function log_images(ims, logbase)
%  log_images(images, logbase)
% Logarithm of images, to base logbase.
%
% If images are not specified, GUI-selection is used,
% if logbase is not specified, base-2 is assumed.

% check spm version:
if exist('spm_select','file') % should be true for spm5
    spm5 = 1;
    select = @(msg) spm_select(inf, 'image', msg);
    float32 = spm_type('float32');
elseif exist('spm_get','file') % should be true for spm2
    spm5 = 0;
    select = @(msg) spm_get(inf, 'img', msg);
    float32 = spm_type('float');
else
    error('log_images:spm_missing',...
        'Failed to find spm_get/spm_select, please add SPM to Matlab path')
end
spm_defaults;

% default to base-2
if ~exist('logbase', 'var') || isempty(logbase)
    logbase = 2;
end

% get filenames
if ~exist('ims', 'var') || isempty(ims)
    ims = select('Select images');
end
N = size(ims, 1);

% process files
for n = 1:N
    invol = spm_vol(ims(n, :));
    [pth fname ext] = fileparts(invol.fname);
    lgvol = invol;
    lgvol.fname = fullfile(pth, ['log_' fname ext]);
    lgvol.descrip = 'Log image';
    if (spm5)
        lgvol.dt(1) = float32;
    else
        lgvol.dim(4) = float32;
    end
    inimg = spm_read_vols(invol);
    lgimg = log2(inimg) / log2(logbase);
    lgimg(~isfinite(lgimg)) = 0; % (treat inf/nan as no change)
    spm_write_vol(lgvol, lgimg);
end