# [R] Fwd: only plot borders of a region in a scatter plot

Jim Lemon drjimlemon at gmail.com
Fri Aug 5 03:40:11 CEST 2016

```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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>>>>> and provide commented, minimal, self-contained, reproducible code.
>>>>>
>>>>
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>>>>
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>>>> and provide commented, minimal, self-contained, reproducible code.
>>>
>>>
>>>
>>>
>>> ______________________________________________
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>
>
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