In finnlindgren/fmesher: Triangle Meshes and Related Geometry Tools

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>",
  dev = "png",
  dev.args = list(type = "cairo-png"),
  fig.width = 5,
  fig.height = 5 * 5 / 7
  #  out.width = 250,
  #  out.height = 250
)
options("bitmapType" = "cairo")

suppressPackageStartupMessages(library(fmesher))
set.seed(1234L)

Mesh construction

domain <- cbind(rnorm(4, sd = 3), rnorm(4))
(mesh2 <- fm_mesh_2d(
  boundary = fm_extensions(domain, c(2.5, 5)),
  max.edge = c(0.5, 2)
))

(mesh1 <- fm_mesh_1d(
  c(0, 2, 4, 7, 10),
  boundary = "free", # c("neumann", "dirichlet"),
  degree = 2
))

Point lookup and evaluation

pts <- cbind(rnorm(400, sd = 3), rnorm(400))

# Find what triangle each point is in, and its triangular Barycentric
# coordinates
bary <- fm_bary(mesh2, loc = pts)
head(bary)
# How many points are outside the mesh?
sum(is.na(bary$index))
bary$where[is.na(bary$index), ]

# Evaluate basis functions
basis <- fm_basis(mesh2, loc = pts) # Raw SparseMatrix
basis_object <- fm_basis(mesh2, loc = pts, full = TRUE) # fm_basis object
sum(!basis_object$ok)

# Construct an evaluator object
evaluator <- fm_evaluator(mesh2, loc = pts)
sum(!fm_basis(evaluator, full = TRUE)$ok)

# Values for the basis function weights; for ordinary 2d meshes this coincides
# with the resulting values at the vertices, but this is not true for e.g.
# 2nd order B-splines on 1d meshes.
field <- mesh2$loc[, 1]
value <- fm_evaluate(evaluator, field = field)
sum(abs(pts[, 1] - value), na.rm = TRUE)

pts1 <- seq(-2, 12, length.out = 1000)

# Find what segment, and its interval Barycentric coordinates
bary1 <- fm_bary(mesh1, loc = pts1)
# Points outside the interval are treated differently depending on the
# boundary conditions:
sum(is.na(bary1$index))
head(bary1)

# Evaluate basis functions
basis1 <- fm_basis(mesh1, loc = pts1) # Raw SparseMatrix
basis1_object <- fm_basis(mesh1, loc = pts1, full = TRUE) # fm_basis object
sum(!basis1_object$ok)

# Construct an evaluator object.
evaluator1 <- fm_evaluator(mesh1, loc = pts1)
# mesh_1d basis functions are defined everywhere
sum(!fm_basis(evaluator1, full = TRUE)$ok)

# Values for the basis function weights; for ordinary 2d meshes this coincides
# with the resulting values at the vertices, but this is not true for e.g.
# 2nd order B-splines on 1d meshes.
field1 <- rnorm(fm_dof(mesh1))
value1 <- fm_evaluate(evaluator1, field = field1)
plot(pts1, value1, type = "l")

Plotting

Base graphics

plot(mesh2)

`ggplot` graphics

suppressPackageStartupMessages(library(ggplot2))
ggplot() +
  geom_fm(data = mesh2)

ggplot() +
  geom_fm(data = mesh1, weights = field1 + 2, xlim = c(-2, 12)) +
  geom_fm(data = mesh1, linetype = 2, alpha = 0.5, xlim = c(-2, 12))

Finite element calculations

fem1 <- fm_fem(mesh1, order = 2)
names(fem1)
fem2 <- fm_fem(mesh2, order = 2)
names(fem2)

Stochastic process simulation

samp <- fm_matern_sample(mesh2, alpha = 2, rho = 4, sigma = 1)[, 1]
evaluator <- fm_evaluator(
  mesh2,
  lattice = fm_evaluator_lattice(mesh2, dims = c(150, 50))
)
image(evaluator$x, evaluator$y, fm_evaluate(evaluator, field = samp), asp = 1)