Add a semi-transparent overlay to an image with {magick}

Because who has time for CSS and gradients?


Thomas Mock


June 22, 2022

I often find myself needing to apply a semi-transparent overlay in slides or reports to provide a better backdrop for text on top of a hero/splash image.

You can apply an overlay with CSS + background-image + gradients, but let’s do it natively in R with magick as that gives us more flexibility outside of a HTML environment.

Get an image

I’ll load magick and read in an example image of my coworker dog Howard.

Linking to ImageMagick
Enabled features: cairo, fontconfig, freetype, heic, lcms, pango, raw, rsvg, webp
Disabled features: fftw, ghostscript, x11

raw_img <- image_read("")

Quick overlay

You can apply a quick overlay of a single color with a simple “one liner” - but we’ll go a bit more custom in case as I’m sure I’ll want to know how someday!

image_colorize(raw_img, 37, "black") |> 

Define overlay function

The basic idea is to draw a rectangle the exact size of the image OVER the image with some transparency.

The create_overlay() function takes a raw image url/path or magick-image and then applies your chosen color to it.

create_overlay <- function(img, overlay_color = "#00000060", out_file = NULL){
  if(!("magick-image" %in% class(img))){
    raw_img <- image_read(img)
  } else if ("magick-image" %in% class(img)){
    raw_img <- img
  # get image dimensions
  img_info <- image_info(raw_img)
  # draw the raw image
  img_overlay <- image_draw(raw_img)
  # draw a rectangle of equal proportion to the raw image
  rect(0, 0, img_info$width, img_info$height, col = overlay_color, border = NA)
  # save the results
  # return it or return + save out
    image_write(img_overlay, path = out_file)
    message(paste("Image saved as", out_file))
  } else {

We can then use the function to apply some colors. Hex codes are typically 6 digits, but you can add transparency by varying the values at the end of string. So #000000 is black but #00000050 is black with 50% transparency.

scales::show_col(c("#000000", paste0("#000000", seq(90, 10, length.out = 5))))

Alternatively, you could use scales::alpha() to convert named colors like "red" to their alpha ranges.

scales::alpha("red", seq(1, 0.1, length.out = 6)) |> 

Compare the image

Now we can use our function to generate the overlay image but it’s rather large… and I’d like to see it side-by-side with the original for easier comparison. We can use magick::image_append() + magick::image_resize().

faded_img <- create_overlay(raw_img, overlay_color = scales::alpha("black", 0.4))
  format width height colorspace matte filesize density
1   JPEG  1536   2048       sRGB  TRUE        0   72x72
  image_resize(c(raw_img, faded_img), 350)

Now, let’s roll that into another function.

compare_img <- function(faded_img, img, px = 350){
    image_resize(c(img, faded_img), px)

faded_compare <- compare_img(faded_img, raw_img)

We can also play around with other colors!

compare_img(create_overlay(raw_img, scales::alpha("red", 0.15)), raw_img)

compare_img(create_overlay(raw_img, scales::alpha("yellow", 0.15)), raw_img)

compare_img(create_overlay(raw_img, scales::alpha("blue", 0.15)), raw_img)

Looks great, but we should see how our text looks as well.

add_text  <- function(img, text = "Hello there!", font_size = 50, color = "white",
                      loc = c("+175+100", "+50+305")){
  img |> 
  image_annotate(text, size = font_size, color = color,
                 location = loc[1], gravity = "center") |> 
  image_annotate(text, size = font_size, color = color,
                 location = loc[2])

faded_compare |> 

In my opinion, it’s quite a bit easier to read the text with the faded overlay!

# save it out for use in another article
create_overlay(raw_img, scales::alpha("black", 0.4)) |> 
  image_resize(500) |> 
  image_annotate("Goodbye Howard!", "center", size = 60, color = "white", 
                 location = "+0+130")

─ Session info ───────────────────────────────────────────────────────────────
 setting  value
 version  R version 4.2.0 (2022-04-22)
 os       macOS Monterey 12.4
 system   aarch64, darwin20
 ui       X11
 language (EN)
 collate  en_US.UTF-8
 ctype    en_US.UTF-8
 tz       America/Chicago
 date     2022-08-01
 pandoc   2.18 @ /Applications/ (via rmarkdown)
 quarto   1.0.37 @ /usr/local/bin/quarto

─ Packages ───────────────────────────────────────────────────────────────────
 package     * version date (UTC) lib source
 magick      * 2.7.3   2021-08-18 [1] CRAN (R 4.2.0)
 scales      * 1.2.0   2022-04-13 [1] CRAN (R 4.2.0)
 sessioninfo * 1.2.2   2021-12-06 [1] CRAN (R 4.2.0)

 [1] /Library/Frameworks/R.framework/Versions/4.2-arm64/Resources/library