Cowplot中的draw_image()函数导致pdf模糊 [英] The draw_image() function from cowplot results in blurred pdfs

问题描述

读取矢量化的pdf有时会导致来自Cowplot的 draw_image()函数严重模糊的图像:

Reading a vectorized pdf sometimes result in a badly blurred image with the draw_image() function from cowplot:

library(ggplot2)
library(cowplot)
library(magick)

# make pdf input as example
p <- ggplot(iris, aes(Sepal.Length, Sepal.Width, shape = Species)) + 
  geom_point() + scale_shape_manual(values = 21:23) + theme_classic()
ggsave("input.pdf", p, width = 6, height = 4.2)

# now draw with draw_image() and then write as png
fig <- ggdraw() + draw_image("input.pdf")
ggsave("output.png", fig, width = 1, height = .7, dpi = 1200) # blurred image

但是，阅读SVG可以正常工作:

However, reading SVGs works fine:

fig <- ggdraw() +
  draw_image("http://jeroen.github.io/images/tiger.svg")
ggsave("output.png", fig, width = 1, height = .7, dpi = 1200)

此外，使用:

magick::image_read_pdf("input.pdf")

导致非模糊输出.

推荐答案

我不完全确定为什么SVG和pdf的处理方式不同，或者用 magick :: image_read()(这是 draw_image()在内部使用的方法)，但是一种解决方案是在 draw_image()内使用 magick :: image_read_pdf() .函数 magick :: image_read_pdf()将pdf转换为光栅图像，我们可以使用 density 参数指定所需的分辨率:

I'm not entirely sure why SVGs and pdfs are handled differently, or what exactly happens when you read pdfs with magick::image_read() (which is what draw_image() uses internally), but one solution is to use magick::image_read_pdf() inside of draw_image(). The function magick::image_read_pdf() converts the pdf into a raster image, and we can specify the resolution we want with the density argument:

fig <- ggdraw() + draw_image(magick::image_read_pdf("input.pdf", density = 600))
ggsave("output.png", fig, width = 1, height = .7, dpi = 1200)

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