[R] transforming output of grid.locator() to coordinates of a leaf viewport

Wittner, Ben, Ph.D. Wittner.Ben at mgh.harvard.edu
Sun Jun 1 22:56:13 CEST 2008

Hadley, In my application the leaf viewports will not overlap, but I guess that
would not necessarily be the case for all applications. In any event, do you
happen to know how to transform coordinates in the top-level viewport to
coordinates in another viewport in the tree? Thanks. -Ben


From: hadley wickham [mailto:h.wickham at gmail.com]
Sent: Sun 6/1/2008 3:24 PM
To: Wittner, Ben, Ph.D.
Cc: r-help at stat.math.ethz.ch
Subject: Re: [R] transforming output of grid.locator() to coordinates of a leaf


What if the click occurs over multiple viewports?


On Mon, Jun 2, 2008 at 6:50 AM, Wittner, Ben, Ph.D.
<Wittner.Ben at mgh.harvard.edu> wrote:
> Short form:
>                How do I transform the output of grid.locator() (or
> grid.locator(unit='npc')) to the native (or npc) coordinates of a viewport
> than the top-level viewport?
>                Thanks in advance. -Ben
> Long form:
>                I would like the user to be able to click anywhere on the R
> graphics window. I would then like to figure out which leaf viewport the user
> clicked in and compute the native coordinates in that leaf viewport of the
> user's click.
>                I saw a 2005 response by Paul Murrell to a post by Gabor
> Grothendieck roughly on this topic in which Paul suggested looking at
> trellis.focus(). I've looked at trellis.focus() and trellis.clickFocus(), but
> they don't appear to do what I want in that a) it would appear that my user
> would have to click twice, once to specify which leaf viewport and then again
> specify which point in that viewport and b) trellis.clickFocus() appears to
> lots of information from the trellis package, which I'm not using.
>                So I decided to try to figure out the native coordinates of the
> click in each leaf viewport, to a) see whether the click is within the
> region for that viewport and b) use those coordinates to take the appropriate
> action if it is. But I don't seem to be able to figure out how to translate
> the top-level viewport coordinates to those of a leaf viewport. I tried
> interpreting the 3X3 matrix returned by current.transform() as an affine
> transformation in the x,y plane in homogeneous coordinates, but could make no
> sense of what I was getting.
>                Thanks for any help. -Ben
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