| Re: Interpolation [message #59887 is a reply to message #2184] |
Sat, 12 April 2008 13:24   |
tarequeaziz
Messages: 20
Registered: November 2007
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Junior Member |
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On Apr 12, 11:57 am, "ben.bighair" <ben.bigh...@gmail.com> wrote:
> On Apr 11, 11:05 pm, tarequea...@gmail.com wrote:
>
>
>
>> Hello All (IDL Gods),
>
>> I am back with yet another problem.
>> I know...I know...its friday night. I apologize for that. But I am
>> really stuck here for a while.
>
>> The problem:
>
>> In a certain part of my code I need to do interpolation. The data that
>> I am dealing with are from a XY grid. I need to convert them to polar
>> coordinate. So, what I do is following:
>
>> a. I generate a radius vector r_vec and a theta array containing
>> values from zero to 2 pi.
>> b. Now use the simple polar-to-rectangular coordinate transform i.e. x
>> = r cos(theta) etc.
>> c. Using these values I have a xy grid generated through a known r-
>> theta values.
>> d. Now think of superimposing the new xy grid(lets call it x'y' ) on
>> to the old xy grid which contains the real data.
>> e. This is the part where I need interpolation. I do interpolations to
>> get the x'y' values from the xy points.
>
>> So here's the question:
>
>> ----- Is there any elegant way of doing this coordinate
>> transformation ? (And in case you are thinking, "well you already have
>> the xy data, so why not just convert to r-theta?", I have to say that
>> the interpolation method actually gives me a way nicer dataset ).
>
>> My 2nd trouble is, and this is probably the biggest and dumbest
>> problem for me.
>
>> ----- i was playing around with several interpolation routines from
>> IDl. My boss's suggestion was to use 'bilinear'.
>> but I thought to give others' a shot too. Problem is, when I am
>> done with interpolation, result is nothing like what I was
>> expecting. A run down version of the code is shown below:
>
>> =======================start of code================================
>
>> Nth= 10.
>> dth= 1/Nth
>> r_vec= findgen(Nth)/Nth
>> theta_vec = findgen(Nth)/Nth * 2.*!Pi
>
>> for i=0L,Nth - 1 do begin
>
>> x[i]= r_vec[i]* cos(theta_vec)
>
>> endfor
>
>> for j=0L,Nth - 1 do begin
>
>> y[j]= r_vec[j]* sin(theta_vec)
>
>> endfor
>
>> ;print,y
>
>> ;plot,x,y
>
>> ------------------------------------------------------------ -----------
>> Now I create the 'main' dataset on which I am going to use
>> interpolation scheme.
>
>> rr = findgen(20.)/30.
>> tht = findgen(20.)/30. *2*!Pi
>
>> m = fltarr(20,20)
>
>> for j=0,19 do begin
>> for i=0,19 do begin
>
>> m[i,j] = rr[i]*cos(tht[j]) + 5.*rr[i]*sin(tht[j])
>
>> ;print,i
>> endfor
>> endfor
>
>> m_p=bilinear(m,x,y)
>
>> End
>> =======================End of Code=================================
>
>> The problem is, as I mentioned above, when I plot m and the
>> interpolated m_p, they do not look like similar at all.
>
>> Any help will be greatly appreciated.
>
>> Thanks in advance.
>
>> ~tareque
>
> Hi,
>
> Coordinate transforms are very easy with CV_COORD().
>
> I don't understand where you want to go with the interpolation. The
> input coordinates to BILINEAR are rectangular and formed as indexed
> based (as in subscript indices in X and Y.) As near as I can
> reconstruct, X ranges from -0.500000 to 0.728115 and Y ranges
> from -0.760845 to 0.285317. My reconstruction of your code is
> below.
>
> Cheers,
> Ben
>
> PRO tareque
>
> Nth= 10L
> dth= 1.0/Nth
> r_vec= findgen(Nth)/Nth
> theta_vec = findgen(Nth)/Nth * 2.*!Pi
> x = fltarr(nth)
> y = fltarr(nth)
>
> for i=0L,Nth - 1 do begin
>
> x[i]= r_vec[i]* cos(theta_vec[i])
>
> endfor
>
> for j=0L,Nth - 1 do begin
>
> y[j]= r_vec[j]* sin(theta_vec[j])
>
> endfor
>
> ;print,y
>
> ;plot,x,y
>
> ;----------------------------------------------------------- ------------
> ;Now I create the 'main' dataset on which I am going to use
> ;interpolation scheme.
>
> rr = findgen(20.)/30.
> tht = findgen(20.)/30. *2*!Pi
>
> m = fltarr(20,20)
>
> for j=0,19 do begin
> for i=0,19 do begin
>
> m[i,j] = rr[i]*cos(tht[j]) + 5.*rr[i]*sin(tht[j])
>
> ;print,i
> endfor
> endfor
>
> m_p=bilinear(m,x,y)
>
> STOP
> End
hi Ben,
Thanks for getting back to me on this.
I guess I could not clearly state my problem before. So, I am going to
give it another try:
The data I deal with comes from a ccd camera and this goes some
tinkering and tweaking (to account for background noise and stuff).
Then this 'processed' data is in need for interpolation. why? well the
reason being this, that these processed data are actually in XY
frame.
We want them to be on a polar coordinate. The reason I cannot use
'cv_coord' is because in that case I will get the data in a r-theta
plane which is
directly correlated to the experimental XY grid. If we use
interpolation, then we can be on a XY grid (yes, it is XY grid) which
is carefully 'designed' according to
our 'own' r-theta values. So in this way we have more leverage over
data manipulation.
Was it any clearer than before?
And one more thing, I could not find much changes in your version
except for an inclusion of 'STOP'. Did i miss something here??
Thanks again for your time.
Much appreciated !
~ Tareque
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