colorize_tiff.R

# Takes a grayscale tiff (with alpha) and converts all dark points (by threshold) to the new color
colorize_tiff <- function(input, threshold, newcolor) {
  # Read original tiff
  orig_tiff <- tiff::readTIFF(input)
  # Convert grayscale to black and white
  orig_tiff[, , 1] <- ifelse(orig_tiff[, , 1] > threshold, 1, 0)
  
  # Create a colorized tiff
  new_tiff <- array(0, dim = c(dim(orig_tiff)[1:2], 4))
  # Set RGB
  new_tiff[, , 1] <- ifelse(orig_tiff[, , 1] == 0, newcolor[1], 1)
  new_tiff[, , 2] <- ifelse(orig_tiff[, , 1] == 0, newcolor[2], 1)
  new_tiff[, , 3] <- ifelse(orig_tiff[, , 1] == 0, newcolor[3], 1)
  # Recover alpha
  new_tiff[, , 4] <- orig_tiff[, , 2]
  
  new_tiff
}

# Plotting to sanity check
plot_tiff <- function(tiff) {
  plot(0:1, 0:1, type = "n", xlab = "", ylab = "")
  rasterImage(as.raster(tiff), 0, 0, 1, 1)
}

# Look up Blue_Transparent.tif
bm_path <- file.choose()
# Convert blue whale to blue
bm_blue <- colorize_tiff(
  bm_path,
  0.4,
  c(48, 112, 173) / 255
)
# Check it out
plot_tiff(bm_blue)
# Can save using tiff::writeTIFF