def update(*args):
global im, dens, dens_prev, u, u_prev, v, v_prev, N, visc, dt, diff
user_step(dens_prev, u_prev, v_prev)
vel_step(N, u, v, u_prev, v_prev, visc, dt)
dens_step(N, dens, dens_prev, u, v, diff, dt)
im.set_data(dens)
def update(*args):
global im, dens, dens_prev, u, u_prev, v, v_prev, N, visc, dt, diff
vel_step(N, u, v, u_prev, v_prev, visc, dt)
dens_step(N, dens, dens_prev, u, v, diff, dt)
im.set_data(dens)
im.set_clim(vmin=dens.min(), vmax=dens.max())