Fill styles in depth

Every filled geom (geom_sketch_col(), geom_sketch_rect(), geom_sketch_tile(), geom_sketch_polygon(), geom_sketch_ribbon(), geom_sketch_area(), geom_sketch_density(), and the boxplot box) takes a fill_style. Under the hood, all of them are produced by a single Active-Edge-Table scan-line filler (hachure_fill() and friends) that works on arbitrary — including concave — polygons.

The ten stroked styles

styles <- c("hachure", "cross_hatch", "zigzag", "zigzag_line", "scribble",
            "dots", "dashed", "stipple", "pencil_shade", "solid")

bars <- do.call(rbind, lapply(styles, function(s) {
  data.frame(style = s, x = c("A", "B", "C"), y = c(4, 6, 3))
}))
bars$style <- factor(bars$style, levels = styles)

# Draw a small panel per style by faceting and re-drawing each with its style.
# (fill_style is a layer parameter, so we build one layer per facet.)
plots <- lapply(styles, function(s) {
  ggplot(subset(bars, style == s), aes(x, y)) +
    geom_sketch_col(fill = "#E8A87C", fill_style = s, seed = 4L) +
    labs(subtitle = s, x = NULL, y = NULL) +
    theme_sketch(base_size = 9)
})

# Show them individually:
for (p in plots) print(p)

Style	Look	Notes
hachure	Parallel diagonal strokes	The default; classic pencil shading.
cross_hatch	Hachure at angle and angle + 90°	Denser, “darker” shading.
zigzag	Hachure lines joined by diagonal connectors	A continuous scribble feel.
zigzag_line	Just the connectors, as one path	Lighter than zigzag.
scribble	One continuous winding stroke that overshoots the edge	Like scribbling to fill a shape.
dots	Tiny rough circles sampled along the lines	Regular dotted texture.
dashed	Hachure broken into dashes	Airy, sketchy texture.
stipple	Tiny rough dots scattered with blue-noise spacing	Denser than dots, engraving-like.
pencil_shade	Overlapping side-of-pencil strokes	Soft graphite tone.
solid	One flat swatch under the roughened outline	A plain fill that keeps the hand-drawn edge.

Watercolour washes

fill_style = "watercolor" is a different beast: instead of scan-line strokes it lays down several translucent, slightly bled and scaled copies of the shape, plus fine granulation specks — a painted wash rather than a drawn hatch. It honours holes (so it works on geom_sketch_contour_filled() bands and donut slices) and applies to every polygon-, ribbon-, ellipse-, and band-filled geom.

ang  <- seq(0, 2 * pi, length.out = 9)[-9]
hex  <- data.frame(x = cos(ang), y = sin(ang))
ggplot(hex, aes(x, y)) +
  geom_sketch_polygon(fill = "#2E86C1", fill_style = "watercolor", seed = 1L) +
  coord_equal() +
  labs(title = "fill_style = \"watercolor\"") +
  theme_sketch()

Because it is a wash, the hachure-tuning parameters below (hachure_angle, hachure_gap, fill_weight) do not apply to it.

Tuning the hachure

Three parameters control the texture.

hachure_angle

The angle (degrees) of the fill lines:

df <- data.frame(x = c("A", "B", "C"), y = c(4, 6, 3))
for (a in c(0, 45, 90, 135)) {
  print(
    ggplot(df, aes(x, y)) +
      geom_sketch_col(fill = "seagreen", hachure_angle = a, seed = 1L) +
      labs(subtitle = paste("hachure_angle =", a), x = NULL) +
      theme_sketch(base_size = 9)
  )
}

hachure_gap

Spacing between fill lines, in data units — smaller means denser shading:

for (g in c(0.6, 1.2, 2.4)) {
  print(
    ggplot(df, aes(x, y)) +
      geom_sketch_col(fill = "#5DADE2", hachure_gap = g, seed = 1L) +
      labs(subtitle = paste("hachure_gap =", g), x = NULL) +
      theme_sketch(base_size = 9)
  )
}

fill_weight

The stroke weight of the fill lines:

for (w in c(0.3, 0.8, 1.6)) {
  print(
    ggplot(df, aes(x, y)) +
      geom_sketch_col(fill = "#AF7AC5", fill_weight = w, seed = 1L) +
      labs(subtitle = paste("fill_weight =", w), x = NULL) +
      theme_sketch(base_size = 9)
  )
}

Fills follow the shape

Because the filler is a true scan-line algorithm, the texture conforms to any polygon outline, not just rectangles:

ang  <- seq(0, 2 * pi, length.out = 9)[-9]
hex  <- data.frame(x = cos(ang), y = sin(ang))
ggplot(hex, aes(x, y)) +
  geom_sketch_polygon(fill = "#E74C3C", fill_style = "cross_hatch",
                      hachure_angle = 30, seed = 1L) +
  coord_equal() +
  labs(title = "Cross-hatch on a hexagon") +
  theme_sketch()

td <- expand.grid(x = 1:5, y = 1:3)
td$z <- td$x
ggplot(td, aes(x, y, fill = z)) +
  geom_sketch_tile(fill_style = "dots", seed = 1L) +
  scale_fill_viridis_c() +
  labs(title = "Dotted tiles") +
  theme_sketch()

Working directly with the filler

The fill geometry comes from pure Layer-1 functions you can call yourself — useful for testing or custom grobs. sketch_fill() dispatches to the styles and returns a list of line segments:

square_x <- c(0, 1, 1, 0)
square_y <- c(0, 0, 1, 1)
segs <- sketch_fill(square_x, square_y, fill_style = "hachure",
                    hachure_gap = 0.15, seed = 1L)
length(segs)      # number of fill segments
#> [1] 8
head(segs[[1]])   # each segment is a matrix of x/y points
#>              x          y
#> [1,] 0.7093499 0.07633409
#> [2,] 0.8440814 0.20310311
#> [3,] 0.9270041 0.27937282