% empty out the final gradient matrix
grad=[];

% in a for-loop, populate matrix one row at a time
for i=1:256
    % in a nested for-loop, populate matrix one column at a time
    for j=1:256
        % the red channel increases with increasing rows (variable i)
        grad(i,j,1) = i;
        % the green channel increases with increasing columns (variable j)
        grad(i,j,2) = j;
        % the blue channel decreases with increasing rows (invert variable
        % i as 255-i)
        grad(i,j,3) = 255 - (i-1);
    end
end

% convert matrix to image data type and display
image(uint8(grad));