On Tue, 2 Sep 2003, kjetil brinchmann halvorsen wrote:
> Hola!
>
> I will want to identify pixels in an image with the mouse, for
> so getting the image data from the matrix(es), for use in subsequent
> discriminant analysis. But the following bombs R:
> (windows XP, rw1071)
>
> > str(baboon)
> list()
> - attr(*, "size")= int [1:2] 512 512
> - attr(*, "cellres")= num [1:2] 1 1
> - attr(*, "bbox")= num [1:4] 0 0 512 512
> - attr(*, "channels")= chr "grey"
> - attr(*, "bbcent")= logi FALSE
> - attr(*, "class")= chr "pixmapGrey"
> - attr(*, "grey")= num [1:512, 1:512] 0.537 0.510 0.345 0.259 0.322
> ...
> > class(baboon)
> [1] "pixmapGrey"
> > library(pixmap)
> > plot(baboon)
> > identify(baboon, n=1)
>
> ... and then R bombs!
>
> What to do?
>
As Kenneth said, give identify() the expanded grid of points, but there is
something wrong here:
> library(pixmap)
> example(pixmap)
....
pixmap> plot(z[1:20, 10:40])
> identify(z, n=1)
Program received signal SIGSEGV, Segmentation fault.
do_set (call=0x869ce7c, op=0x8299648, args=0x869ce98, rho=0x8f2496c)
at eval.c:1308
1308 switch (NAMED(s)) {
The identify.default() function is passing the pixmap object through to
xy.coords(), which returns two empty x and y vectors, which are checked
for length in the R code - xy.coords() treats the pixmap object as a list,
does:
} else if (is.list(x)) {
xlab <- paste(ylab, "$x", sep = "")
ylab <- paste(ylab, "$y", sep = "")
y <- x[["y"]]
x <- x[["x"]]
} else {
and y and x are the same length, so returns to identify.default() with
nothing, which is passed on to the .Internal() undetected. Two
possibilities - an identify.pixmap() in pixmap, or a test for the
(package, S4) pixmap class in xy.coords. But as it stands, it's a quick
way to exit the program. xy.coords() seems to be trusting the user to have
a list with x and y components, and here it has neither. The list is
indeed empty, being an S4 class - it just has attributes.
> version
_
platform i686-pc-linux-gnu
arch i686
os linux-gnu
system i686, linux-gnu
status
major 1
minor 7.1
year 2003
month 06
day 16
language R
Package: pixmap
Version: 0.3-2
> Kjetil Halvorsen
>
--
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Breiviksveien 40, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 93 93
e-mail: [EMAIL PROTECTED]
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