rgb2ycbcr.m

conver rgb image to yuv

function ycbcr = rgb2ycbcr(varargin)
%RGB2YCBCR Convert RGB color values to YCbCr color space.
%   YCBCRMAP = RGB2YCBCR(MAP) converts the RGB values in MAP to the YCBCR
%   color space. MAP must be a M-by-3 array. YCBCRMAP is a M-by-3 matrix
%   that contains the YCBCR luminance (Y) and chrominance (Cb and Cr) color
%   values as columns.  Each row represents the equivalent color to the
%   corresponding row in the RGB colormap.
%
%   YCBCR = RGB2YCBCR(RGB) converts the truecolor image RGB to the
%   equivalent image in the YCBCR color space. RGB must be a M-by-N-by-3
%   array.
%
%   If the input is uint8, then YCBCR is uint8 where Y is in the range [16
%   235], and Cb and Cr are in the range [16 240].  If the input is a double,
%   then Y is in the range [16/255 235/255] and Cb and Cr are in the range
%   [16/255 240/255].  If the input is uint16, then Y is in the range [4112
%   60395] and Cb and Cr are in the range [4112 61680].
%
%   Class Support
%   -------------
%   If the input is an RGB image, it can be uint8, uint16, or double. If the
%   input is a colormap, then it must be double. The output has the same class
%   as the input.
%
%   Examples
%   --------
%   Convert RGB image to YCbCr.
%
%      RGB = imread('board.tif');
%      YCBCR = rgb2ycbcr(RGB);
%
%   Convert RGB color space to YCbCr.
%
%      map = jet(256);
%      newmap = rgb2ycbcr(map);
%
%   See also NTSC2RGB, RGB2NTSC, YCBCR2RGB.

%   Copyright 1993-2006 The MathWorks, Inc.  
%   $Revision: 1.13.4.7 $  $Date: 2006/11/08 17:49:05 $

%   References: 
%     C.A. Poynton, "A Technical Introduction to Digital Video", John Wiley
%     & Sons, Inc., 1996, p. 175
% 
%     Rec. ITU-R BT.601-5, "STUDIO ENCODING PARAMETERS OF DIGITAL TELEVISION
%     FOR STANDARD 4:3 AND WIDE-SCREEN 16:9 ASPECT RATIOS",
%     (1982-1986-1990-1992-1994-1995), Section 3.5.

rgb = parse_inputs(varargin{:});

%initialize variables
isColormap = false;

%must reshape colormap to be m x n x 3 for transformation
if (ndims(rgb) == 2)
  %colormap
  isColormap=true;
  colors = size(rgb,1);
  rgb = reshape(rgb, [colors 1 3]);
end

% This matrix comes from a formula in Poynton's, "Introduction to
% Digital Video" (p. 176, equations 9.6). 

% T is from equation 9.6: ycbcr = origT * rgb + origOffset;
origT = [65.481 128.553 24.966;...
     -37.797 -74.203 112; ...
     112 -93.786 -18.214];
origOffset = [16;128;128];

% The formula ycbcr = origT * rgb + origOffset, converts a RGB image in the range
% [0 1] to a YCbCr image where Y is in the range [16 235], and Cb and Cr
% are in that range [16 240]. For each class type (double,uint8,
% uint16), we must calculate scaling factors for origT and origOffset so that
% the input image is scaled between 0 and 1, and so that the output image is
% in the range of the respective class type.

scaleFactor.double.T = 1/255;      % scale output so in range [0 1].
scaleFactor.double.offset = 1/255; % scale output so in range [0 1].
scaleFactor.uint8.T = 1/255;       % scale input so in range [0 1].
scaleFactor.uint8.offset = 1;      % output is already in range [0 255].
scaleFactor.uint16.T = 257/65535;  % scale input so it is in range [0 1]  
                                   % and scale output so it is in range 
                                   % [0 65535] (255*257 = 65535).
scaleFactor.uint16.offset = 257;   % scale output so it is in range [0 65535].

% The formula ycbcr = origT*rgb + origOffset is rewritten as 
% ycbcr = scaleFactorForT * origT * rgb + scaleFactorForOffset*origOffset.  
% To use imlincomb, we rewrite the formula as ycbcr = T * rgb + offset, where T and
% offset are defined below.
classIn = class(rgb);
T = scaleFactor.(classIn).T * origT;
offset = scaleFactor.(classIn).offset * origOffset;

%initialize output
ycbcr = zeros(size(rgb),classIn);

for p = 1:3
  ycbcr(:,:,p) = imlincomb(T(p,1),rgb(:,:,1),T(p,2),rgb(:,:,2), ...
                         T(p,3),rgb(:,:,3),offset(p));
end  

if isColormap
   ycbcr = reshape(ycbcr, [colors 3 1]);
end

%%%
%Parse Inputs
%%%
function X = parse_inputs(varargin)

iptchecknargin(1,1,nargin,mfilename);
X = varargin{1};

if ndims(X)==2
  % For backward compatibility, this function handles uint8 and uint16
  % colormaps. This usage will be removed in a future release.

  iptcheckinput(X,{'uint8','uint16','double'},{'nonempty'},mfilename,'MAP',1);
  if (size(X,2) ~=3 || size(X,1) < 1)
    eid = sprintf('Images:%s:invalidSizeForColormap',mfilename);
    error(eid,'MAP must be a m x 3 array.');
  end
  if ~isa(X,'double')
    wid = sprintf('Images:%s:notAValidColormap',mfilename);
    warning(wid,...
            'MAP should be a double m x 3 array with values in the range %s',...
            '[0,1]. Converting your map to double using IM2DOUBLE.');
    X = im2double(X);
  end

elseif ndims(X)==3
  iptcheckinput(X,{'uint8','uint16','double'},{},mfilename,'RGB',1);
  if (size(X,3) ~=3)
    eid = sprintf('Images:%s:invalidTruecolorImage',mfilename);
    error(eid,'RGB must a m x n x 3 array.');
  end
else
    eid = sprintf('Images:%s:invalidInputSize',mfilename);
    error(eid,'%s only accepts two-dimensional or three-dimensional inputs.', ...
        upper(mfilename));
end