R:将Alpha值添加到png图像 [英] R: add alpha-value to png-image
问题描述
是否有一种方法可以使rasterGrob对象部分透明,从而为其添加alpha因子?我在ggplot2
图中使用徽标作为水印,方法是通过annotation_custom
插入png图像(作为rasterGrob).但是,与annotate
不同的是,alpha
选项在这里不起作用,因此我认为必须提前更改图像.
Is there a way to make a rasterGrob-object partly transparent, so to add an alpha-factor to it? I'm using a logo as a watermark within a ggplot2
plot by inserting a png-image (as rasterGrob) by annotation_custom
. However, unlike with annotate
, the alpha
option does not work here, so I guess the image has to be changed in advance.
作为一个简单的示例,该示例基于baptiste在他的博客中的建议,到目前为止,我是这样做的:
As a simple example based on what baptiste suggests in his blog, so far I'm doing it this way:
img.path <- readPNG("logo.png")
pngob <- rasterGrob(img.path)
qplot(1:10, rnorm(10), geom = "blank") +
annotation_custom(pngob, xmin=6.8, xmax=Inf, ymin=1, ymax=Inf) +
geom_point()
上面的例子很好用.
但是,在控制台中键入dim(pngob)
会返回NULL
.因此,以下有关如何设置alpha
值的建议不起作用:
However, typing dim(pngob)
into the console returns NULL
. Thus, the suggestion below on how to set an alpha
-value does not work:
m <- pngob
w <- matrix(rgb(m[,,1],m[,,2],m[,,3], m[,,4] * 0.2), nrow=dim(m)[1])
这将返回错误Error in m[, , 1]: wrong number of dimensions
推荐答案
Straight from the ggplot2 blog by @baptiste. You can adjust alpha when you create w
.
library(png)
library(gridExtra)
m <- readPNG(system.file("img", "Rlogo.png", package="png"), FALSE)
w <- matrix(rgb(m[,,1],m[,,2],m[,,3], m[,,4] * 0.2), nrow=dim(m)[1]) #0.2 is alpha
qplot(1:10, rnorm(10), geom = "blank") +
annotation_custom(xmin=-Inf, ymin=-Inf, xmax=Inf, ymax=Inf,
rpatternGrob(motif=w, motif.width = unit(1, "cm"))) +
geom_point()
或者如果您只想拥有一张图片:
Or if you want to have a single image:
qplot(1:10, rnorm(10), geom = "blank") +
annotation_custom(xmin=-Inf, ymin=-Inf, xmax=Inf, ymax=Inf,
rasterGrob(w)) +
geom_point()
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