On Wed, 19 Apr 2006 21:25:15 -0700, chen123.dian wrote:
> Hi, I want to use nearest-neighbor method to interpolate 2-D data.
> After reading some posters here and idl references, I am still confused
> how to do. I use a 1-D data to explain my problem. For example, there
> is x-arrary, [1.0, 2.9, 5.1], the corresponding y-arrary is [10, 50,
> 80]. Now we want to get the y-values at x-value [0.5, 2.6, 3.1, 4.6]
> using nearest-neighbor method. The result of y-values should be [10,
> 50, 50, 80]. Can the IDL function of interpolate.pro do this work? The
> input of interpolate.pro, X,Y,Z, arrays of numeric type containing the
> locations for which interpolated are desired, is very confuse. Is there
> any function like the function interp1 (interp2 for 2-D data) for
> matlab? Like, y_values=interp1(x, y, x-values, 'nearest', 'extrap').
Doesn't it depend on how your 2D array is sorted? Are the values
along rows and columns always monotonically increasing, e.g.:
0.1 1.2 2.4
0.2 1.3 2.5
...........
Otherwise, the problem is undefined.
That said, I think you are not asking a value lookup problem at all,
but a simple 2D interpolation on a regular grid, where instead of
linear nearest neighbor intepolation, you want "plain" nearest
neihbor. VALUE_LOCATE can do this for you, assuming "nearest in x"
and "nearest in y" are separable.. Let 'yarray' be your data array
you'd like to intepolate on, and 'curve_x' be the monotonic, but
non-regular grid of x positions, and 'curve_y' similarly, e.g.:
IDL> yarray=randomu(sd,100,100)
IDL> curvex=findgen(100)^2
IDL> curvey=alog10(1.+findgen(100))
such that in the 2D 'yarray', all rows have the same X vector
coordinates 'curvex', and all the columns the same Y vector 'curvey'.
Now you have two vectors describing the X,Y positions in this funny
space where you would like nearest neighbor interpolates:.
IDL> x=(randomu(sd,10)*100)^2
IDL> print,x
6544.61 2552.47 2239.70 9695.07 144.029 919.768
933.261 3231.17 151.407 761.378
IDL> y=alog10(1.+randomu(sd,10)*100)
IDL> print,y
1.63828 1.52555 1.80391 0.983095 1.26320 1.86245
1.79494 1.67543 1.89285 1.37616
IDL> xpos=value_locate(curvex,x)
IDL> xpos=round(xpos+(x-curvex[xpos])/(curvex[xpos+1]-curvex[xpos ]))
IDL> ypos=value_locate(curvey,y)
IDL> ypos=round(ypos+(y-curvey[ypos])/(curvey[ypos+1]-curvey[ypos ]))
IDL> interp=yarray[xpos,ypos]
JD
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