2点を結ぶとき線分が多角形の境界をまたぐかの判定

crossing_border.R

#ref https://qiita.com/zu_rin/items/e04fdec4e3dec6072104
Judge <- function(a, b, c, d) {
  s = (a$x - b$x) * (c$y - a$y) - (a$y - b$y) * (c$x - a$x);
  t = (a$x - b$x) * (d$y - a$y) - (a$y - b$y) * (d$x - a$x);
  if (s * t > 0){
    return(FALSE)
  }
  s = (c$x - d$x) * (a$y - c$y) - (c$y - d$y) * (a$x - c$x);
  t = (c$x - d$x) * (b$y - c$y) - (c$y - d$y) * (b$x - c$x);
  if (s * t > 0){
    return(FALSE)
  }
  return(TRUE)
}

crossborder <- function(seg, df_v){
  seg = simplify2array(seg)
  a = as.list(seg[1,])
  b = as.list(seg[2,])
  logi = FALSE
  for(i in seq_len(nrow(df_v)-1L)){
    c = as.list(df_v[i,])
    d = as.list(df_v[i+1,])
    logi = logi | Judge(a,b,c,d)
    if(logi){
      return(TRUE)
    }
  }
  c = as.list(df_v[nrow(df_v),])
  d = as.list(df_v[1,])  
  logi = logi | Judge(a,b,c,d)
  return(logi)
}

df_v <- data.frame(
  x = c(1:9, 8:1),
  y = c(1, 2*(5:3), 2, -1, 17, 9, 8, 2:9)
)

plot(range(polygon$x), range(polygon$y), type = "n", xlab="x", ylab="y")
polygon(polygon$x, polygon$y, col = "orange", lty = 2, lwd = 2, border = "red")

seg1 <- locator(2)
segments(x0 = seg1$x[1], y0 = seg$y[1], x1 = seg1$x[2], y1 = seg1$y[2], lty=2)
crossborder(seg1, df_v)

seg2 <- locator(2)
segments(x0 = seg2$x[1], y0 = seg2$y[1], x1 = seg2$x[2], y1 = seg2$y[2], lty=2)
crossborder(seg2, df_v)