R/ggplot2：添加带有透明度信息的 png

Question

I want to stack PNG images on a ggplot2 plot. One of the pngs has alpha/transparency info, but it gets ignored.

How can I add the second image (image1.png) on top of bg.png, so that transparency gets preserved and only the black text is visible on the yellow background?

Example

library(ggplot2)
library(png)
library(patchwork)

img1 <- readPNG("bg.png", native = TRUE)
img2 <- readPNG("image1.png", native = TRUE)

ggp <- ggplot(data.frame()) +
  geom_point() + 
  theme_nothing() +
  inset_element(p = img1,
                left = 0,
                bottom = 0,
                right = 1,
                top = 1,
                align_to = "full") +
  inset_element(p = img2,
                left = 0,
                bottom = 0,
                right = 0.5,
                top = 0.5,
                align_to = "full")

ggp

Example files:

• bg.png: https://cloud.linuxfabrik.io/index.php/s/3iBTARTtgWBnL5c
• image1.png: https://cloud.linuxfabrik.io/index.php/s/G5P958XpJeEY7No

Answer 1

I think that the alpha channel is being respected here. It's just that inset_element draws a white canvas element first. What you are trying to do arguably isn't an inset, more of an annotation.

I think using annotation_custom from ggplot2 is more appropriate here (and means one less package to load).

library(ggplot2)
library(png)

img1 <- grid::rasterGrob(readPNG("bg.png"))
img2 <- grid::rasterGrob(readPNG("image1.png"))

ggp <- ggplot(data.frame()) +
  geom_point() + 
  theme_void() +
  annotation_custom(img1) +
  annotation_custom(img2, xmax = 0.5, ymax = 0.5)

ggp