imapprox.m

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function [cout,map] = imapprox(a,cm,n,flag)
%IMAPPROX Approximate indexed image by one with fewer colors.
%   [Y,NEWMAP] = IMAPPROX(X,MAP,N) approximates the colors in the
%   indexed image X and associated colormap MAP by using minimum
%   variance quantization. IMAPPROX returns indexed image Y with
%   colormap NEWMAP, which has at most N colors.
%
%   [Y,NEWMAP] = IMAPPROX(X,MAP,TOL) approximates the colors in X
%   and MAP through uniform quantization. NEWMAP contains at most
%   (FLOOR(1/TOL)+1)^3 colors.  TOL must be between 0 and 1.0.
%
%   Y = IMAPPROX(X,MAP,NEWMAP) approximates the colors in MAP by
%   using colormap mapping to find the colors in NEWMAP that best
%   match the colors in MAP.
%
%   Y = IMAPPROX(...,DITHER_OPTION) enables or disables
%   dithering. DITHER_OPTION is a string that can have one of
%   these values:
%
%       'dither'   dithers, if necessary, to achieve better color
%                  resolution at the expense of spatial
%                  resolution (default)
%
%       'nodither' maps each color in the original image to the
%                  closest color in the new map. No dithering is
%                  performed.
%
%   Class Support
%   -------------
%   The input image X can be of class uint8 or double. The output
%   image Y is of class uint8 if the length of NEWMAP is less
%   than or equal to 256. If the length of NEWMAP is greater than
%   256, X is of class double.
%
%   See also CMUNIQUE, DITHER, RGB2IND.

%   Clay M. Thompson 2-25-93
%   updated by Chris Griffin 8-28-96
%   Copyright 1993-1998 The MathWorks, Inc.  All Rights Reserved.
%   $Revision: 5.10 $  $Date: 1997/11/24 15:35:18 $

%   Grandfathered syntax:
%   [Y,NEWMAP] = IMAPPROX(X,MAP) uniformly removes colors
%   from the colormap if necessary so that NEWMAP has fewer
%   than 256 colors.  The removed colors are approximated
%   by other colors in the colormap.

if nargin<2, error('Not enough input arguments.'); end

if nargin==2, % imapprox(a,cm) (remove colors)
  if size(cm,1)<=256,
    c = a; map = cm;
  else
    % Grandfathered syntax
    n = size(cm,1)-256; 
    ndx = floor((1:n)/(n+1)*size(cm,1)+1);
    map = cm; map(ndx,:) = [];
    rgb = ind2rgb(a,cm);
    c = dither(rgb,map);
  end

elseif isstr(n), % imapprox(a,cm,'dither_option')
  flag = [lower(n) ' ']; % Protect against short string
  if size(cm,1)<=256,
    c = a; map = cm;
  elseif flag(1)=='n', % 'nodither'
    n = size(cm,1)-256; 
    ndx = floor((1:n)/(n+1)*size(cm,1)+1);
    map = cm; map(ndx,:) = [];

    % Manhattan Distance to colors in reduced colormap
    M = size(map,1); N = length(ndx);
    [v,loc] = min(abs(repmat(map(:,1),1,N)-repmat(cm(ndx,1)',M,1)) + ...
                  abs(repmat(map(:,2),1,N)-repmat(cm(ndx,2)',M,1)) + ...
                  abs(repmat(map(:,3),1,N)-repmat(cm(ndx,3)',M,1)));
    dpos = ones(size(cm,1),1); 
    dpos(ndx) = zeros(size(ndx));
    pos = cumsum(dpos); pos(ndx) = loc;
    if isa(a, 'uint8')   % promotion to double is necessary until we have
        a = double(a)+1; % the utility function for uint8 indexing.
    end
    c = reshape(pos(a),size(a));
  else
    n = size(cm,1)-256; 
    ndx = floor((1:n)/(n+1)*size(cm,1)+1);
    map = cm; map(ndx,:) = [];
    rgb = ind2rgb(a,cm);
    c = dither(rgb,map);
  end

elseif prod(size(n))==1, % imapprox(a,cm,tol) or imapprox(a,cm,n)
  if nargin<4, flag = 'dither'; end
  if n>size(cm,1),
    c = a; map = cm;
  else
    % Convert colormap to RGB so that rgb2ind can do all the work
    rgb(:,:,1) = cm(:,1); rgb(:,:,2) = cm(:,2); rgb(:,:,3) = cm(:,3);

    [ndx,map] = rgb2ind(rgb,n,'n');
    if flag(1)=='n', % 'nodither'
      if isa(a, 'uint8')   % promotion to double is necessary until we have
          a = double(a)+1; % the utility function for uint8 indexing.
      end
      c = reshape(ndx(a), size(a));
    else
      rgb = ind2rgb(a,cm);
      c = dither(rgb,map);
    end
  end

elseif size(n,2)==3, % imapprox(a,cm,map)
  if nargin<4, flag = 'dither'; end
  flag = [lower(flag) ' ']; % Protect against short string
  map = n;
  if flag(1)=='n', % 'nodither'
    % Manhattan Distance to colors in new colormap
    M = size(map,1); N = size(cm,1);
    
    [v,loc] = min(abs(repmat(map(:,1),1,N)-repmat(cm(:,1)',M,1)) + ...
                  abs(repmat(map(:,2),1,N)-repmat(cm(:,2)',M,1)) + ...
                  abs(repmat(map(:,3),1,N)-repmat(cm(:,3)',M,1)));
    if isa(a, 'uint8')   % promotion to double is necessary until we have
        a = double(a)+1; % the utility function for uint8 indexing.
    end
    c = reshape(loc(a),size(a));
  else
    rgb = ind2rgb(a,cm);
    c = dither(rgb,map);
  end    
end

if nargout==0,
  imshow(c,map)
  return
end

if isa(c, 'double') & size(map,1) <= 256
    c = uint8(c-1);
end

cout = c;