%% Returns a model grid with alternating checkerboard pattern of values +1 and -1 
%% ======================================================================================
%%
%% Parameters:
%%	model - structure describing the 2D rectangular model.
%%	inc_rows - increment of the pattern in rows (depth)
%%	inc_cols - increment of the pattern in columns (horizontal distance)

%% Returns:
%%	pattern - grid containing values +1 or -1

function pattern = checkerboard(model,inc_rows,inc_cols)

  pattern = ones(model.nz,model.nx);

  nr = size(pattern,1);
  nc = size(pattern,2);

	% change sign, first in alternating groups of rows, 
	% and then in alternating groups of columns

  for i=1:2*inc_rows:nr
	ind = i:min( [(i+inc_rows-1), nr]);		% range of rows to flip sign

  	pattern(ind,:) = -pattern(ind,:);
  end

  for i=1:2*inc_cols:nc
	ind = i:min( [(i+inc_cols-1), nc]);		% range of columns to flip sign

  	pattern(:,ind) = -pattern(:,ind);
  end

end