## Copyright (C) 1994-2015 John W. Eaton
##
## This file is part of Octave.
##
## Octave is free software; you can redistribute it and/or modify it
## under the terms of the GNU General Public License as published by
## the Free Software Foundation; either version 3 of the License, or (at
## your option) any later version.
##
## Octave is distributed in the hope that it will be useful, but
## WITHOUT ANY WARRANTY; without even the implied warranty of
## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
## General Public License for more details.
##
## You should have received a copy of the GNU General Public License
## along with Octave; see the file COPYING.  If not, see
## <http://www.gnu.org/licenses/>.

## -*- texinfo -*-
## @deftypefn  {Function File} {@var{yiq_map} =} rgb2ntsc (@var{rgb_map})
## @deftypefnx {Function File} {@var{yiq_img} =} rgb2ntsc (@var{rgb_img})
## Transform a colormap or image from red-green-blue (RGB) color space to
## luminance-chrominance (NTSC) space.  The input may be of class uint8,
## uint16, single, or double.  The output is of class double.
##
## Implementation Note:
## The reference matrix for the transformation is
##
## @example
## @group
## /Y\     0.299  0.587  0.114  /R\
## |I|  =  0.596 -0.274 -0.322  |G|
## \Q/     0.211 -0.523  0.312  \B/
## @end group
## @end example
##
## @noindent
## as documented in @url{http://en.wikipedia.org/wiki/YIQ} and truncated to 3
## significant figures.  Note: The FCC version of NTSC uses only 2
## significant digits and is slightly different.
## @seealso{ntsc2rgb, rgb2hsv, rgb2ind}
## @end deftypefn

## Author: Tony Richardson <arichard@stark.cc.oh.us>
## Created: July 1994
## Adapted-By: jwe

function yiq = rgb2ntsc (rgb)

  if (nargin != 1)
    print_usage ();
  endif

  cls = class (rgb);
  if (! any (strcmp (cls, {"uint8", "uint16", "single", "double"})))
    error ("rgb2ntsc: invalid data type '%s'", cls);
  elseif (isfloat (rgb) && (any (rgb(:) < 0) || any (rgb(:) > 1)))
    error ("rgb2ntsc: floating point images may only contain values between 0 and 1");
  endif

  ## If we have an image convert it into a color map.
  if (isreal (rgb) && ndims (rgb) == 3)
    is_image = true;
    sz = size (rgb);
    rgb = [rgb(:,:,1)(:), rgb(:,:,2)(:), rgb(:,:,3)(:)];
    ## Convert to a double image.
    if (isinteger (rgb))
      low = double (intmin (cls));
      high = double (intmax (cls));
      rgb = (double (rgb) - low) / (high - low);
    elseif (isa (rgb, "single"))
      rgb = double (rgb);
    endif
  else
    is_image = false;
  endif

  if (! isreal (rgb) || columns (rgb) != 3 || issparse (rgb))
    error ("rgb2ntsc: input must be a matrix of size Nx3 or NxMx3");
  endif

  ## Reference matrix for transformation from http://en.wikipedia.org/wiki/YIQ
  ## and truncated to 3 significant figures.  Matlab uses this matrix for their
  ## conversion.
  trans = [ 0.299,  0.596,  0.211;
            0.587, -0.274, -0.523;
            0.114, -0.322,  0.312 ];

  ## Convert data.
  yiq = rgb * trans;

  ## If input was an image, convert it back into one.
  if (is_image)
    yiq = reshape (yiq, sz);
  endif

endfunction


## Test pure RED, GREEN, BLUE colors
%!assert (rgb2ntsc ([1 0 0]), [.299  .596  .211])
%!assert (rgb2ntsc ([0 1 0]), [.587 -.274 -.523])
%!assert (rgb2ntsc ([0 0 1]), [.114 -.322  .312])

%!test
%! rgb_map = rand (64, 3);
%! assert (ntsc2rgb (rgb2ntsc (rgb_map)), rgb_map, 1e-3);

%!test
%! rgb_img = rand (64, 64, 3);
%! assert (ntsc2rgb (rgb2ntsc (rgb_img)), rgb_img, 1e-3);

## Test input validation
%!error rgb2ntsc ()
%!error rgb2ntsc (1,2)
%!error <invalid data type 'cell'> rgb2ntsc ({1})
%!error <must be a matrix of size Nx3 or NxMx3> rgb2ntsc (ones (2,2))

