| Re: A new puzzle for histogram [message #50142] |
Fri, 15 September 2006 15:58  |
gknoke
Messages: 9
Registered: September 2006
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Junior Member |
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Hm... not quite what I had in mind. I've solved the half of the
problem dealing with x and y, what I have now is:
cos_table = transpose(cos(angles))
sin_table = transpose(sin(angles))
;Calculate x and y in meters
x = r_pts#cos_table
y = r_pts#sin_table
;Find corresponding pixel on mapped grid
ix = round((x-x0)/cellsize)+xysize/2
jy = round((y-y0)/cellsize)+xysize/2
count = hist_2d(ix, jy, max1=(xysize-1),max2=(xysize-1),min1=0,min2=0)
Which does away with the need for the for loops. The only line I can't
figure out how to replace is:
map(ix,jy)=map(ix,jy) + data(i_range,j_theta)
Is there a hist_2d equivalent to reverse_indices? If I could simply
figure out the indices of all the points I've pulled out of ix and jy I
suddenly have all of the indices of the data points I need. From there
it's a simple problem of summing the points that are in the same
histogram bins.
Thanks,
--Greg
Jean H. wrote:
> Hi,
>
> If I corectly understand your problem, you might want to look at the
> rebin function:
>
> a = indgen(4) ;could be your angle
> b = transpose(indgen(4)) ;could be your range
>
> print, rebin(a,4,4) * rebin(b,4,4)
>
>>> 0 0 0 0
> 0 1 2 3
> 0 2 4 6
> 0 3 6 9
>
> hope that helps...
>
> Jean
>
> gknoke wrote:
>> So, I've got this piece of code which is horribly horribly inefficient.
>> I know the solution lies in a clever application of the histogram
>> function, but being Friday afternoon my brain isn't seeing it. Anyone
>> else have any insight on how I might approach it?
>>
>> This particular routine is mapping a piece of data from polar to
>> cartesian coordinates. Currently the code generates sin/cos angle
>> tables and calculates the x,y coordinates in meters for each
>> range/angle, and then converts that to an x,y coordinate in terms of
>> pixels from the center. I realize the calculation of the x,y
>> coordinates can be replaced with a simple vector operation, but I can't
>> see how to turn the separate resulting arrays for x and y into a single
>> array I can use the histogram function to match to the mapped grid.
>>
>> ;Setup the output grid
>> map = fltarr(xysize, xysize)
>> count = intarr(xysize, xysize)
>>
>> cos_table = cos(angles)
>> sin_table = sin(angles)
>>
>> for j_theta = 0, n_elements(angles)-1 do begin
>> for i_range = 0, n_range-1 do begin
>> ;Calculate x and y in meters
>> x = r_pts(i_range)*cos_table(j_theta)
>> y = r_pts(i_range)*sin_table(j_theta)
>>
>> ;Find corresponding pixel on mapped grid
>> ix = round((x-x0)/cellsize)+xysize/2
>> jy = round((y-y0)/cellsize)+xysize/2
>>
>> ;If the pixel coord is inside the image put the data point there
>> if(ix ge 0 and ix le xysize-1) then begin
>> if(jy ge 0 and jy le xysize-1) then begin
>> map(ix,jy)=map(ix,jy) + data(i_range,j_theta)
>> count(ix,jy)=count(ix,jy)+1
>> endif
>> endif
>> endfor
>> endfor ;End of nearest neighbor loops
>>
>> Thanks,
>>
>> --Greg
>>
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