function matrixbox( m, s )
% visualizes the effect of a matrix m on a set of vectors forming a box
% m: a 2x2 matrix
% s: a string plotted as the title of the plot
% this function is called by matrix2dtrafos.m

    % the 5 vectors that point into the cornes of a unit square:
    v = [ 0 1 1 0 0; 
          0 0 1 1 0 ];
    % transform v by means of m into new vector w:
    w = m*v;
    clf;
    hold on;
    % axis:
    plot( [-2 2], [0 0], 'k', 'LineWidth', 1 );
    plot( [0 0], [-2 2], 'k', 'LineWidth', 1 );
    % old vectors:
    plot( v(1,:), v(2,:), 'k', 'LineWidth', 1.0 )
    quiver( v(1,1), v(2,1), v(1,3), v(2,3), 1.0, 'k', 'LineWidth', 2.0 );
    scatter( v(1,2), v(2,2), 60.0, 'filled', 'k' );
    % transformed vectors:
    plot( w(1,:), w(2,:), 'b', 'LineWidth', 2.0 )
    quiver( w(1,1), w(2,1), w(1,3), w(2,3), 1.0, 'b', 'LineWidth', 3.0 );
    scatter( w(1,2), w(2,2), 100.0, 'filled', 'b' );
    % eigenvectors:
    [ev ed] = eig(m);
    n = ev*ed;
    if isreal( n )
        quiver( [0 0], [0 0], n(1,:), n(2,:), 1.0, 'r', 'LineWidth', 2.0 );
        text( 0.1, 0.2, sprintf( '\\lambda_1 = %.3g', ed(1,1) ), 'Units', 'normalized' )
        text( 0.1, 0.1, sprintf( '\\lambda_2 = %.3g', ed(2,2) ), 'Units', 'normalized' )
    end
    hold off;
    xlim( [ -2 2 ] );
    ylim( [ -2 2 ] );
    axis( 'equal' );
    m( abs(m) < 1e-3 ) = 0.0;  % make zeros really a zero
    text( 0.7, 0.15, 'A =', 'Units', 'normalized' )
    text( 0.8, 0.2, sprintf( '%.3g', m(1,1) ), 'Units', 'normalized' )
    text( 0.9, 0.2, sprintf( '%.3g', m(1,2) ), 'Units', 'normalized' )
    text( 0.8, 0.1, sprintf( '%.3g', m(2,1) ), 'Units', 'normalized' )
    text( 0.9, 0.1, sprintf( '%.3g', m(2,2) ), 'Units', 'normalized' )
    title( s );
    waitforbuttonpress;
end