raster_alpha: Combine a Matrix of Colors and a Matrix of Alpha Values
In plothelper: New Plots Based on 'ggplot2' and Functions to Create Regular Shapes

The function is a wrapper of scales::alpha. While the latter only works on vectors, the former can combine a matrix of colors and a matrix of alpha values as long as the two have the same numbers of rows and columns.

1	raster_alpha(color, alpha, result = "raster", res = 144)

`color`	a matrix of colors, a raster or an image read into R by `magick::image_read`.
`alpha`	either a single value (e.g., 0.4) or a matrix of alpha values. The matrix should have the same numbers of rows and columns as `color`.
`result`	if it is "raster", the result will be a matrix which can be used by `annotation_raster` (default), if it is "magick", the result is a magick image.
`res`	the `res` parameter used by `magick::image_graph` when `result` is "magick". Default is 144.

# A color matrix
co=c("red", "yellow", "green", "blue")
co=rbind(co, co, co)
# An alpha matrix
alp=c(1, 0.6, 0.3, 0.1)
alp=rbind(alp, alp, alp)
# Now combine the two
result=raster_alpha(co, alp)