Using Masks
In nara: Native Raster Image Tools

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)

set.seed(1)
library(nara)

{nara} supports masked rendering of most drawing operations.

blit_mask_begin() and blit_mask_end() are used to delineate the use of a mask on an image. Within these bounds, any operation will only affect the pixels in the image corresponding to where the mask is not transparent.

Note: masks to not need to be rectangular or contiguous. Anywhere the mask is opaque is part of the valid masked area for drawing.

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Setup a drawing canvas and an image to use as a mask
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
w <- 300
h <- 300
canvas <- nr_new(w, h, fill = 'lightblue')
mask   <- nr_new(w, h, fill = 'transparent')

# Set the top-left area of the mask to be the active area by making it 
# not transparent
nr_rect(mask, x=0, y=0, w=w/2, h=h/2)
plot(mask)

# Attach the mask to the canvas
nr_mask_begin(canvas, mask)
{
  # Draw random circles
  xs <- runif(50, 0, w)
  ys <- runif(50, 0, h)
  nr_circle(canvas, xs, ys, 20, rainbow(50))
}
nr_mask_end(canvas) # Finalise the masked operation


plot(canvas)