R help - Dec 2009 - Dividing a pixel image into factors - (cut.im(), cut.default())

Hi,

 

I have a numeric pixel image which I would like to divide into factors
for analysis in Spatstat.  I have found that I can use cut.im() function
to divide the range of pixel values into a series of equal length
intervals (e.g. if my pixels values range from 0 to 60,
cut.im(X.im,breaks=2) will produce two factors one containing pixel
values 0-30 and one containing pixel values of 30 - 60, or thereabouts).


 

However, I would like to specify the pixel value at which the factors
are created - e.g. create one factor containing pixel values of 0-5 and
another factor containing all other pixel values.  I have been
struggling to work out how to do this using either cut.im() or
cut.default().  Can anyone help?

 

Many thanks,

 

Lauren

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Lauren,

I think (?) you mean to say that you wish to create a factor and control the
range of values assigned to each level.

The "breaks" argument can specify either the number of intervals you
desire
OR the values you wish to use to define levels of your factor.  See
'?cut'
and the following example:
> #A numeric vector
> x <- 1:10
> 
> #Assign equal number of values
> #2 each of 2 automatically 
> #determined intervals
> levels1 <- cut(x,breaks=2)
> 
> #User-specified intervals
> levels2 <- cut(x,breaks=c(0,3,Inf))
> 
> data.frame(x,levels1,levels2)
    x     levels1 levels2
1   1 (0.991,5.5]   (0,3]
2   2 (0.991,5.5]   (0,3]
3   3 (0.991,5.5]   (0,3]
4   4 (0.991,5.5] (3,Inf]
5   5 (0.991,5.5] (3,Inf]
6   6    (5.5,10] (3,Inf]
7   7    (5.5,10] (3,Inf]
8   8    (5.5,10] (3,Inf]
9   9    (5.5,10] (3,Inf]
10 10    (5.5,10] (3,Inf]

'Inf' is an easy way to create an interval that contains all values
larger
than some cutoff.

Good luck with your analysis,

Glen


Gough Lauren wrote:
> 
> I have a numeric pixel image which I would like to divide into factors
> for analysis in Spatstat.  I have found that I can use cut.im() function
> to divide the range of pixel values into a series of equal length
> intervals (e.g. if my pixels values range from 0 to 60,
> cut.im(X.im,breaks=2) will produce two factors one containing pixel
> values 0-30 and one containing pixel values of 30 - 60, or thereabouts).
> 
> However, I would like to specify the pixel value at which the factors
> are created - e.g. create one factor containing pixel values of 0-5 and
> another factor containing all other pixel values.  I have been
> struggling to work out how to do this using either cut.im() or
> cut.default().  Can anyone help?
> 

-----
Glen Sargeant
Research Wildlife Biologist
-- 
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On 4/12/2009, at 10:10 PM, Gough Lauren wrote:
> Hi,
>
>
>
> I have a numeric pixel image which I would like to divide into factors
> for analysis in Spatstat.  I have found that I can use cut.im()  
> function
> to divide the range of pixel values into a series of equal length
> intervals (e.g. if my pixels values range from 0 to 60,
> cut.im(X.im,breaks=2) will produce two factors one containing pixel
> values 0-30 and one containing pixel values of 30 - 60, or  
> thereabouts).
>
>
>
>
> However, I would like to specify the pixel value at which the factors
> are created - e.g. create one factor containing pixel values of 0-5  
> and
> another factor containing all other pixel values.
	I think you mean a factor with two ***levels***, one encompassing
	values from 0 to 5, the other encompassing values from 5 to 60.
> I have been
> struggling to work out how to do this using either cut.im() or
> cut.default().  Can anyone help?
Unless I'm terribly confused, the obvious solution to your question is:

	cut.im(X.im,breaks=c(0,5,60))

Uhhh, why was this difficult?

	cheers,

		Rolf Turner

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