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Motion-trail layer effects

Source: R/effects.R
motion.Rd

Draw a fading trail of a stroked or point mark: n copies marching off along a direction (x, y), each further out and fainter than the last, composited beneath the crisp original — a speed-blur or animation-still look. motion() defaults to many close, low-opacity copies (a smooth blur streak); echo() defaults to a few wider-spaced, more-opaque copies (discrete ghost repeats). Both build the same effect, differing only in defaults, and apply to the same marks as glow().

Usage

motion(
  x = 3,
  y = 0,
  n = 8L,
  alpha = 0.15,
  decay = 1,
  spread = 0,
  blend = "normal",
  color = NULL
)

echo(
  x = 4,
  y = 0,
  n = 3L,
  alpha = 0.45,
  decay = 1,
  spread = 0,
  blend = "normal",
  color = NULL
)

Arguments

x, y

Trail direction and length in millimetres (+x right, +y up): the offset of the furthest copy. Defaults to a rightward streak.

n

Number of trail copies.

alpha

Opacity of the nearest (strongest) copy; copies fade toward the tail.

decay

Fade exponent shaping the opacity ramp along the trail (higher = faster fade toward the tail; 0 = no fade).

spread

Optional widening, in millimetres, applied progressively toward the tail (0 keeps a constant width).

blend

Blend mode compositing the trail copies (any CSS mix-blend-mode name); "normal" for opaque ghosts.

color

Trail colour, or NULL (default) to inherit the mark's own resolved colour.

Value

A MotionSpec object for a mark's effects list.

Details

The direction is an absolute distance in millimetres (+x right, +y up), resolved device-side via vellum's compound npc + mm unit, so the trail keeps the same physical length and stays isotropic regardless of the panel's size or aspect (as shadow()'s offset does).

See also

glow(), shadow(), outline()

Examples

df <- data.frame(x = 1:20, y = cumsum(rnorm(20)))
vplot(df) |> mark_line(x = x, y = y, effects = list(motion(x = 4)))


vplot(mtcars) |>
  mark_point(x = wt, y = mpg, size = 4, effects = list(echo(x = 5)))