function?[updown]?=?envelope（xyinterpMethod）

%ENVELOPE?gets?the?data?of?upper?and?down?envelope?of?the?known?input?（xy）.
%????interpMethod????the?interpolation?method
%
%???Input?parameters輸入?yún)?shù):
%????x???????????????the?abscissa橫坐標?of?the?given?data
%????y???????????????the?ordinate縱坐標?of?the?given?data
%
%???Output?parameters輸出參數(shù):
%????up??????the?upper?envelope?which?has?the?same?length?as?x.
%????down????the?down?envelope?which?has?the?same?length?as?x.
%
%???See?also?DIFF?INTERP1

%???Designed?by:?Lei?Wang??11-Mar-2003.
%???Last?Revision:?21-Mar-2003.
%???Dept.?Mechanical?&?Aerospace?Engineering?NC?State?University.
%???$Revision:?1.1?$??$Date:?3/21/2003?10:33?AM?$

if?length（x）?~=?length（y）
????error（‘Two?input?data?should?have?the?same?length兩個輸入數(shù)據(jù)長度需要一樣.‘）;
end

strmethod={‘nearest‘?‘linear‘?‘spline‘?‘cubic‘};??%將插值方法定義為單元數(shù)組
%插值方法（nearest臨近點插值;linear線性;spline三次樣條插值;‘pchip’?or?‘cubic’立方插值）

if?（nargin??3）???%nargin是用來判斷輸入變量個數(shù)的函數(shù)，這樣就