The filterdemo computes a FIR lowpass filter by solving a linear program. It solves a discretization of the problem minimize d2 subject to -1/d1 <= H(wk) <= d1, 0 <= w <= wc max(abs(H(wk))) <= d2, ws <= w <= pi where H(w) = h_0 + sum_{i=1}^{n-1} h_i*cos(2*pi/n*i*w). The variables are h and d2. 'filterdemo_cli' uses a simple a simple command-line interface. 'filterdemo_gui' uses a graphical user interface based on GTK.