draw_image() does not position transparent images correctly

[gauravdiwan89]

I am trying to draw a few transparent images over my ggplot. I wish to position the transparent image at the very bottom left of the plot. I used x = 0 and y = 0 to position this transparent image. However draw_image() somehow does not recognise transparency and instead positions the bottom left corner of the entire image at those co-ordinates (first plot below).

To try and improve this, I tried the image_trim function of the magick package. However, even here the left-most part (in this case the blue dice) is not quite at x = 0 (second plot below). I understand there is an hjust option, but I am running this operation for hundreds of images and cannot define the hjust in every case.

Is there a way of getting the true left most edge of a transparent image to x = 0?

Thank you

library(cowplot,quietly = T)
#> 
#> ********************************************************
#> Note: As of version 1.0.0, cowplot does not change the
#>   default ggplot2 theme anymore. To recover the previous
#>   behavior, execute:
#>   theme_set(theme_cowplot())
#> ********************************************************
library(ggplot2,quietly = T)

##basic version
ggplot(data.frame(x = 0:3, y = 0:3), aes(x, y)) +
  geom_point(size = 3) + 
  draw_image("https://upload.wikimedia.org/wikipedia/commons/4/47/PNG_transparency_demonstration_1.png",x=0,y=0)

##trimmed version
img <- magick::image_read("https://upload.wikimedia.org/wikipedia/commons/4/47/PNG_transparency_demonstration_1.png")
img <- magick::image_trim(img)

ggplot(data.frame(x = 0:3, y = 0:3), aes(x, y)) +
  geom_point(size = 3) + 
  draw_image(img,x=0,y=0)

^{Created on 2019-08-29 by the reprex package (v0.3.0)}

[clauswilke]

The problem is not the transparency, it's the aspect ratio of the image and how that interacts with the way images are placed. There is no easy fix, and a proper fix would require an entirely different placement API. Maybe look into the ggimage package and see if that better fits your needs.

[gauravdiwan89]

Thank you for the suggestion. ggimage is indeed more useful (even though a little finicky).

[clauswilke]

Let's keep the issue open. After some reflection, I think a fix is needed, and I'll look into it when I can.

[clauswilke]

This is now possible in the development version, via additional paramters halign and valign (which in most cases should have the same values as hjust and vjust).

library(ggplot2)
library(cowplot)

img <- magick::image_read("https://upload.wikimedia.org/wikipedia/commons/4/47/PNG_transparency_demonstration_1.png")
img <- magick::image_trim(img)

ggplot(data.frame(x = 0:3, y = 0:3), aes(x, y)) +
  geom_point(size = 3) + 
  draw_image(img, x=0, y=0, valign = 0, halign = 0)

^{Created on 2020-08-21 by the reprex package (v0.3.0)}