DU et al.: PLTD: FOR HSI 69
projection [54]. Although, at first glance, these methods may
appear suitable for HSI compression and reconstruction, they
are basically designed for a set of tensor objects, and they can-
not perform feature reduction with only single tensor data as the
input.
To fully address the advantages of tensor representation for
HSI compression, we propose a novel patch-based low-rank
tensor decomposition (PLTD) method, which combines patch-
based tensor representation and tensor dictionary learning. In-
stead of compressing the HSI by spectral channel or by pixel,
we represent each local patch of the HSI as a third-order tensor,
which helps to preserve both the spatial and spectral correlations.
Matrix dictionary learning [55] aims to decompose the observed
data matrix to a dictionary and a sparse coefficient, respectively,
which leads to a low-rank representation of the input matrix. In
order to share the nonlocal similarity over the spatial domain
[56], [57], the similar tensor patches are grouped by a cluster-
ing algorithm, and we construct a fourth-order tensor per cluster.
Since the grouped tensor is assumed to be redundant, by extend-
ing the matrix dictionary learning to tensor dictionary learning,
each cluster can be low-rank decomposed to a sparse coefficient
tensor and three dictionary matrices, respectively. Since the co-
efficient tensor is sparse, we proposed to extract all the nonzero
entries to form an intrinsic dense tensor whose dimensions along
the two spatial modes and an additional spectral mode are much
smaller than the original cluster, thus leading to simultaneous
compression of both the spatial and spectral domains. Finally,
the reconstructed HSI is recovered by the combination of the
reconstructed patches, which are obtained by the tensor product
of the coefficient and dictionaries per cluster. In this way, the
proposed PLTD algorithm optimally removes the redundancy in
both the spatial and spectral domains in a unified framework.
The proposed PLTD algorithm can overcome the shortcom-
ings of the existing gray-level based image compression meth-
ods. For example, JPEG and SPIHT can only deal with the
spatial redundancy, while the high dimension of the spectral
channel is left unchanged; PCA treats each pixel vector as an
individual observation, and thus only the spectral dimension is
considered while the redundancy in the spatial dimensions is ig-
nored in the compression, which results in a poor compression
ratio in practice; MPCA is not suitable for single tensor data
as the input if it is used to address both the spatial and spec-
tral redundancies. In summary, these methods cannot take full
advantage of the characteristics of HSIs. In contrast, the pro-
posed PLTD algorithm is expected to be able to overcome these
limitations. Patch-based tensor representation helps to preserve
both the spatial and spectral correlations, and the tensor dictio-
nary learning and low-rank decomposition make it possible to
simultaneously remove the redundancy in both the spectral and
spatial domains.
The main contributions of the proposed PLTD algorithm are
summarized as follows:
1) A patch-based tensor representation for HSI data com-
pression is proposed, and the nonlocal similarity measure
of nonlocal patches in the spatial domain is employed. A
tensor can describe 3D data as an entirety, keeping the
spatial and spectral correlations at the same time. In the
PLTD algorithm, the HSI cube is split into many over-
lapping tensor patches in the spatial domain by a fixed
local window, while the spectral signature is unchanged
in the patch-based tensor representation. In this way, the
proposed method can keep as much of the spatial and
spectral correlation as possible, which may be very im-
portant in the subsequent HSI analysis. Through the for-
mulated nonlocal similarity, similar patches are grouped
together. Patches in a certain cluster can be considered as a
fourth-order tensor, which helps to share the nonlocal sim-
ilarity among patches and leads to a better compression
performance.
2) The tensor dictionary learning is generalized based on the
matrix dictionary learning, and a new definition of tensor
sparsity is provided to support this generalization. By us-
ing the suggested tensor sparsity, we can simultaneously
enforce the sparse constraint of an HSI data patch on the
height, width, and spectral modes. Based on this sparsity,
the matrix dictionary learning is extended to the tensor-
based application, and the tensor cluster is decomposed to
the sparse coefficient tensor and a few matrix dictionaries.
With the constraint that the coefficient should be sparse
in both the spatial and spectral modes, the data size of the
intrinsic dense tensor is far less than the original tensor,
and thus both the spatial and spectral redundancies can be
compressed.
3) A low-rank tensor decomposition based optimization al-
gorithm is developed. In the proposed method, in order to
find the optimal coefficient tensor and the corresponding
dictionaries for each cluster, we extend the matrix-based
dictionary learning to a tensor version according to multi-
linear algebra, and we further develop an optimization ap-
proach based on low-rank tensor decomposition. Through
such tensor decomposition, an HSI is split into numerous
clusters, while each of the clusters is represented by a co-
efficient tensor and its dictionaries, by which the data size
of the coefficient tensor is far less than the original cluster
from the HSI.
The rest of this paper is organized as follows. Section II
introduces the notation and some basic multilinear algebra.
Section III presents the PLTD algorithm for HSI compression
and reconstruction. The experimental results and analysis are
provided in Section IV, followed by the summary and conclu-
sion in Section V.
II. N
OTATION AND BASIC MULTILINEAR ALGEBRA
In this section, we introduce the notation and multilinear
algebra used in the paper. Since a tensor is a multidimensional
array of numbers that transforms linearly under a coordinate
transformation, we distinguish the vectors (first-order tensor),
matrices (second-order tensor), and tensors which are higher
than the second-order as follows. A vector is denoted as an
italic lowercase letter such as a, a matrix is denoted as a bold
italic uppercase letter such as A, and an Nth-order tensor is
denoted as a calligraphic uppercase letter A∈R
I
1
× I
2
× ···× I
N
.
The elements of an object (including a vector and matrix) are