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670 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 57, NO. 2, FEBRUARY 2019
Fig. 1. Architecture of the proposed HSI-DeNet.
as ADMM [38], by introducing the auxiliary variable A to
solve (2)
A
(k+1)
= shrink
α
(DX
(k)
+ αJ
(k)
) (3)
where D is the sparse transformation operator, A is the
auxiliary variable that can be approximately equivalent to X,
J can be regarded as the compensating variation, α is the
regularization parameter, shrink
α
is the soft shrinkage operator,
and k is the iteration number.
We can observe that (1) and (3) are very similar to each
other. Both of them obtain the desired solution gradually via
a linear transformation and then nonlinear activation function.
The number of the recursion depends on the depth of the
deep model and the iteration of the optimization method. This
intrinsic similarity can partially explain why the deep model is
also suitable for the image restoration task. However, the trans-
formation parameters in the CNN model are adaptively learned
to implicitly fit the distribution of the training data set, which
makes them more professional for a specific task.
B. Problem Formulation
The noise degradation model in this paper is formulated as
Y = X + N (4)
where Y ∈ R
R×C×B
is the measured HSI, R, C,andB stand
for the numbers of the row, column, and band respectively,
X is the desired clear HSI, and N is the noise in HSI, which
includes various noise components. Note that the goal of this
paper is to estimate the residual noise component N, not the
clear image, from the degraded image Y. The main reason is
that we adopt the residual learning idea from [19] and [22] to
train a residual mapping F (Y) = N. The restoration problem
is formulated as a regression task as follows:
J
Recon
=
1
2
F (Y ) − N
2
(5)
where F is the composite network mapping function of S.
Fig. 2. Illustration of one block. Each block contains the convolution, BN,
and nonlinear response.
C. Architecture of HSI-DeNet
In the proposed HSI-DeNet, we use a very deep con-
volutional network followed by [19], [50], and [53]. Each
convolutional layer consists of M
d
filters with the size of
3×3× N , except the first and the last output layer. The channel
of the first and the last output layer has to match the spectral
dimension of the input HSI. We use a 3×3 filters throughout
the network with stride 1, which has been demonstrated that
the decomposition of larger size filters into small-size filters
with deeper layers would make the model more discriminative
[19], [20], [22], [50], [53]. To avoid the boundary effect and
preserve the spatial size, we pad each layer with the same size
as the original image.
The architecture of the HSI-DeNet is shown in Table I.
Each block contains three components: convolutional, batch
normalization (BN), and RELU, as shown in Fig. 2. We denote
the Convolutional(C) + Batch normalization(B) + RELU(R)
block as CBR. The depth D of HSI-DeNet is 19 (including the
L
2
loss layer). The main reasons for us to choose the depth as
19 are threefold. On the one hand, the depth in the CNN model
is similar to the iteration number in optimization-based meth-
ods. Many works [20], [54] have discussed their relationship
and design their deep architecture based on the optimization
solvers. Since the iteration number of the nonconvex problem