https://github.com/mumax/3/pull/335

src/engine/ext_make3dgrains.go

@@ -120,23 +120,32 @@ func (t *tesselation3d) tabulateCells() []cellLocs {
 	return cells
 }
 
+var grainCutShape = false
+
 // Find the nearest Voronoi center to the point (x, y, z). Only points inside the given shape will be
 // assigned a region.
 func (t *tesselation3d) RegionOf(x, y, z float64) int {
-	if t.shape(x, y, z) {
-		nearest := center3d{}
-		mindist := math.Inf(1)
-		for _, c := range t.centers {
-			dist := sqr(x-c.x) + sqr(y-c.y) + sqr(z-c.z)
-			if dist < mindist {
-				nearest = c
-				mindist = dist
-			}
+	if !(t.shape(x, y, z) || grainCutShape) {
+		return -1 // Regions < 0 won't be rastered
+	}
+
+	// Find the nearest center point to the (x, y, z) position
+	nearest := center3d{x, y, z, 0}
+	mindist := math.Inf(1)
+	for _, c := range t.centers {
+		dist := sqr(x-c.x) + sqr(y-c.y) + sqr(z-c.z)
+		if dist < mindist {
+			nearest = c
+			mindist = dist
 		}
+	}
+
+	// Check if the nearest point's region should be returned
+	if (t.shape(x, y, z) && !grainCutShape) || (t.shape(nearest.x, nearest.y, nearest.z) && grainCutShape) {
 		return int(nearest.region)
-	} else {
-		return -1 //When the regions are rendered, any region < 0 will not be rastered.
 	}
+
+	return -1
 }
 
 // Generate normally distributed numbers; mean = lambda, variance = lambda. If generated number < 0, return 1.

src/test/make3dgrains_shape_edge.mx3

@@ -0,0 +1,32 @@
+N:=32
+SetMesh(N, N, N, 1e-9,1e-9,1e-9, 0, 0, 0)
+
+//set material params
+alpha=1.
+Msat=350e3
+Aex=10e-12
+Ku1=1.0e6
+
+//define a spherical shape 
+diam:=16e-9
+sphere := Ellipsoid(diam,diam,diam)
+defregion(1, sphere)
+
+seed:=238948790
+randSeed(seed) 
+GrainCutShape=true
+ext_make3Dgrains(8e-9, 2, 250, sphere, seed)
+
+for i:=2; i<254; i+=1 {
+	anisU.setregion(i, vector(randNorm(),randNorm(),randNorm()))
+}
+
+
+//a cell in the sphere for which the nearest voronoi centre also lies in the sphere:  nonzero region nr expected
+expect("region", regions.getcell(15,15,15), 191, 0)
+
+//a cell in the sphere for which the nearest voronoi centre lies outside the sphere: region nr 1 expected as it is not affected by make3Dgrains
+expect("region", regions.getcell(9,15,15), 1, 0)
+
+//a cell outside of the sphere for which the nearest voronoi centre lies inside the sphere: nonzero region nr expected as it was completed by make3Dgrains
+expect("region", regions.getcell(16,4,15), 64, 0)