Is it possible to plot images in a ggplot2 plot, that don't get distorted when you save to any non-standard aspect ratio? - r

Question

I'm looking for any solution to this problem, regardless of packages used.
The problem at hand is that plotted images get distorted when you save them using ggsave. Let me give an example:
image_links = data.frame(id = c(1,2,3,4,5),
image = c("https://cdn.shopify.com/s/files/1/1061/1924/products/Smiling_Emoji_with_Eyes_Opened_large.png",
"https://cdn.shopify.com/s/files/1/1061/1924/products/Smiling_Emoji_with_Smiling_Eyes_large.png",
"https://cdn.shopify.com/s/files/1/1061/1924/products/Hushed_Face_Emoji_large.png",
"https://cdn.shopify.com/s/files/1/1061/1924/products/Disappointed_but_Relieved_Face_Emoji_large.png",
"https://cdn.shopify.com/s/files/1/1061/1924/products/Expressionless_Face_Emoji_large.png"))
mydata = data.frame(x = rnorm(100, mean = 50, sd = 20),
y = rnorm(100, mean = 50, sd = 5),
id = rep(c(1,2,3,4,5), 20))
mydata$y = mydata$y - 10*mydata$id
mydata = mydata %>% left_join(image_links, by='id')
g <- ggplot(mydata) + geom_image(aes(x=x, y=y, image=image), size=0.05)
ggsave(g, filename='[INSERT PATH HERE].png', width=width, height=height, dpi=300)
This works fine:
The problem arises when you adjust the width and height parameters of ggsave, for instance because you want the x and y-axis to be in the correct proportion:
width = (max(mydata$x) - min(mydata$x))/10
height = (max(mydata$y) - min(mydata$y))/10
ggsave(g, filename='[INSERT PATH HERE].png', width = width, height=height, dpi=300)
The x and y-axis are now fine, but the images are distorted:
This happens in ANY situation where you plot an image but the width/height aspect ratio is different than what was the original aspect ratio of the image you want to add.
I'm looking for any solution to this problem, not necessarily restricted to ggimage. It seems very weird to me that you can't properly add images to a ggplot, as I image it's very common for people to want to do that.

Answer 1

I don't know a lot about ggsave, but this seems like an issue related to relative versus absolute units. Probably the geom_image() calculates positions relative to the axes, which get distorted when the axes get resized (such as within ggsave). For example:
ggplot(mydata) + geom_image(aes(x=x, y=y, image=image), size=0.05)
Can look like:
Or can look like:
Depending on the device window that I can resize at will.
There are two ways I can see this getting fixed, both of which will involve re-calculating the sizes of the rasters at drawtime. The easier fix will be the one below.
# Get plot
g <- ggplot(mydata) + geom_image(aes(x=x, y=y, image=image), size=0.05)
# Convert to gtable
gt <- ggplotGrob(g)
# Get the imagegrobs, correct slots found by trial and error
imagegrobs <- gt$grobs[[6]]$children[[3]]$children
# Re-class them to a custom, made-up class
imagegrobs <- lapply(imagegrobs, function(image) {
class(image) <- c("fixasp_raster", class(image))
image
})
# Put them back into the gtable
gt$grobs[[6]]$children[[3]]$children <- imagegrobs
So now that we have a custom class for these images, we can write a piece of code that gets executed at the time of drawing by writing a method for our class using the S3 generic makeContent from the grid package.
library(grid)
makeContent.fixasp_raster <- function(x) {
# Convert from relative units to absolute units
h <- convertHeight(x$height, "cm", valueOnly = TRUE)
w <- convertWidth(x$width, "cm", valueOnly = TRUE)
# Decide how the units should be equal
x$height <- x$width <- unit(sqrt(h * w), "cm")
x
}
Note that taking the square root of the product is improvised, I don't know if this is the optimal procedure.
When we plot the data now, we'll have a consistent size of the images, regardless of the aspect ratio:
grid.newpage(); grid.draw(gt)
The second way to fix this is to file an issue in the github page of the ggimage package, motivating your use case and convice them to implement something that adresses your concerns. If they want, they could make a fix at the ggproto level, so that you don't have dabble with gtables.

Answer 2

#teunbrand's answer has been implemented in dev version 0.2.4 of ggimage. You can install the latest dev version like this:
setRepositories(ind=1:2)
## install.packages("devtools")
devtools::install_github("GuangchuangYu/ggimage")
This should fix the aspect ratio issues.