R help - Mar 2005 - Sv: Using locator() to digitise

Hi,
Splus allows pasting a graphics object into the plotting window, which makes it
possible to do what you describe below.
Now I use R which doesn't seem to allow pasting the picture into the
graphics window, so I copy the graph onto a transparency
sheet, and stick it onto my screen using tape. The coordinates need to be
converted to make sense (as you describe).

My methodology with the transparency has a "stoneage" appearance, but
it works quite well, if I may say so myself.

I have written down some notes that describes what I did, but it's in
Swedish so I guess it's not of much help.

Kind regards,
Sixten.

>>> Ted Harding <Ted.Harding at nessie.mcc.ac.uk> 2005-03-20
17:09 >>>
Hi Folks,

I'm contemplating using locator() to digitise external
graphics. To set context, I would be using X11 display
on Linux.

To pre-empt the obvious comment: I've found on the R site
the suggestion to use the 'pixmap' package. I've tried
this, and it works; but it involves building a big R
object (the internal pixmap representation), and this
chokes my somewhat puny laptop (e.g. it can take about
1 minute to draw the graphic inside a plot area using
addlogo(), with mucho swappo, and subsequently working
knee-deep in treacle). The following idea would be a lot
slicker.

For examples: I have something like

a) A scatterplot of data printed in a journal (but the
   data values are not available;

b) A contour map (on paper) of a region.

So, I can scan the document, and obtain a file in some
graphics format (jpeg, pbm or png, say).

Now: an idea which I find attractive is to be able to
overlay an R plot with axes onto a display of the graphics
file (produced as an X window by any suitable program such
as 'xv' or 'display') so that (if the overlay were possible)
clicking on the points of the graphic would in fact be
clicking on the R plot and, via locator(), generate the
R-plot coordinates of the mouse clicks which would correspond
to the selected points on the graphic.

Provided the coordinate system of the R plot were properly
related to the graphic, the results would be a digitisation
of the selected points on the graphic.

What seems to be needed for this idea to work is that
the R-plot should be displayed in an X11() device whose
background was completely transparent, so that when
moved over the (independently generated) display of the
graphic the latter would be visible (but locator() would
still be working on the R-plot itself). Window resizing
could look after the correspondence between graphic coordinates
and R-plot coordinates.

The R plot itself could be empty (apart from coordinate axes)
or could contain "helper" elements such as grid lines, circles
(e.g. I want to digitise graphics points within a certain circle),
etc. "Helper" elements could be added to the R-plot by subsequent
'lines' or 'points' commands (e.g. I identify two points on
the graphic, R-plot the line joining them, and then pick off
graphic-points which lie on the R-line).

So this question is really about producing a "bare" R plot
on, as it were, a virtual acrylic transparency. It's certainly
possible to do such a thing in X: e.g. the cute "xteddy" is
in fact a picture of a bear on a completely transparent
rectangular background, though you'd never know by looking!

Any comments?

With thanks, and best wishes to all,
Ted.


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Fax-to-email: +44 (0)870 094 0861
Date: 20-Mar-05                                       Time: 16:09:11
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On 21-Mar-05 Sixten Borg wrote:
> Hi,
> Splus allows pasting a graphics object into the plotting
> window, which makes it possible to do what you describe below.
> Now I use R which doesn't seem to allow pasting the picture
> into the graphics window, so I copy the graph onto a transparency 
> sheet, and stick it onto my screen using tape. The coordinates
> need to be converted to make sense (as you describe).
> 
> My methodology with the transparency has a "stoneage" appearance,
but
> it works quite well, if I may say so myself.
Hi Sixten,

Such things often work well! You take me back many years to
my earliest digitising, using a flat-bed pen plotter.

I bought a cheap little "pocket microscope" (the top and
bottom lenses folded out from the upright), plucked two
hairs from my head, and fixed these as *literal* cross-hairs
on the lower lens with sellotape. To use this, I would
remove the pen from its holder on the plotter arm and attach
the microscope to the pen holder with a rubber band. The
sheet to be digitised was laid on the plotter bed in the
usual way.

I then wrote a program (BASIC for CP/M) which would move
the pen-holder under control of the arrow-keys on the
computer, and keep an internal record of the point moved to.
Pressing "Return" would store the plotter coordinates of
the current point in an array.

Any other anecdotes, anyone?

Best wishes,
Ted.


--------------------------------------------------------------------
E-Mail: (Ted Harding) <Ted.Harding at nessie.mcc.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 21-Mar-05                                       Time: 09:55:58
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