On Apr 12, 4:24 pm, tarequea...@[EMAIL PROTECTED]
wrote:
> On Apr 12, 11:57 am, "ben.bighair" <ben.bigh...@[EMAIL PROTECTED]
> wrote:
>
>
>
> > On Apr 11, 11:05 pm, tarequea...@[EMAIL PROTECTED]
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
Hi Again,
The only thing I did other than STOP (which I forgot to remove) was
redefine X and Y, which in your example were missing. I thought it
would be helpful for others if the whole thing worked. I should have
been clearer.
Speaking of clearer... I think I am more confused now. You might
need the intervention from the IDL gods that you originally
petitioned. Regardless of the parts I don't understand, the thing I
do get is that BILINEAR is looking for you to provide interpolation
coords that fit within the dimensions of the input array to be
sampled.
Take a peek at the second example in the online help for BILINEAR...
(slightly modified here...)
P = FINDGEN(3,3)
IX = [[0.5, 1.9], [1.1, 2.2]] ;Define the X subscripts.
JY = [[0.1, 0.9], [1.2, 1.8]] ;Define the Y subscripts.
Z = BILINEAR(P, IX, JY, MISSING = !VALUES.F_NAN) ;Interpolate.
PRINT, P
PRINT,Z
P prints as...
0.00000 1.00000 2.00000
3.00000 4.00000 5.00000
6.00000 7.00000 8.00000
Z prints as ...
0.800000 4.60000
4.70000 NaN
Note that IX and JY are mostly falling within the 0-2 range of indices
for a 3x3 array. BILINEAR can extrapolate - just remove the MISSING
keyword - but it can only extrapolate a little ways. In your example
IX are sampled at
0.00000 0.0809017 0.0618034 -0.0927051 -0.323607
-0.500000 -0.485410 -0.216312 0.247214 0.728115
and JY are sampled at ...
0.00000
0.0587785
0.190211
0.285317
0.235114
-4.37114e-08
-0.352671
-0.665740
-0.760845
-0.529007
Which to my way of thinking your are interpolating all around either
side of the [0,0] location. Is that what you intended? It is
possible that in my redo of your example I corrupted the X and Y
values you intended. In which case I would be not helping very much
at all!
Cheers,
Ben
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