only plot borders of a region in a scatter plot

Hi Zun Yin,
The first problem requires something like this:

pixel8<-function(x,y,pixsize=1) {
 nsteps<-length(x)-1
 newx<-x[1]
 newy<-y[1]
 for(i in 1:nsteps) {
  dx<-diff(x[i:(i+1)])
  dy<-diff(y[i:(i+1)])
  if(dx && dy) {
   newx<-c(newx,x[i]+dx,x[i]+dx)
   newy<-c(newy,y[i],y[i]+dy)
  }
  else {
   newx<-c(newx,x[i+1])
   newy<-c(newy,y[i+1])
  }
 }
 return(list(x=newx,y=newy))
}

I think that this does part of what you want. Your points seem to be
in the middle of a pixel edge, so an offset would have to be added to
align the resulting points with the corners. The other thing to work
out is the order of changing x and y on a slope, which I think is a
function of whether the line is "inside" or "outside" the overall area
enclosed. I'll post again if I have any brilliant ideas.

Jim


On Fri, Aug 5, 2016 at 6:08 AM, Duncan Murdoch <murdoch.duncan at gmail.com> wrote:

On 04/08/2016 11:46 AM, Zun Yin wrote:

Dear William, Duncan and Bert,

Thanks a lot for your help and tips :D In fact the contour() plot still
cannot solve my problem perfectly. As the resolution of my plot is very
coarse, I want to a contour perfectly surround all grid cells in the river
basin. In another word, I want all angle of the contour to be 90 degree.
Another problem for the contour() is that the contour line doesn't close
at
where the value of neighbouring grid cell is NA. See the right side of the
two contour lines (attachment). Do you know how I can get what I want?
Thanks a lot :P

The second problem is easy:  just change your NA values to some new ID
value, or change the test from

basiinID == ID

to

!is.na(basiinID) & basiinID == ID

The first one looks harder.

Duncan Murdoch

Cheers,


Zun Yin

On Thu, Aug 4, 2016 at 5:09 PM, Bert Gunter <bgunter.4567 at gmail.com>
wrote:

... note the typo. It's:

contour( basiinID == ID, level=0.5)

:-)


-- Bert



Bert Gunter

"The trouble with having an open mind is that people keep coming along
and sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )


On Thu, Aug 4, 2016 at 8:03 AM, William Dunlap via R-help
<r-help at r-project.org> wrote:

If 'basinID' is the matrix of basin identifiers you could draw an
outline
of the basin with identifier 'ID' with
   coutour( basiinID == ID, level=0.5)
Add 'add=TRUE' if you are overlaying this on an existing plot.

Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Thu, Aug 4, 2016 at 1:51 AM, Zun Yin <yinzun2000 at gmail.com> wrote:

D
ear all,

I have a matrix with ID of river basins (integer numbers). Now I want
to
highlight one river basin in a map by plotting only the border. Like
the
attached figure. Two river basins are highlighted by polygons. It is
created by ferret, but I prefer to implement it by R. Anybody know how

to

do it? Thanks a lot.

Cheers,

Zun Yin

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