selection so this should also do what you want.

Answer 2

For people interested in a scripted version. This will set every selected vert of the active object to the 'mask_value'

import bpy
import bmesh
import numpy as np
obj = bpy.context.active_object
bm = bmesh.new()
bm.from_mesh(obj.data)

#get selected verts
v_sel = np.empty(len(obj.data.vertices), dtype=bool)
obj.data.vertices.foreach_get('select', v_sel)
sel_idx, = np.where(v_sel)

#get custom data layer paint_mask
mask_layer= bm.verts.layers.paint_mask.verify() #strange way, but that gets you custom data layer
bm.verts.ensure_lookup_table()
#set every selected vert to mask_value

mask_value = 1.0
for idx in sel_idx:
    bm.verts[idx][mask_layer]=mask_value

bm.to_mesh(obj.data)

Answer 3

It appears that using the mask brush will also obey face sets, as long as the Face Sets and/or Face Set Boundary are selected when painting the mask.

By enabling this, you can paint a masked area constricted by the face set. If you then disable the face set auto-masking you will have your typical masking behavior (where your brush obeys the mask no matter where the stroke starts).

This could also be useful for temporarily combining multiple face sets without destroying them.

I sympathize with OP that it would be great to have a "create face sets from vertex groups" option, but that doesn't account for the fact that vertex group selections can overlap while face sets cannot.