def power_method(P, PT, data, n_it=10):
'''
Calculates the norm of operator K = [grad, P],
i.e the sqrt of the largest eigenvalue of K^T*K = -div(grad) + P^T*P :
||K|| = sqrt(lambda_max(K^T*K))
P : forward projection
PT : back projection
data : acquired sinogram
'''
x = PT(data)
for k in range(0, n_it):
x = PT(P(x)) - div(gradient(x))
s = sqrt(norm2sq(x))
x /= s
return sqrt(s)
def chambolle_pock(P, PT, data, Lambda, L, n_it, return_energy=True):
"""
Chambolle-Pock algorithm for the minimization of the objective function
||P*x - d||_2^2 + Lambda*TV(x)
P : projection operator
PT : backprojection operator
Lambda : weight of the TV penalization (the higher Lambda, the more sparse is the solution)
L : norm of the operator [P, Lambda*grad] (see power_method)
n_it : number of iterations
return_energy: if True, an array containing the values of the objective function will be returned
"""
sigma = 1.0 / L
tau = 1.0 / L
x = 0 * PT(data)
p = 0 * gradient(x)
q = 0 * data
x_tilde = 0 * x
theta = 1.0
if return_energy:
en = np.zeros(n_it)
for k in range(0, n_it):
# Update dual variables
p = proj_l2(p + sigma * gradient(x_tilde), Lambda)
q = (q + sigma * P(x_tilde) - sigma * data) / (1.0 + sigma)
# Update primal variables
x_old = x
x = x + tau * div(p) - tau * PT(q)
x_tilde = x + theta * (x - x_old)
# Calculate norms
if return_energy:
fidelity = 0.5 * norm2sq(P(x) - data)
tv = norm1(gradient(x))
energy = 1.0 * fidelity + Lambda * tv
en[k] = energy
if VERBOSE and k % 10 == 0:
print(
"[%d] : energy %e \t fidelity %e \t TV %e"
% (k, energy, fidelity, tv)
)
if return_energy:
return en, x
else:
return x