R help - Sep 2003 - using identify() together with plot () and pixmap()

Dear R users
I have a two-dimensional array, whose values I want to plot, using the
pixmapGrey class. Plotting works fine, and now I would like to be able
to identify some of the points in the plot using identify(). But I get
the following message while pressing the left mouse button:
> plot(pixmapGrey(fmri.vtc[,,slice,volume]))
> identify(fmri.vtc[,,slice,volume])
warning: no point with 0.25 inches


pressing the right mouse button I get:
numeric(0)

what is the problem  here and how can I solve it?

many thanks for your help

Cheers! 

Christoph


-- 
Christoph Lehmann <christoph.lehmann at gmx.ch>

Christoph Lehmann wrote:
>>plot(pixmapGrey(fmri.vtc[,,slice,volume]))
>>identify(fmri.vtc[,,slice,volume])
> 
> warning: no point with 0.25 inches
> 
> 
> pressing the right mouse button I get:
> numeric(0)
> 
> what is the problem  here and how can I solve it?
  The problem is that there is no method for identifying points on a 
pixmap (and anyway, you are feeding identify() a matrix).

  You'll need to write an "identify.pixmapGrey" function. This
would do
something like use the 'locator()' function to get clicks on the screen,
and then it would compute which grid cell the coordinates of those 
clicks were in. pixmap objects store enough information for you to work 
this out from the attributes - look at the 'bbox' attribute for example.

  It would make a nice addition to the pixmap library. Although one 
thing that just suprised me was that the different flavours of pixmap - 
pixmapGrey, pixmapIndexed, pixmapRGB - dont all inherit from a 'pixmap' 
class, which might make it easier to write a single identify function 
for all the types.

Baz

>>>>> On Tue, 30 Sep 2003 10:16:11 +0200,
>>>>> Christoph Lehmann (CL) wrote:
  > Dear R users
  > I have a two-dimensional array, whose values I want to plot, using the
  > pixmapGrey class. Plotting works fine, and now I would like to be able
  > to identify some of the points in the plot using identify(). But I get
  > the following message while pressing the left mouse button:

  >> plot(pixmapGrey(fmri.vtc[,,slice,volume]))
  >> identify(fmri.vtc[,,slice,volume])
  > warning: no point with 0.25 inches


identify() needs x and y values, you are passing it the z values of
the image.

Maybe you want to use locator()?

.f