[R-gui] ggraphics and retrieving mouse coordinate as npc coordinates

Mark Heckmann <mark.heckmann <at> gmx.de> writes:

Hi all,

i tried to retrieve the mouse coordinates from a ggraphics widget  
using a handler. With base graphics system it works fine, but using  
grid, the returned values do not make sense to me. How can I transform  
the returned values to e.g. npc coordinates?

	library(RGtk2)
	library(gWidgets)

	w = gwindow("ggraphics example")
	g = ggraphics(cont=w, expand=T)
	size(g) <- c(900,900)
	Sys.sleep(1)
	print(grid.rect())

	addHandlerClicked(g, handler = function(h,...) {
   		x <- h$x; y <- h$y
		print(c("pressed at:", c(x,y)))
	})	

There are two ways. The first is to mimic the handler call in gWidgetsRGtk2
skipping the call to pltToUsr:


f <- function(h,...) {
  theArgs <- list(...)
  w <- theArgs[[1]]
  e <- theArgs[[2]]
  allocation <- w$GetAllocation()
  xclick <- e$GetX()
  yclick <- e$GetY()
  x <- xclick/allocation$width
  y <- 1 - (allocation$height - yclick)/allocation$height
  print(c(x,y))
}

addHandler(g, signal="button-press-event", handler =f)

(Changing print(c(x,y)) as needed.)

The other is to invert this function which is what converts the (x,1-y) value
above into the output of the handler:

pltToUsr <- function(x,y) {
      plt <- par("plt"); usr = par("usr")
      c( (usr[2]-usr[1])/(plt[2]-plt[1])*(x - plt[1]) + usr[1],
         (usr[4] - usr[3])/(plt[4] - plt[3])*(y - plt[3]) + usr[3]
       )
}



Hope that helps!

Thanks and merry X-mas
Mark

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Mark Heckmann
Dipl. Wirt.-Ing. cand. Psych.
Vorstra?e 93 B01
28359 Bremen
Blog: www.markheckmann.de
R-Blog: http://ryouready.wordpress.com