In baptiste/minixcali: Excalidraw Document Builder

library(minixcali)
library(purrr)
library(knitr)
opts_chunk$set(message = FALSE, 
               warning=FALSE,
               fig.width = 7,
               fig.height = 4, 
               out.width='50%')

Let's import a volcano in Excalidraw; since there's no support for raster graphics, we'll use the {isobands} package to create polygonal contour lines.

The R object

The {isobands} package takes a matrix of elevations and returns a number of contour lines at different heights. Adapting the documentation's example with the R volcano dataset,

library(minixcali)
library(isoband)

m <- volcano
b <- isobands((1:ncol(m))/(ncol(m)+1), (nrow(m):1)/(nrow(m)+1), m, 10*(9:19), 10*(10:20))
str(b, list.len = 3)

We have a number of polygons with (x,y) coordinates to pass to xkd_draw() elements, but note that some of these polygons are not simply connected: this will be a problem as Excalidraw will join the pieces that should be disjoint. We'll therefore break down each path into separate groups using the id attribute returned by isobands().

p <- split(data.frame(b[[2]]), b[[2]]$id)
str(p)

We now proceed to build the Excalidraw scene one subpath at a time, colouring the levels with both fill and colour.

d <- Excali_doc()

library(scales)

fills <- scales::viridis_pal(alpha = 0.2)(length(b))
cols <- scales::viridis_pal(alpha = 1)(length(b))

scale <- 500

for(ii in seq_along(b)) {
  fill <- fills[ii]
  col <- cols[ii]

  l <- b[[ii]]
  spl <- split(data.frame(x = l$x, y = -l$y), l$id)

  for (s in spl) {
    r_x <- range(s$x)
    r_y <- range(s$y)

    m <- scale * cbind(s$x - r_x[1], s$y - r_y[1])

    shape <- xkd_draw(
      x = scale * r_x[1],
      y =  scale * r_y[1],
      width = scale * diff(r_x),
      height = scale * diff(r_y),
      strokeWidth = 0.8,
      roughness = 2L,
      strokeSharpness = "sharp",
      groupIds = list("levelplot", 
                 paste0('level-', ii)),
      fillStyle = "solid",
      strokeColor = col,
      backgroundColor = fill,
      points  = m
    )

    d$add(shape)
  }
}

d$export('volcano.json')