function [vol avi mov] = vol2avi(vol, avi, fps, ax, step, rw, rev)

if ~exist('vol', 'var') || isempty(vol)
    vol = spm_select(1, 'image');
end
if ischar(vol)
    vol = spm_vol(vol);
end

if ~exist('avi', 'var') || isempty(avi)
    [pth fnm] = fileparts(vol.fname);
    avi = fullfile(pth, [fnm '.avi']);
end

if ~exist('fps', 'var') || isempty(fps)
    fps = 24;
end

if ~exist('ax', 'var') || isempty(ax)
    ax = 3;
end

if ~exist('step', 'var') || isempty(step)
    step = 1;
end

if ~exist('rw', 'var') || isempty(rw)
    rw = false;
end

if ~exist('rev', 'var') || isempty(rev)
    rev = false;
end

img = spm_read_vols(vol);

f = figure; colormap gray

dim = vol.dim(ax);
n = 0;
for d = 1:step:dim
    n = n + 1;
    switch ax
        case 1, slice = img(d, :, :);
        case 2, slice = img(:, d, :);
        case 3, slice = img(:, :, d);
    end
    slice = rot90(squeeze(slice));
    imagesc(slice);
    axis image; axis off;
    mov(n) = getframe;
end

% rewind sequence
if rw
    for d = dim-1:-step:2
        n = n + 1;
        switch ax
            case 1, slice = img(d, :, :);
            case 2, slice = img(:, d, :);
            case 3, slice = img(:, :, d);
        end
        slice = rot90(squeeze(slice));
        imagesc(slice);
        axis image; axis off;
        mov(n) = getframe;
    end
end

% reverse whole sequence (not very neatly...)
if rev
    tmp = mov;
    N = length(mov);
    for n = 1:N
        mov(n) = tmp(N+1-n);
    end
end

movie2avi(mov, avi, 'FPS',fps);
close(f);