impixel.m

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function varargout = impixel(varargin)
%IMPIXEL Determine pixel color values.
%   IMPIXEL returns the red, green, and blue color values of
%   specified image pixels. In the syntaxes below, IMPIXEL
%   displays the input image and waits for you to specify the
%   pixels with the mouse:
%
%        P = IMPIXEL(I)
%        P = IMPIXEL(X,MAP)
%        P = IMPIXEL(RGB)
%
%   If you omit the input arguments, IMPIXEL operates on the
%   image in the current axes.
%
%   Use normal button clicks to select pixels. Press <BACKSPACE>
%   or <DELETE> to remove the previously selected pixel. A
%   shift-click, right-click, or double-click adds a final pixel
%   and ends the selection; pressing <RETURN> finishes the
%   selection without adding a pixel.
%
%   When you finish selecting pixels, IMPIXEL returns an M-by-3
%   matrix of RGB values in the supplied output argument. If you
%   do not supply an output argument, IMPIXEL returns the matrix
%   in ANS.
%
%   You can also specify the pixels noninteractively, using these
%   syntaxes:
%
%        P = IMPIXEL(I,C,R)
%        P = IMPIXEL(X,MAP,C,R)
%        P = IMPIXEL(RGB,C,R)
%
%   R and C are equal-length vectors specifying the coordinates
%   of the pixels whose RGB values are returned in P. The k-th
%   row of P contains the RGB values for the pixel (R(k),C(k)).
%
%   If you supply three output arguments, IMPIXEL returns the
%   coordinates of the selected pixels:
%
%        [C,R,P] = IMPIXEL(...)
%
%   To specify a nondefault spatial coordinate system for the
%   input image, use these syntaxes:
%
%        P = IMPIXEL(x,y,I,xi,yi)
%        P = IMPIXEL(x,y,X,MAP,xi,yi)
%        P = IMPIXEL(x,y,RGB,xi,yi)
%
%   x and y are 2-element vectors specifying the image XData and
%   YData. xi and yi are equal-length vectors specifying the
%   spatial coordinates of the pixels whose RGB values are
%   returned in P. If you supply three output arguments, IMPIXEL
%   returns the coordinates of the selected pixels:
%
%        [xi,yi,P] = IMPIXEL(x,y,...)
%
%   Class Support
%   -------------
%   The input image can be of class uint8 or double. All other
%   inputs and outputs are of class double.
%
%   Example
%   -------
%       RGB = imread('flowers.tif');
%       c = [12 146 410];
%       r = [104 156 129];
%       pixels = impixel(RGB,c,r)
%
%   See also IMPROFILE.

%   Clay M. Thompson 1-27-93
%   Revised by Chris Griffin, Steve Eddins for IPT v2
%   Copyright 1993-1998 The MathWorks, Inc.  All Rights Reserved.
%   $Revision: 5.12 $  $Date: 1997/11/24 15:35:29 $

[a,cm,xi,yi,x,y] = parse_inputs(varargin{:});

RGB_img = (ndims(a)==3 & size(a,3)==3);

[m,n,o] = size(a);

% Minimum and maximum x & y values
xmin = min(x(:)); ymin = min(y(:));
xmax = max(x(:)); ymax = max(y(:));

dx = (xmax-xmin)/max(n-1,1);
dy = (ymax-ymin)/max(m-1,1);
xx = xmin:dx:xmax;
yy = ymin:dy:ymax;

if prod(size(a))~=1,
   if RGB_img
      rgb(:,:,1) = interp2(xx,yy,a(:,:,1),xi,yi,'*nearest');
      rgb(:,:,2) = interp2(xx,yy,a(:,:,2),xi,yi,'*nearest');
      rgb(:,:,3) = interp2(xx,yy,a(:,:,3),xi,yi,'*nearest');
   else
      rgb = interp2(xx,yy,a,xi,yi,'*nearest');
   end
else
   rgb = repmat(a,length(xi),1);
end

% xi,yi may not be in pixel coordinates.  
xi_pix = axes2pix(n, [xmin xmax], xi);
yi_pix = axes2pix(m, [ymin ymax], yi);

% Find any points which are not in image
pixOutOfImage = find(xi_pix<1 | xi_pix>n | yi_pix<1 | yi_pix>m);

if isempty(cm),   % If we have grayscale or RGB image
   if RGB_img    
      rgb = reshape(rgb, length(xi), 3);
   else           % Grayscale
      rgb = [rgb(:) rgb(:) rgb(:)];
   end
   if isa(rgb, 'uint8')  % Promote to double, insert NAN's in right places
      rgb = double(rgb);
      if ~isempty(pixOutOfImage)  
         rgb(pixOutOfImage,:) = NaN;
      end
   end
else
   if isa(rgb,'uint8')
      rgb = double(rgb)+1;  % RGB contains colormap values
   end
   % If NaN's, replace temporarily with 1, then index
   % into colormap, then change 'em back to NaN's.
   % isnan doesn't take uint8's, so promote rgb to double
   if ~isempty(pixOutOfImage), 
      rgb(pixOutOfImage) = 1; 
   end
   rgb = cm(rgb(:),:);
   if ~isempty(pixOutOfImage), 
      rgb(pixOutOfImage,:) = NaN; 
   end
end

switch nargout
case {0, 1}
   varargout{1} = rgb;
case 3
   varargout{1} = xi;
   varargout{2} = yi;
   varargout{3} = rgb;
otherwise
   error('invalid number of output arguments');
end



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
%  Function: parse_inputs
%

function [a,cm,xi,yi,x,y] = parse_inputs(varargin)
% a    The image
% cm   The colormap (possibly empty)
% xi   X Indices of selected pixels - in rounded Pixel Coordinates
% yi   Y Indices of selected pixels - in rounded pixel coordinates
% x    vector returned by X=get(image_hdl, 'XData')
% y    vector returned by Y=get(image_hdl, 'YData') 

switch nargin
case 0,                 % Get information from the current figure
   [x,y,a,kind] = getimage;
   if ~kind, 
      error('The current axes must contain an image.');
   end
   if kind==1, 
      cm = colormap; 
   else 
      cm = []; 
   end
   
   [M,N,O] = size(a);
   
   % Get info from user
   [xi,yi] = getpts(gcf);
   xi = round(axes2pix(N, x, xi));
   yi = round(axes2pix(M, y, yi));
   x = [1 N]; y = [1 M];  % Pixel Coordinates
   
case 1         % impixel(I) or impixel(RGB)
    a = varargin{1};
    cm = [];
    h = imshow(a);
    
    % Get info from user
    [xi,yi] = getpts(get(h,'Parent'));
    [M,N,P] = size(a);
    x = [1 N];
    y = [1 M];
    xi = round(axes2pix(N, x, xi));
    yi = round(axes2pix(M, y, yi));
    
case 2         % impixel(X,map)
    a = varargin{1};
    cm = varargin{2};
    h = imshow(a,cm);
    
    % Get info from user
    [xi,yi] = getpts(get(h,'Parent'));
    [M,N,P] = size(a);
    x = [1 N];
    y = [1 M];
    xi = round(axes2pix(N, x, xi));
    yi = round(axes2pix(M, y, yi));
   
case 3,        % impixel(I,xi,yi) or impixel(RGB,xi,yi)
   a = varargin{1}; 
   xi = varargin{2}; 
   yi = varargin{3};
   x = [1 size(a,2)]; 
   y = [1 size(a,1)];
   cm = [];
   
case 4, % impixel(X,map,xi,yi)
   a = varargin{1}; 
   cm = varargin{2}; 
   xi = varargin{3};
   yi =  varargin{4};
   x = [1 size(a,2)]; 
   y = [1 size(a,1)];
   
case 5, % impixel(x,y,I or RGB ,xi,yi)
   x = varargin{1}; 
   y = varargin{2}; 
   a = varargin{3}; 
   xi = varargin{4}; 
   yi = varargin{5};
   cm = [];
   
case 6, % impixel(x,y,X,map,xi,yi)
   x = varargin{1}; 
   y = varargin{2}; 
   a = varargin{3}; 
   cm = varargin{4};
   xi = varargin{5}; 
   yi = varargin{6};
   
otherwise,
   error('Wrong number of input arguments.');
   
end

if length(xi) ~= length(yi)
   error('Xi and Yi must have the same length.');
end