我有一个从互联网上传的形状。基于其计算出的质心，我想从中绘制一条 50 度线并找到他与轮廓相交的坐标。知道我该怎么做吗？
谢谢。
脚本：
library(ggplot2)
df = read.table("E:/cloud1.txt") #stored at https://ufile.io/zq679
colnames(df)<- c("x","y")
meanX <- mean(df$x)
meanY <- mean(df$y)
ggplot(df, aes(x, y))+ geom_point()+ geom_point(aes(meanX, meanY),colour="green",size=2)
这是一个解决方案sf：
df <- read.table("~/Bureau/cloud1.txt") #stored at https://ufile.io/zq679
colnames(df) <- c("x","y")
meanX <- mean(df$x)
meanY <- mean(df$y)
# Transform your data.frame in a sf polygon (the first and last points
# must have the same coordinates)
library(sf)
poly <- st_sf(st_sfc(st_polygon(list(as.matrix(df)))))
# Choose the angle (in degrees)
angle <- 50
# Find the minimum length for the line segment to be always
# outside the cloud whatever the choosen angle
maxX <- max(abs(abs(df[,"x"]) - abs(meanX)))
maxY <- max(abs(abs(df[,"y"]) - abs(meanY)))
line_length = sqrt(maxX^2 + maxY^2) + 1
# Find the coordinates of the 2 points to draw a line with
# the intended angle.
# This is the gray line on the graph below
line <- rbind(c(meanX,meanY),
c(meanX + line_length * cos((pi/180)*angle),
meanY + line_length * sin((pi/180)*angle)))
# Transform into a sf line object
line <- st_sf(st_sfc(st_linestring(line)))
# Intersect the polygon and line. The result is a two points line
# shown in black on the plot below
intersect_line <- st_intersection(poly, line)
# Extract only the second point of this line.
# This is the intersecting point
intersect_point <- st_coordinates(intersect_line)[2,c("X","Y")]
# Visualise this with ggplot
# You might need to install the latest github version :
# devtools::install_github("tidyverse/ggplot2")
library(ggplot2)
ggplot() + geom_sf(data=poly, fill = NA) +
geom_sf(data=line, color = "gray80", lwd = 3) +
geom_sf(data=intersect_line, color = "gray20", lwd = 1) +
geom_point(aes(meanX, meanY), colour="orangered", size=2) +
geom_point(aes(intersect_point["X"], intersect_point["Y"]),
colour="orangered", size=2) +
theme_bw()
编辑 2018-07-03
原始问题的数据集现已消失。
这是一个完全可重现的示例，带有一个心形，并且没有使用 geom_sf 作为图形来回答评论中的问题。
# Generate a heart shape
t <- seq(0, 2*pi, by=0.1)
df <- data.frame(x = 16*sin(t)^3,
y = 13*cos(t)-5*cos(2*t)-2*cos(3*t)-cos(4*t))
df <- rbind(df, df[1,]) # close the polygon
meanX <- mean(df$x)
meanY <- mean(df$y)
# Transform your data.frame in a sf polygon (the first and last points
# must have the same coordinates)
library(sf)
#> Linking to GEOS 3.5.1, GDAL 2.1.3, proj.4 4.9.2
poly <- st_sf(st_sfc(st_polygon(list(as.matrix(df)))))
# Choose the angle (in degrees)
angle <- 50
# Find the minimum length for the line segment to be always
# outside the cloud whatever the choosen angle
maxX <- max(abs(abs(df[,"x"]) - abs(meanX)))
maxY <- max(abs(abs(df[,"y"]) - abs(meanY)))
line_length = sqrt(maxX^2 + maxY^2) + 1
# Find the coordinates of the 2 points to draw a line with
# the intended angle.
# This is the gray line on the graph below
line <- rbind(c(meanX,meanY),
c(meanX + line_length * cos((pi/180)*angle),
meanY + line_length * sin((pi/180)*angle)))
# Transform into a sf line object
line <- st_sf(st_sfc(st_linestring(line)))
# Intersect the polygon and line. The result is a two points line
# shown in black on the plot below
intersect_line <- st_intersection(poly, line)
# Extract only the second point of this line.
# This is the intersecting point
intersect_point <- st_coordinates(intersect_line)[2,c("X","Y")]
# Visualise this with ggplot and without geom_sf
# you need first transform back the lines into data.frame
line <- as.data.frame(st_coordinates(line))[,1:2]
intersect_line <- as.data.frame(st_coordinates(intersect_line))[,1:2]
library(ggplot2)
ggplot() + geom_path(data=df, aes(x = x, y = y)) +
geom_line(data=line, aes(x = X, y = Y), color = "gray80", lwd = 3) +
geom_line(data=intersect_line, aes(x = X, y = Y), color = "gray20", lwd = 1) +
geom_point(aes(meanX, meanY), colour="orangered", size=2) +
geom_point(aes(intersect_point["X"], intersect_point["Y"]),
colour="orangered", size=2) +
theme_bw()
由reprex 包（v0.2.0）于 2018 年 7 月 3 日创建。