function obj = add_matrix(obj, mat3d, mat, imgno)
% Add 3d matrix image vol to slice overlay
% FORMAT obj = add_matrix(obj, mat3d, mat, imgno)
%
% Inputs
% obj          - object
% mat3d        - 3D matrix to add as img
% mat          - optional 4x4 voxel->world translation
% imgno        - optional img no to add to (defaults to last in object)
%
% Ouputs
% obj          - modified object
%__________________________________________________________________________

% Matthew Brett
% $Id: add_matrix.m 6623 2015-12-03 18:38:08Z guillaume $

if nargin < 2
    return
end
if nargin < 3
    mat = [];
end
if nargin < 4
    imgno = [];
end
if isempty(imgno)
    imgno = length(obj.img);
end
if ~isempty(mat3d)
    obj.img(imgno).vol = pr_matrix2vol(mat3d, mat);
end