rgb与lab互换

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Mark Ruzon发来的邮件：
代码如下：
===========================rgb2lab.m

function[L,a,b]=RGB2Lab(R,G,B)
%function[L,a,b]=RGB2Lab(R,G,B)
%RGB2LabtakesmatricescorrespondingtoRed,Green,andBlue,and
%transformsthemintoCIELab.ThistransformisbasedonITU-R

%RecommendationBT.709usingtheD65whitepointreference.
%TheerrorintransformingRGB->Lab->RGBisapproximately
%10^-5.RGBvaluescanbeeitherbetween0and1orbetween0and255.
%ByMarkRuzonfromCcodebyYossiRubner,23September1997.

%UpdatedforMATLAB528January1998.
%
%Ifyourimageisloadedintouint8formatasanMxNx3tensor,you

%canpassitinasoneargument.Ifyoubreakitinto3pieces,convert

%themintodoublebeforecallingthisfunction.

if(nargin==1)

B=double(R(:,:,3));

G=double(R(:,:,2));

R=double(R(:,:,1));
end

if((max(max(R))>1.0)|(max(max(G))>1.0)|(max(max(B))>1.0))
R=R/255;
G=G/255;
B=B/255;
end
[M,N]=size(R);
s=M*N;
%Setathreshold
T=0.008856;
RGB=[reshape(R,1,s);reshape(G,1,s);reshape(B,1,s)];
%RGBtoXYZ
MAT=[0.4124530.3575800.180423;
0.2126710.7151600.072169;
0.0193340.1191930.950227];
XYZ=MAT*RGB;
X=XYZ(1,:)/0.950456;
Y=XYZ(2,:);
Z=XYZ(3,:)/1.088754;
XT=X>T;
YT=Y>T;
ZT=Z>T;
fX=XT.*X.^(1/3)+(~XT).*(7.787.*X+16/116);
%ComputeL
Y3=Y.^(1/3);
fY=YT.*Y3+(~YT).*(7.787.*Y+16/116);
L=YT.*(116*Y3-16.0)+(~YT).*(903.3*Y);
fZ=ZT.*Z.^(1/3)+(~ZT).*(7.787.*Z+16/116);
%Computeaandb
a=500*(fX-fY);
b=200*(fY-fZ);
L=reshape(L,M,N);
a=reshape(a,M,N);
b=reshape(b,M,N);

if((nargout==1)|(nargout==0))
L=cat(3,L,a,b);
end

==========================================lab2rgb.m

function[R,G,B]=Lab2RGB(L,a,b)
%function[R,G,B]=Lab2RGB(L,a,b)

%Lab2RGBtakesmatricescorrespondingtoL,a,andbinCIELabspace

%andtransformsthemintoRGB.ThistransformisbasedonITU-R

%RecommendationBT.709usingtheD65whitepointreference.

%andtheerrorintransformingRGB->Lab->RGBisapproximately
%10^-5.ByMarkRuzonfromCcodebyYossiRubner,23September1997.

%UpdatedforMATLAB528January1998.

%Fixedabuginconversionbacktouint89September1999.

if(nargin==1)
b=L(:,:,3);
a=L(:,:,2);
L=L(:,:,1);
end
%Thresholds
T1=0.008856;
T2=0.206893;
[M,N]=size(L);
s=M*N;
L=reshape(L,1,s);
a=reshape(a,1,s);
b=reshape(b,1,s);
%ComputeY
fY=((L+16)/116).^3;
YT=fY>T1;
fY=(~YT).*(L/903.3)+YT.*fY;
Y=fY;

%AlterfYslightlyforfurthercalculations
fY=YT.*(fY.^(1/3))+(~YT).*(7.787.*fY+16/116);
%ComputeX
fX=a/500+fY;
XT=fX>T2;
X=(XT.*(fX.^3)+(~XT).*((fX-16/116)/7.787));
%ComputeZ
fZ=fY-b/200;
ZT=fZ>T2;
Z=(ZT.*(fZ.^3)+(~ZT).*((fZ-16/116)/7.787));
X=X*0.950456;
Z=Z*1.088754;
MAT=[3.240479-1.537150-0.498535;
-0.9692561.8759920.041556;
0.055648-0.2040431.057311];
RGB=max(min(MAT*[X;Y;Z],1),0);
R=reshape(RGB(1,:),M,N)*255;
G=reshape(RGB(2,:),M,N)*255;
B=reshape(RGB(3,:),M,N)*255;

if((nargout==1)|(nargout==0))
R=uint8(round(cat(3,R,G,B)));
end