Rotate raster annotation ggplot2

Question

Is there a way I can plot a raster annotation in a ggplot, rotated at a specified angle (which is not necessarily a multiple of 90 degrees)? In particular, I need the scale of the image to appear unchanged.

What I've tried so far: based on the question How to rotate an image R raster I created a function to rotate an image and save it as a temp file (seems to be the only thing I can do to store the image created by persp), with the idea of then loading it in with ggplot2::annotation_raster.

raster_rotate <- function(img, theta, width) {
  # only works with square imgs right now
  theta <- theta %% 90
  b <- cos(theta) + sin(theta)

  x1 <- 0:ncol(img)
  y1 <- 0:nrow(img)
  z <- matrix(1, nrow = length(x1), ncol = length(y1))
  col_mat <- t(apply(matrix(rgb(getValues(img)/255), nrow=nrow(img), byrow=TRUE), 2, rev))

  tmppath = tempfile(pattern = "img", fileext = ".png", tmpdir = "tmp/imgrot")
  side = round(b * width)
  png(filename = tmppath, width = side, height = side)
  persp(x1, y1, z, zlim = c(0, 2), theta = theta, phi = 90,
        col = col_mat, scale = FALSE, border = NA, box = FALSE)
  dev.off()
}

Because rotating the image inside the same sized canvas will appear to shrink the image as angles approach 45 degrees, I have to rescale the canvas size by a factor of cos(theta) + sin(theta) as I rotate. However, when I add this scaling to the png function I get an error: Error in png(filename = tmppath, width = side, height = side) : invalid 'width' or 'height'

I would accept a solution to this error to help me fix my messy hack, but if there's a cleaner way to do this directly into ggplot that would be even better.

Answer 1

Here is how I rotate maps, maybe tweaking it a bit will address your issue?

library(tidyverse) 
rotate.axis <- function(xy,theta){
       pimult <- (theta * 2 * pi) / 360
       newx <- c(cos(pimult), sin(pimult))
       newy <- c(-sin(pimult), cos(pimult))
       XY <- as.matrix(xy) %*% cbind(newx, newy)
       as.data.frame(XY)
}

ak <- map_data('world','USA:Alaska')

newd <- data.frame(longitude=ak$long, latitude=ak$lat)
rotate <- rotate.axis(newd,30)
newak <- bind_cols(ak,rotate)

No rotation of object

newak %>% 
  filter(long<0) %>% 
  ggplot() + geom_polygon(aes(long,lat,group=group),fill=8,color="black")

Rotated object

newak %>% 
  filter(long<0) %>% 
  ggplot() + geom_polygon(aes(newx,newy,group=group),fill=8,color="black")