r - 展平ggplot2中的alpha通道

Question

ggplot2 的一些对象（函数）支持 alpha 通道。

http://docs.ggplot2.org/current/geom_point.html

这是一个不错的功能，但 pdf 和 png 文件中的透明颜色通常是各种麻烦的来源。

如何使用 alpha 选项并获得扁平化的 pdf 输出文件？

Answer 1

I had ongoing problems with a large Latex doc created with knitr + ggplot that was perfectly readable on my mac but was unopenable for Windows users. After numerous attempts at optimizing and flattening the pdf, I finally isolated the issue to a single alpha-heavy plot. In my case, switching from pdf to png output made the difference (though, based on your question, it sounds like pngs may not solve your problem).

If you are using knitr, you can automatically create a png version by setting the global (or chunk-specific) image output option dev to "png" instead of the default of "pdf". Depending on the plot, this can shrink the size & complexity of an alpha-heavy ggplot significantly. I've had a 10x reduction in plot file size from ~700KB to ~70KB (some simpler plots may actually increase in size, though).

The trade-off is there is likely to be some loss of resolution for the plots. For circulating a draft by email or for certain types of plots this may be fine. If the loss of resolution is too great, consider adjusting the dpi of the output (though, of course, this will increase the file size but may still work better in cross-platform settings).

To set the global image output to "png" you could use code like this:

    library(knitr)
    opts_chunk$set(dev="png", dpi=200)

To set the output to "png" within a specific chunk (e.g., just the plot that is alpha-heavy), use the dev="png" option. The example below generates a regular, non-alpha heavy plot. With knitr + Latex, the include=FALSE option will prevent a pdf version from automatically being included (I'm not sure if this is necessary for RMarkdown).

    <<myplot, dev="png", dpi=200, include=FALSE>> 
    library(ggplot2)
    x <- 1:1000
    y <- 2*x + rnorm(1000, 0, 100)
    df <- data.frame(x,y)  
    ggplot(df, aes(x=x, y=y)) + geom_point(alpha=.3)
    @

The above code will generate a file called myplot.png which, in Latex, can then easily be included in the document with an includegraphics command nested in a figure environment.

    \begin{figure}[h!]
    \centering
    \scalebox{.5}{\includegraphics{myplot.png}}
    \caption{Some caption \label{fig:myplot}} 
    \end{figure}

Another resource that might be helpful is this blog post hosted at R Bloggers:

Fast-track publishing using knitr: exporting images for sharing and press

Also, see the Plots section of knitr options.

Answer 2

为了避免在 LaTex 中出现类似图形的问题，我使用 R 生成 png 文件。在那里你可以使用该type = c("cairo", "cairo-png", ...)选项，对我来说，“windows”在那里工作得很好。之后，我使用 imagemagick 将其转换为 jpg 并将此 jpg 包含在我的 LaTeX 文件中，pdfLaTeX 结果然后是扁平化的 pdf 输出。