Inverse of rgb2ycbcr() in Octave? (No ycbcr2rgb)

Question 1

I just finished rewriting rgb2ycbcr and implementing ycbcr2rgb for Octave's image package. It will be part of the next image package release (likely to happen at the end of the summer). Just clone the image package repository or download the individual functions manually:

The ycbcr2rgb function;
The new rgb2ycbcr function;
The private function that actually does the all the calculations.

Question 2

There is a useful function here to do this (copied below)

%Copyright (C) 2005  Berge-Gladel

%This program 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 2
%of the License, or (at your option) any later version.

%This program 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.

function nouvmat2 = ycbcr2rgb(in)

%   YCBCR2RGB Convert YCbCr values to RGB color space.
%   RGBMAP = YCBCR2RGB(YCBCRMAP) converts the YCbCr values in the
%   colormap YCBCRMAP to the RGB color space. If YCBCRMAP is M-by-3 and
%   contains the YCbCr luminance (Y) and chrominance (Cb and Cr) color
%   values as columns, then RGBMAP is an M-by-3 matrix that contains
%   the red, green, and blue values equivalent to those colors.
%
%   RGB = YCBCR2RGB(YCBCR) converts the YCbCr image to the equivalent
%   truecolor image RGB
%   

    % cas ou on a une image (s est la taille de l'image en pixels)
    %matrice(:,:,:) = colormap(map) ; 
    % attention, on a des valeurs entre 0 et 1
    m=size(in);

    nbligmat = m(1);
    nbcolmat = m(2);

    nouvmat2 = ones(nbligmat,nbcolmat);

    % on multiplie par 255
    matrice = in *255;

    for i = 1 : nbligmat
        %  R = Y + 1.402*(Cr-128)
        nouvmat2(i,1) = matrice(i,1,:) + 1.402 * (matrice(i,3,:) - 128);
        %  G = Y - 0.34414 (Cb-128) - 0.71414 (Cr-128)
        nouvmat2(i,2) = matrice(i,1,:) - 0.34414 * (matrice(i,2,:) - 128) - 0.71414 * (matrice(i,3,:) - 128);
        %  B = Y + 1.772 * (Cb-128)
        nouvmat2(i,3) = matrice(i,1,:) + 1.772 * (matrice(i,2,:) - 128);
    end


    % on divise par 255 pour rendre une matrice avec des valeurs entre 0 et 1
        nouvmat2 = nouvmat2 / 255;

    for i = 1 : nbligmat
        for j = 1 : nbcolmat
            if nouvmat2(i,j) < 0
                nouvmat2(i,j) = 0;
            end
        end
    end
 endfunction