Sketchy engraving: tonal shading by hatch-line density

geom_sketch_engrave() shades a surface the way an etcher or banknote engraver does: continuous tone is built from the density of hand-drawn hatch lines, with cross-hatching deepening the shadows. It takes a regular grid of x, y, and z (like ggplot2::geom_raster()); z is mapped to tone (high = dark by default). Unlike the fill-pattern packages, the tone is computed from geometry (a engrave_fill() ladder), not tiled from a motif.

Usage

GeomSketchEngrave

GeomSketchShade

geom_sketch_engrave(
  mapping = NULL,
  data = NULL,
  stat = "identity",
  position = "identity",
  ...,
  levels = 5L,
  base_gap = 0.08,
  gap_ratio = 0.62,
  base_angle = 45,
  cross_after = 3L,
  reverse = FALSE,
  roughness = 0.5,
  bowing = 0.3,
  min_gap_in = 0.012,
  seed = NULL,
  na.rm = FALSE,
  show.legend = NA,
  inherit.aes = TRUE
)

geom_sketch_shade(
  mapping = NULL,
  data = NULL,
  stat = "identity",
  position = "identity",
  ...,
  levels = 5L,
  base_gap = 0.08,
  gap_ratio = 0.62,
  base_angle = 45,
  cross_after = 3L,
  roughness = 0.5,
  bowing = 0.3,
  min_gap_in = 0.012,
  seed = NULL,
  na.rm = FALSE,
  show.legend = NA,
  inherit.aes = TRUE
)

Arguments

mapping

Aesthetic mappings created by ggplot2::aes(). geom_sketch_engrave() needs x, y, z; geom_sketch_shade() needs x, y and uses the tone aesthetic (default 0.6).

data

Data to display.

stat

Statistical transformation. Default "identity".

position

Position adjustment. Default "identity".

...

Other arguments passed on to the layer.

levels

Number of hatch layers in the ladder. Default 5.

base_gap

Pitch of the sparsest layer (npc-x fraction). Default 0.08.

gap_ratio

Multiplicative pitch shrink per layer (smaller = densens faster). Default 0.62.

base_angle

Angle of the first hatch layer (degrees). Default 45.

cross_after

Layer index at which cross-hatching begins. Default 3.

reverse

(geom_sketch_engrave() only) Invert the tone mapping so low z is dark. Default FALSE.

roughness

Non-negative stroke roughness. Default 0.5.

bowing

Non-negative bowing multiplier. Default 0.3.

min_gap_in

Pitch floor in inches; finer ladder layers are dropped. Default 0.012.

seed

Integer seed. NULL uses getOption("ggsketch.seed", 1L).

na.rm

Remove missing values silently? Default FALSE.

show.legend

Logical; include in legend?

inherit.aes

Override default aesthetics?

Value

A ggplot2 layer object.

Details

geom_sketch_shade() shades each polygon region with a uniform density set by a tone aesthetic in [0, 1], so a mapped value reads directly as darkness – a hand-drawn alternative to a solid fill scale.

The hatch ladder is a stack of layers of increasing density and rotating angle (levels, base_gap, gap_ratio, base_angle, cross_after); each layer is drawn only where the tone reaches its threshold, so light areas keep the sparse base layer and shadows accumulate every layer. min_gap_in is a pitch floor (inches) that keeps the darkest tones from exploding into a runaway number of strokes.

See also

Other sketch-geoms: GeomSketchAbline, GeomSketchArrow, GeomSketchBoxplot, GeomSketchBracket, GeomSketchCallout, GeomSketchChicklet, GeomSketchCol, GeomSketchContourFilled, GeomSketchCurve, GeomSketchDotplot, GeomSketchDumbbell, GeomSketchEdge, GeomSketchEllipse, GeomSketchGantt, GeomSketchHex, GeomSketchLine, GeomSketchLinerange, GeomSketchLollipop, GeomSketchMarkCircle, GeomSketchMarkHull, GeomSketchPath, GeomSketchPoint, GeomSketchPolygon, GeomSketchRect, GeomSketchRibbon, GeomSketchRug, GeomSketchSegment, GeomSketchSfPolygon, GeomSketchSlope, GeomSketchSmooth, GeomSketchSpoke, GeomSketchTextRepel, GeomSketchViolin, StatSketchBeeswarm, StatSketchCalendar, StatSketchDensityRidges, StatSketchFunnel, StatSketchPie, StatSketchPyramid, StatSketchRadar, StatSketchStream, StatSketchTreemap, StatSketchWaffle, StatSketchWaterfall, annotate_sketch(), annotate_sketch_arrow(), annotate_sketch_callout(), annotate_sketch_highlight(), geom_sketch_alluvial(), geom_sketch_arc_diagram(), geom_sketch_bin2d(), geom_sketch_bump(), geom_sketch_chord(), geom_sketch_contour(), geom_sketch_count(), geom_sketch_dendrogram(), geom_sketch_density(), geom_sketch_density2d(), geom_sketch_ecdf(), geom_sketch_function(), geom_sketch_histogram(), geom_sketch_jitter(), geom_sketch_marimekko(), geom_sketch_mosaic(), geom_sketch_parallel(), geom_sketch_qq(), geom_sketch_quantile(), geom_sketch_rose(), geom_sketch_sunburst(), geom_sketch_text(), sketch_graph()

Examples

library(ggplot2)
ggplot(faithfuld, aes(waiting, eruptions, z = density)) +
  geom_sketch_engrave(seed = 1L) +
  theme_sketch()


# A real surface: an engraved relief map of Maungawhau (Mt Eden),
# the way a 19th-century atlas shades elevation by hatch density.
volcano_df <- expand.grid(
  easting  = seq_len(nrow(volcano)),
  northing = seq_len(ncol(volcano))
)
volcano_df$height <- as.vector(volcano)

ggplot(volcano_df, aes(easting, northing, z = height)) +
  geom_sketch_engrave(
    levels = 8L, base_gap = 0.04, gap_ratio = 0.72,
    roughness = 0.35, bowing = 0.2, seed = 1L
  ) +
  coord_equal() +
  labs(title = "Maungawhau", subtitle = "engraved by elevation") +
  theme_sketch()