给指定区域着色? [英] Color a designated area?

问题描述

我想知道是否有一种方法可以只为我使用 grid.circle 创建的圆的一半着色.

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))

我想让上半部变成蓝色，下半部留白.

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

感谢您的帮助！

推荐答案

使用 grid.polygon() 和一些基本的三角函数，你可以定义一个函数来做这个

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

需要小心谨慎，以便在视口非方形时填充的半圆不会扭曲.为了以匹配 grid.circle() 使用的规则的方式完成此操作，我将原点设置为 "npc" 单位，圆半径设置为 <代码>snpc"个单位.(关于 "npc" 和 "snpc" 的更多含义，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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