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function_rgb2LumChrom.m
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function_rgb2LumChrom.m
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function [y A l2normLumChrom]=function_rgb2LumChrom(x,colormode)
% Forward color-space transformation ( inverse transformation is function_LumChrom2rgb.m )
%
% Alessandro Foi - Tampere University of Technology - 2005 - 2016 Public release v1.43 (15 May 2016)
% -----------------------------------------------------------------------------------------------------------------------------------------------
%
% SYNTAX:
%
% [y A l2normLumChrom] = function_rgb2LumChrom(x,colormode);
%
% INPUTS:
% x is RGB image with range [0 1]^3
%
% colormode = 'dct', 'opp', 'yuv', 'pca', or a custom 3x3 matrix
%
% 'dct' or 'opp' Opponent color space ('dct' is orthonormal version, 'opp' is equirange version)
% 'yuv' standard YUV (e.g. for JPEG images)
% 'pca' Principal components (note that this transformation is renormalized to be equirange)
%
% OUTPUTS:
% y is color-transformed image (with range typically included in or equal to [0 1]^3, depending on the transformation matrix)
%
% l2normLumChrom (optional) l2-norm of the transformation (useful for noise std calculation)
% A transformation matrix (used necessarily if colormode='pca')
%
% NOTES: - If only two outputs are used, then the second output is l2normLumChrom, unless colormode='pca';
% - 'opp' is used by default if no colormode is specified.
%
%
% USAGE EXAMPLE FOR PCA TRANSFORMATION:
% %%%% -- forward color transformation --
% if colormode=='pca'
% [zLumChrom colormode]=function_rgb2LumChrom(zRGB,colormode);
% else
% zLumChrom=function_rgb2LumChrom(zRGB,colormode);
% end
%
% %%%% [ ... ] Processing [ ... ]
%
% %%%% -- inverse color transformation --
% zRGB=function_LumChrom2rgb(zLumChrom,colormode);
%
if nargin==1
colormode='opp';
end
change_output=0;
if size(colormode)==[3 3]
A=colormode;
l2normLumChrom=sqrt(sum(A.^2,2));
else
if strcmp(colormode,'dct')
% A=dct(eye(3))/sqrt(3);
A=[1/3 1/3 1/3;1/sqrt(6) 0 -1/sqrt(6);1/sqrt(18) -sqrt(2)/3 1/sqrt(18)];
end
if strcmp(colormode,'opp')
% A=dct(eye(3));
% A=A./repmat(sum(A.*(A>0),2)-sum(A.*(A<0),2),[1 3]); %% ranges are normalized to unitary length;
A=[1/3 1/3 1/3; 0.5 0 -0.5; 0.25 -0.5 0.25];
end
if strcmp(colormode,'yuv')
A=[0.29900000000000 0.58700000000000 0.11400000000000; -0.16873660714285 -0.33126339285715 0.50000000000000; 0.50000000000000 -0.41868750000000 -0.08131250000000];
end
if strcmp(colormode,'pca')
A=princomp(reshape(x,[size(x,1)*size(x,2) 3]))';
A=A./repmat(sum(A.*(A>0),2)-sum(A.*(A<0),2),[1 3]); %% ranges are normalized to unitary length;
else
if nargout==2
change_output=1;
end
end
end
l2normLumChrom=sqrt(sum(A.^2,2)); %% l2-norms
%%% COLOR TRANSFORMATION %%%%%%%
y(:,:,1)=x(:,:,1)*A(1,1) + x(:,:,2)*A(1,2) + x(:,:,3)*A(1,3) + (1-sum(A(1,:).*(A(1,:)>0),2)-sum(A(1,:).*(A(1,:)<0),2))/2;
y(:,:,2)=x(:,:,1)*A(2,1) + x(:,:,2)*A(2,2) + x(:,:,3)*A(2,3) + (1-sum(A(2,:).*(A(2,:)>0),2)-sum(A(2,:).*(A(2,:)<0),2))/2;
y(:,:,3)=x(:,:,1)*A(3,1) + x(:,:,2)*A(3,2) + x(:,:,3)*A(3,3) + (1-sum(A(3,:).*(A(3,:)>0),2)-sum(A(3,:).*(A(3,:)<0),2))/2;
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if change_output
A=l2normLumChrom;
end