Is st_voronoi using the envelope argument?

[zross]

Perhaps my expectations are wrong. I was assuming that including the envelope argument in st_voronoi() would limit the resulting voronoi polygons to those in the envelope. Essentially, I thought the voronoi polygons would be clipped by the envelope. But I'm finding that the results with and without an envelope argument are the same, is this a bug or am I wrong about what it should be doing?

Green is the convex hull and grey is the voronoi

set.seed(100)
df <- data.frame(ID = 1:10, X = sample(1:10, 10), Y = sample(1:10, 10))
d <- st_geometry(st_as_sf(df,coords = c("X", "Y")))
d <- st_union(d)
hull <- st_convex_hull(d)
plot(hull)
plot(d, add = TRUE)

# No envelope
v <- st_voronoi(d)
plot(v, col = "grey")
plot(hull, add = TRUE, col = rgb(0, 1, 0, 0.5))
plot(d, add = TRUE)

# Envelope
v <- st_voronoi(d, envelope = hull)
plot(v, col = "grey")
plot(hull, add = TRUE, col = rgb(0, 1, 0, 0.5))
plot(d, add = TRUE)

[edzer]

It does, but only if it is larger than the default envelope:

pol = st_polygon(list(rbind(c(-10,-10), c(30,-10), c(30,30), c(-10,30), c(-10,-10))))
 v <- st_voronoi(d, env)
plot(v, col = "grey", axes = TRUE)

See lines 702-703 of geos_c.h. I will add this to the docs.

[zross]

OK. Is there a reason why it doesn't work for a smaller envelope? The default envelope seems to be bigger than necessary in many instances and I would have thought that including a smaller envelope should work the same as a larger envelope no?

[edzer]

I guess because the algorithm tesselates infinite space, and GEOS nor simple features can represent that. If you need a smaller section, cut it out:

plot(st_intersection(st_cast(v), hull), col = 'grey')

[zross]

Will do, much appreciated!