Fetch outer dimensions to draw a bounding box from an irregular polygon grob in `R` using `grid`

Question

I am trying to draw a bounding box around different polygon shapes in R with the grid functions grobX and gorbY. The polygons are drawn using the grid primitive function polygonGrob or grid.polygon.

x <- c(0.65, 0.614906666467847, 0.42604722665004, 0.425, 0.359046106882114,
       0.259046106882114, 0.425, 0.526047226650039, 0.614906666467847)
y <- c(0.5, 0.692836282905962, 0.895442325903662, 0.759807621135332,
       0.602606042997701, 0.397393957002299, 0.240192378864668, 0.204557674096338,
       0.307163717094038)

library(grid)

pg <- polygonGrob(x=x, y=y,
                  default.units = "native",
                  gp=gpar(fill="gray"))
pntg <- pointsGrob(x=x, y=y,
                   pch = 20,
                   size = grid::unit(3, "mm"),
                   default.units = "native",
                   gp=gpar(col="red"))

rcg <- rectGrob(x=0.33, height=0.7, width=0.2,
                gp=gpar(fill="gray"))

I am able to fetch the outer dimensions with regular shapes like a rectangle. But with an irregular polygon shape, the values are off.

pushViewport(plotViewport(c(5, 4, 2, 2)))
pushViewport(dataViewport(x, y))
grid.rect()
grid.xaxis()
grid.yaxis()

grid.draw(pg)

grid.draw(pntg)

x1 <- grid::convertWidth(grid::grobX(pg, "west"), "npc", TRUE)
x2 <- grid::convertWidth(grid::grobX(pg, "east"), "npc", TRUE)
y1 <- grid::convertHeight(grid::grobY(pg, "south"), "npc", TRUE)
y2 <- grid::convertHeight(grid::grobY(pg, "north"), "npc", TRUE)


grid::grid.polyline(x = c(x1, x1, x2, x2, x1),
                    y = c(y1, y2, y2, y1, y1),
                    default.units = "npc",
                    gp = gpar(col = "blue"))

pushViewport(plotViewport(c(5, 4, 2, 2)))
pushViewport(dataViewport(x, y))
grid.rect()
grid.xaxis()
grid.yaxis()

grid.draw(rcg)

x1 <- grid::convertWidth(grid::grobX(rcg, "west"), "npc", TRUE)
x2 <- grid::convertWidth(grid::grobX(rcg, "east"), "npc", TRUE)
y1 <- grid::convertHeight(grid::grobY(rcg, "south"), "npc", TRUE)
y2 <- grid::convertHeight(grid::grobY(rcg, "north"), "npc", TRUE)

grid::grid.polyline(x = c(x1, x1, x2, x2, x1),
                    y = c(y1, y2, y2, y1, y1),
                    gp = gpar(col = "red"))

How to get the accurate outer dimensions from an irregular polygon grob ?

Answer 1

I'm not sure why you are using grobX and grobY here. You can simply get the minimum and maximum values of the co-ordinates and convert them to npc with convertX and convertY

grid.newpage()
pushViewport(plotViewport(c(5, 4, 2, 2)))
pushViewport(dataViewport(x, y))

grid.rect()
grid.xaxis()
grid.yaxis()

grid.draw(pg)

grid.draw(pntg)

x1 <- convertX(min(pg$x), "npc", TRUE)
x2 <- convertX(max(pg$x), "npc", TRUE)
y1 <- convertY(min(pg$y), "npc", TRUE)
y2 <- convertY(max(pg$y), "npc", TRUE)


grid::grid.polyline(x = c(x1, x1, x2, x2, x1),
                    y = c(y1, y2, y2, y1, y1),
                    default.units = "npc",
                    gp = gpar(col = "blue"))