如何旋转图像 R 光栅 [英] How to rotate an image R raster

问题描述

我有下面的代码，可以将图像保存到我的电脑.我想将该图像围绕其中心(或左下角)旋转 45,90 和 135 度，然后另存为 3 个不同的图像.我怎么能这样做?

I have code below which saves an image to my pc. I would like to rotate that image by 45,90 and 135 degrees around its center (or bottom left hand corner) and then save as 3 different images. How could I do that?

library(raster)
r1 <- brick(system.file("external/rlogo.grd", package="raster"))
x <- crop(r1, extent(0,50,0,50))
plotRGB(x)
png(width=50, height=50)
par(mai=c(0,0,0,0))
image(x)
dev.off()

---------update1-------------

---------update1-------------------------

根据接受的答案，工作代码如下

Based upon the accepted answer the working code is as below

library(raster)
r1 <- brick(system.file("external/rlogo.grd", package="raster"))
r1
x <- crop(r1, extent(0,ncol(r1),0,nrow(r1)))
plotRGB(x)

x1 <- 0:ncol(x)
y1 <- 0:nrow(x)
z <- matrix(1, nrow=length(x1), ncol=length(y1))

col.mat <- t(apply(matrix(rgb(getValues(x)/255), nrow=nrow(x), byrow=TRUE), 2, rev))

# Rotate 45 degrees
persp(x1, y1, z, zlim=c(0,2), theta = 20, phi = 90, 
      col = col.mat, scale=FALSE, border=NA, box=FALSE)
png("SaveThisPlot.png")
persp(x1, y1, z, zlim=c(0,2), theta = 20, phi = 90, 
      col = col.mat, scale=FALSE, border=NA, box=FALSE)
dev.off()

推荐答案

对于 90 度旋转，这是一个简单的解决方案:

For the 90 degree rotations, this is an easy solution:

image(t(flip(x, 1)))
image(t(flip(x, 2)))
plotRGB(t(flip(x, 1)))
plotRGB(t(flip(x, 2)))

对于 45 度和 135 度旋转，会有点棘手.可能还有其他方法，但我将使用 persp 函数并为 theta 参数提供不同的角度.

For the 45 degree and 135 degree rotation, it will be a bit trickier. There are probably other ways, but I'll use the persp function and give different angles to the theta argument.

只需正确设置对 persp 的调用即可.x1、y1 和 z 只是 persp 函数的输入(参见 ?persp 以了解有关该函数的参数的更多信息).col.mat 是一个保存颜色值的矩阵.

It's just a matter of setting up the call to persp properly. x1, y1, and z are just inputs for the persp function (see ?persp for more about the arguments to that function). col.mat is a matrix holding color values.

x1 <- 0:ncol(x)
y1 <- 0:nrow(x)
z <- matrix(1, nrow=length(x1), ncol=length(y1))
col.mat <- t(apply(matrix(rgb(getValues(x)/255), nrow=nrow(x), byrow=TRUE), 2, rev))
# the transposing and reversing are just to get the colors in the same 
# spots as they're in when viewing plotRGB(x).
#
# getValues(x) is how you get the rgb colors, in the form of a 3-column matrix.
# rgb(getValues(x)/255) converts them into hex code, which is convenient enough here.

如果您发现这是您正在寻找的镜像，请尝试以不同的方式填充颜色矩阵.例如:

If you find that this is the mirror image of what you're looking for, try filling up the color matrix differently. For example:

col.mat <- matrix(rgb(getValues(x)/255), nrow=nrow(x))

如您所知，正确填充颜色矩阵是使这种方法适合您的关键.我将把它作为练习留给读者，以了解如何对颜色矩阵进行任何其他操作.

As you can tell, filling up the color matrix properly is the key to making this approach work for you. I'll leave it as an exercise to the reader to figure out how to do any other manipulations to the color matrix.

现在，调用 persp.这里我设置了zlim的值，所以有一个范围包括1.因为我把所有的z值都设置为1，所以你需要设置一个有效的范围，否则，persp 将抛出有关无效限制的错误.它不喜欢从 1 到 1 的范围.

Now, call persp. Here, I set the zlim values so there is a range including 1. Because I made all the z values 1, you need to set a valid range, otherwise, persp will throw an error about invalid limits. It doesn't like a range from 1 to 1.

# Rotate 45 degrees
persp(x1, y1, z, zlim=c(0,2), theta = 45, phi = 90, 
    col = col.mat, scale=FALSE, border=NA, box=FALSE)

这里是 135 度:

# Rotate 135 degrees
persp(x1, y1, z, zlim=c(0,2), theta = 135, phi = 90, 
    col = col.mat, scale=FALSE, border=NA, box=FALSE)

可以按照您在问题中显示的相同方式保存图:

Saving the plots can be done in the same way you show in your question:

png("SaveThisPlot.png")
persp(x1, y1, z, zlim=c(0,2), theta = 135, phi = 90, 
    col = col.mat, scale=FALSE, border=NA, box=FALSE)
dev.off()

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