# Forward-Backward Splitting¶

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In :
using PyPlot


This numerical tour presents the Forward-Backward (FB) algorithm to minimize the sum of a smooth and a simple function. It shows an application to sparse deconvolution.

## Forward-Backward Algorithm¶

We consider the problem of minimizing the sum of two functions $$E^\star = \umin{x \in \RR^N} E(x) = f(x) + g(x).$$

So, we want to find a vector $x^\star$ solution to the problem, i.e. a minimizer of $E=f+g$.

We assume that $f$ is a $C^1$ function with $\beta$-Lipschitz gradient.

We also assume that $g$ is "simple", in the sense that one can compute exactly and quickly its proximity operator, which is defined as $$\text{prox}_{\ga g}(x) = \uargmin{y \in \RR^N} \frac{1}{2}\norm{x-y}^2 + \ga g(y).$$ for any $\ga > 0$.

The forward-backward algorithm reads, after initializing $x^{(0)} \in \RR^N$, $$x^{(k+1)} = \text{prox}_{\ga g}\pa{ x^{(k)} - \ga \nabla f( x^{(k)} ) }.$$

If $0 < \ga < 2/\beta$, then this scheme converges to a minimizer of $f+g$.

## Sparse Regularization of Inverse Problems¶

We consider a linear inverse problem $$y = A x^\sharp + w \in \RR^P$$ where $x^\sharp \in \RR^N$ is the (unknown) signal to recover, $w \in \RR^P$ is a noise vector, and $A \in \RR^{P \times N}$ models the acquisition device.

To recover an estimate of the signal $x^\sharp$, we consider basis pursuit denoising, which makes use of the $\ell^1$ norm as sparsity enforcing penalty: $$\umin{x \in \RR^N} \frac{1}{2} \norm{A x-y}^2 + \la \norm{x}_1,$$ where the $\ell^1$ norm is defined as $$\norm{x}_1 = \sum_i \abs{x_i}.$$

The parameter $\la$ should be set in accordance to the noise level $\norm{w}$.

This minimization problem can be cast in the form of minimizing $f+g$ where $$f(x) = \frac{1}{2} \norm{Ax-y}^2 \qandq g(x) = \la \norm{x}_1.$$

$f$ is smooth; we have $$\nabla f(x) = A^* (A x - y),$$ which is $\beta$-Lipschitz continuous, with $$\beta = \norm{ A^* A }.$$

The $\ell^1$-norm is "simple", because its proximal operator is soft thresholding: $$\big(\text{prox}_{\ga g}(x)\big)_n = \max\pa{ 0, 1 - \frac{\la \ga}{\abs{x_n}} } x_n.$$

## Signal Deconvolution on Synthetic Sparse Data¶

A simple linearized model of seismic acquisition considers a linear filtering operator (convolution): $$A x = h \ast x$$

The filter $h$ is called the impulse response, or the poind spread function, of the acquisition process $x\mapsto Ax$.

In :
N = 1024;


We define the width of the filter $h$.

In :
s = 5;


We define $h$ as the second derivative of a Gaussian.

In :
t = -N/2 : N/2 - 1
h = (1-t.^2./s^2).*exp(-(t.^2)./(2*s^2))
h = h - mean(h);


We define the operator $A$. For simplicity, here periodic boundary conditions are used, so that the convolution is efficiently implemented as a product in Fourier domain.

In :
h_tf = fft(fftshift(h))
opA = u -> real(ifft(fft(u) .* h_tf));


We display the filter $h$ and its spectrum (amplitude of its Fourier transform).

In :
plot(t, h)
xlim(-100, 100) Out:
(-100,100)
In :
plot(t, fftshift(abs(h_tf))) Out:
1-element Array{Any,1}:
PyObject <matplotlib.lines.Line2D object at 0x324f51c50>

We generate a synthetic sparse signal $x^\sharp$, with only a small number of nonzero coefficients.

In :
srand(80);

In :
s = Int(round(N*.01)) # number of nonzero elements of xsharp
sel = randperm(N)
sel = sel[1 : s]   # indices of the nonzero elements of xsharp
xsharp = zeros(N)
xsharp[sel] = sign(randn(s)) .* (1 - 0.3*rand(s));

In :
noiselevel = 0.2;


Compute the measurements $y=A x^\sharp + w$ where $w$ is a realization of white Gaussian noise.

In :
y = opA(xsharp) + noiselevel * randn(N);

In :
fig = figure(figsize = (14, 5))
stem(sel, xsharp[sel])
xlim(0, N - 1)
title(L"signal $x^\sharp$") Out:
PyObject <matplotlib.text.Text object at 0x3250d1b50>
In :
fig = figure(figsize = (14, 5))
plot(0 : N - 1, y)
xlim(0, N - 1)
title(L"signal $y$") Out:
PyObject <matplotlib.text.Text object at 0x328cae7d0>

## Deconvolution¶

We now implement the foward-backward algorithm to recover an estimate of the sparse signal

We define the regularization parameter $\la$.

In :
Lambda = 3;


We define the proximity operator of $\ga g$.

In :
prox_gamma_g = (x, gamma, Lambda) -> x - x./max(abs(x)./(Lambda.*gamma), 1);


We define the gradient operator of $f$. Note that $A^*=A$ because the filter $h$ is symmetric.

### __Exercise: write the code of the function grad_f.__

In :
grad_f = x -> opA(opA(x)-y);


We define the Lipschitz constant $\beta$ of $\nabla f$.

In :
beta = maximum(abs(fft(h)))^2
beta

Out:
85.03218404550671

We define the stepsize $\ga$, which must be smaller than $2/\beta$.

In :
gamma = 1.9 / beta;


We compute the solution of $\ell_1$ deconvolution (basis pursuit denoising). We keep track of the energy $E_k=f(x^{(k)})+g(x^{(k)})$.

### __Exercise: write the code of the forward-backward iteration.__

In :
nbiter = 2000
x = y
En_array = zeros(nbiter + 1)

En_array = vecnorm(opA(x) - y)^2/2 + Lambda*maximum(sum(abs(x), 1))
for iter in 1 : nbiter  # iter goes from 1 to nbiter
x = prox_gamma_g(x - gamma.*grad_f(x), gamma, Lambda)
En_array[iter + 1] = vecnorm(opA(x) - y)^2/2 + Lambda*maximum(sum(abs(x), 1))
end
x_restored = x;


We display the result.

In :
fig, (subfig1, subfig2) = subplots(1, 2, figsize = (16, 7)) # one figure with two horizontal subfigures
subfig1[:stem](xsharp)
subfig2[:stem](x_restored)
subfig1[:set_title](L"$x^\sharp$")
subfig2[:set_title](L"$x_\mathrm{restored}$") Out:
PyObject <matplotlib.text.Text object at 0x32c77d850>

We plot the relative error $(E_k-E^\star)/(E_0-E^\star)$ in log-scale with respect to $k$.

In :
plot(log10((En_array[1 : 1800] - En_array[end])./(En_array - En_array[end]))) Out:
1-element Array{Any,1}:
PyObject <matplotlib.lines.Line2D object at 0x32f6d2c10>

## Over-relaxed Forward-Backward¶

It is possible to introduce a relaxation parameter $\rho$ with $0 < \rho < 1$. The over-relaxed foward-backward algorithm initializes $x^{(0)} \in \RR^N$, and then iterates, for $k=1,2,\ldots$ $$z^{(k)} = \text{prox}_{\ga g}\pa{ x^{(k-1)} - \ga \nabla f( x^{(k-1)} ) }.$$ $$x^{(k)} = z^{(k)} + \rho \pa{ z^{(k)} - x^{(k-1)} }$$

Let us assume $\gamma=1/\beta$. Convergence of the iterates $x^{(k)}$ and $z^{(k)}$ to a solution is guaranteed for $0 < \rho < 1/2$. The weaker property of convergence of $E(x^{(k)})$ to $E^\star$ is proved, when $1/2\leq \rho <1$.

### __Exercise: write the code of the over-relaxed forward-backward iteration.__

In :
gamma = 1/beta
nbiter = 1700
rho = 0.95
x = y
En_array_overrelaxed = zeros(nbiter + 1)
En_array_overrelaxed = vecnorm(opA(x) - y)^2/2 + Lambda*maximum(sum(abs(x), 1))
for iter in 1 : nbiter
xprevious = x
x = prox_gamma_g(x - gamma.*grad_f(x), gamma, Lambda)
x += rho .* (x - xprevious)
En_array_overrelaxed[iter + 1] = vecnorm(opA(x) - y)^2/2 + Lambda*maximum(sum(abs(x), 1))
end
x_restored = x;

In :
plot(log10((En_array[1 : 1700] - En_array[end])./(En_array - En_array[end])))
plot(log10((En_array_overrelaxed[1 : 1700] - En_array[end])./(En_array - En_array[end])))
# I changed 1800 to 1700 because En_array_overrelaxed is of length 1701 Out:
1-element Array{Any,1}:
PyObject <matplotlib.lines.Line2D object at 0x32f7df3d0>

As we see, in this example, over-relaxation does not bring any speedup, because $\gamma$ is lower than without over-relaxation. There are other setting parameters or other problems, for which over-relaxation does bring a significant speedup.

## FISTA-like Accelerated Forward-Backward Algorithm¶

We consider the FISTA algorithm introduced in:

A. Beck and M. Teboulle, "A Fast Iterative Shrinkage-Thresholding Algorithm for Linear Inverse Problems", SIAM Journal on Imaging Sciences, 2009.

More precisely, we consider a slightly modified version of FISTA, whose convergence is proved, see A. Chambolle and C. Dossal, "On the convergence of the iterates of "FISTA"", preprint, 2015.

Given an initial estimate $x^{(0)}$ of the solution and a parameter $a>2$, the algorithm sets $\gamma=1/\beta$, sets $z^{(0)}=x^{(0)} \in \RR^N$, and iterates, for $k=1,2,\ldots$ $$x^{(k)} = \text{prox}_{\ga g}\pa{ z^{(k-1)} - \ga \nabla f( z^{(k-1)} ) }.$$ $$\alpha_k=(k-1)/(k+a)$$ $$z^{(k)} = x^{(k)} + \alpha_k \pa{ x^{(k)} - x^{(k-1)} }$$

It is proved that the iterates $x^{(k)}$ converge to a solution $x^\star$ of the problem. Moreover, the optimal convergence rate for this class of problems is reached, namely $$E_k - E^\star = O(1/k^2),$$ whereas the convergence rate for the normal forward-backward is only $O(1/k)$.

Note the difference between the over-relaxed forward-backward and the accelerated forward-backward: the later is based on an inertia mechanism, of different nature than over-relaxation.

### __Exercise: write the code of the accelerated forward-backward iteration.__

In :
gamma = 1/beta
nbiter = 1700
a = 10
x = y
En_array_fista = zeros(nbiter + 1)
En_array_fista = vecnorm(opA(x) - y)^2/2 + Lambda*maximum(sum(abs(x), 1))
z = x
for iter in 1 : nbiter
xprevious = x
x = prox_gamma_g(z - gamma.*grad_f(z), gamma, Lambda)
alpha = iter/(iter + 1 + a)
z = x + alpha .* (x - xprevious)
En_array_fista[iter + 1] = norm(opA(x) - y)^2/2 + Lambda*maximum(sum(abs(x), 1))
end
x_restored = x;

In :
plot(log10((En_array[1 : 1700] - En_array[end])./(En_array - En_array[end])))
plot(log10((En_array_overrelaxed[1 : 1700] - En_array[end])./(En_array - En_array[end])))
plot(log10((abs(En_array_fista[1 : 1700] - En_array[end]))./(En_array - En_array[end]))) Out:
1-element Array{Any,1}:
PyObject <matplotlib.lines.Line2D object at 0x32f9947d0>

### __Exercise: try different values of a. You can try a=3,10,30,50,100.__

We can note that the accelerated forward-backward is not monotonic: the cost function E is not decreasing along with the iterations and some oscillations are present.