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Dist 函数详解
1、Dist函数的算法如下(很显然该算法的本质就是常说的欧氏距离算法)
R(i,j) = SQRT(F(i)^2 + G(j)^2) where:
F(i) = i IF 0 <= i <= n/2
= n-i IF i > n/2
G(i) = i IF 0 <= i <= m/2
= m-i IF i > m/2
图解：tvscl,dist(200)
扩展：SURFACE, DIST(20), /SAVE
IDL实现源码：
; $Id: //depot/Release/ENVI51_IDL83/idl/idldir/lib/dist.pro#1 $;; Copyright (c) 1982-2013, Exelis Visual Information Solutions, Inc. All; rights reserved. Unauthorized reproduction is prohibited.;;+; NAME:; DIST;; PURPOSE:; Create a rectangular array in which each element is proportional; to its frequency. This array may be used for a variety; of purposes, including frequency-domain filtering and; making pretty pictures.;; CATEGORY:; Signal Processing.;; CALLING SEQUENCE:; Result = DIST(N [, M]);; INPUTS:; N = number of columns in result.; M = number of rows in result. If omitted, N is used to return; a square array.;; OUTPUTS:; Returns an (N,M) floating array in which:;; R(i,j) = SQRT(F(i)^2 + G(j)^2) where:; F(i) = i IF 0 <= i <= n/2; = n-i IF i > n/2; G(i) = i IF 0 <= i <= m/2; = m-i IF i > m/2;; SIDE EFFECTS:; None.;; RESTRICTIONS:; None.;; PROCEDURE:; Straightforward. The computation is done a row at a time.;; MODIFICATION HISTORY:; Very Old.; SMR, March 27, 1991 - Added the NOZERO keyword to increase efficiency.; (Recomended by Wayne Landsman); DMS, July, 1992. - Added M parameter to make non-square arrays.; CT, RSI, March 2000: Changed i^2 to i^2. to avoid overflow.;-function dist,n,m ;Return a rectangular array in which each pixel = euclidian ;distance from the origin.compile_opt idl2on_error,2 ;Return to caller if an error occursn1 = n[0]m1 = (n_elements(m) le 0) ? n1 : m[0]x=findgen(n1) ;Make a rowx = (x < (n1-x)) ^ 2 ;column squaresa = FLTARR(n1,m1,/NOZERO) ;Make arrayfor i=0L, m1/2 do begin ;Row loop y = sqrt(x + i^2.) ;Euclidian distance a[0,i] = y ;Insert the row if i ne 0 then a[0, m1-i] = y ;Symmetricalendforreturn,aend
2、用法：
Dist(20,20)生成一个20×20的数组，当用户输入Dist(20),则默认行与列都是20。
3、IDL帮助的解释：
DIST
The DIST function creates an array in which each array element value is proportional to its frequency. This array may be used for a variety of purposes, including frequency-domain filtering.
This routine is written in the IDL language. Its source code can be found in the file dist.pro in the lib subdirectory of the IDL distribution.
Examples
; Display the results of DIST as an image:
TVSCL, DIST(100)
Syntax
Result = DIST(N [, M])
Return Value
Returns a rectangular array in which the value of each element is proportional to its frequency.
Arguments
N
The number of columns in the resulting array.
M
The number of rows in the resulting array. If M is omitted, the resulting array will be N by N.
Keywords
None.
Dist 函数详解