[R] identify with image

[Roger Bivand]

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: Roger.Bivand at nhh.no