为指定区域着色? [英] Color a designated area?

问题描述

我试图找出是否有一种方法可以仅对使用grid.circle创建的半圆进行着色.

I am trying to figure out if there is a way to color only half of a circle that I created using grid.circle.

    library(grid)
    grid.circle(x=.5, y=.5, r=.25,gp=gpar(lwd=10))

我想将上半部分设为蓝色，而下半部分保留为白色.

I want to make the top half blue, and the bottom half leave white.

谢谢您的帮助！

推荐答案

使用grid.polygon()和一些基本的三角函数，您可以定义一个可以执行此操作的函数

Using grid.polygon() and some basic trigonometry, you can define a function that'll do this

需要采取一些谨慎的措施，以确保当视口为非正方形时，填充的半圆不会变形.为了以与grid.circle()所使用的规则相匹配的方式完成此操作，我以"npc"单位设置了原点，并以"snpc"单位设置了圆半径. (有关"npc"和"snpc"的含义的更多信息，请参见?unit和vignette("grid")):

Some picky care needs to be taken so that the filled semicircle is not distorted when the viewport is non-square. To accomplish this in a way that matches the rules used by grid.circle(), I've set the origin in "npc" units, and the circle radius in "snpc" units. (For more on the meanings of "npc" and "snpc", see ?unit and vignette("grid")):

library(grid)

filledSemiCircle <- function(x_origin, y_origin, radius, fillcolor, top=TRUE) {
    theta <- seq(0, pi, length = 100)
    if(!top) theta <- theta + pi     ## To fill the bottom instead
    x <- unit(x_origin, "npc") + unit(cos(theta) * radius, "snpc")
    y <- unit(y_origin, "npc") + unit(sin(theta) * radius, "snpc")
    grid.polygon(x, y, gp = gpar(fill = fillcolor))
}

filledSemiCircle(0.5, 0.5, 0.25, "dodgerblue")
filledSemiCircle(0.5, 0.5, 0.25, "gold", top=FALSE)
grid.circle(x = .5, y=.5, r=.25,gp=gpar(lwd=10))

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