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Deep plug-and-play priors for spectral snapshot compressive imag...
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We propose a plug-and-play (PnP) method that uses deep-learning-based denoisers as regularization priors for spectral snapshot compressive imaging (SCI). Our method is efficient in terms of reconstruction quality and speed trade-off, and flexible enough to be ready to use for different compressive coding mechanisms. We demonstrate the efficiency and flexibility in both simulations and five different spectral SCI systems and show that the proposed deep PnP prior could achieve state-of-the-art res
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Deep plug-and-play priors for spectral snapshot
compressive imaging
SIMING ZHENG,
1,2,†
YANG LIU,
3,†
ZIYI MENG,
4
MU QIAO,
5
ZHISHEN TONG,
6,7
XIAOYU YANG,
1,2
SHENSHENG HAN,
6,7
AND XIN YUAN
8,
*
1
Computer Network Information Center, Chinese Academy of Sciences, Beijing 100190, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
3
Computer Science and Artificial Intelligence Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
4
Beijing University of Posts and Telecommunications, Beijing 100876, China
5
New Jersey Institute of Technology, Newark, New Jersey 07102, USA
6
Key Laboratory for Quantum Optics and Center for Cold Atom Physics of CAS, Shanghai Institute of Optics and Fine Mechanics,
Chinese Academy of Sciences, Shanghai 201800, China
7
Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
8
Nokia Bell Labs, Murray Hill, New Jersey 07974, USA
*Corresponding author: xyuan@bell-labs.com
Received 5 October 2020; revised 20 November 2020; accepted 23 November 2020; posted 23 November 2020 (Doc. ID 411745);
published 21 January 2021
We propose a plug-and-play (PnP) method that uses deep-learning-based denoisers as regularization priors for
spectral snapshot compressive imaging (SCI). Our method is efficient in terms of reconstruction quality and speed
trade-off, and flexible enough to be ready to use for different compressive coding mechanisms. We demonstrate
the efficiency and flexibility in both simulations and five different spectral SCI systems and show that the pro-
posed deep PnP prior could achieve state-of-the-art results with a simple plug-in based on the optimization frame-
work. This paves the way for capturing and recovering multi- or hyperspectral information in one snapshot,
which might inspire intriguing application s in remote sensing, biomedical science, and material science. Our
code is available at: https://github.com/zsm12 11/PnP-CASSI.
© 2021 Chinese Laser Press
https://doi.org/10.1364/PRJ.411745
1. INTRODUCTION
Real scenes are spectrally rich. Capturing the color, and thus the
spectral information, has been a central issue since the dawn of
photography. Correspondingly, many strategies have been con-
sidered. Since the advent of solid-state imaging, the color filter
array and especially the red–green–blue (RGB) bayer filter have
been the dominant strategy [1]. These filter arrays usually only
capture red, green, and blue bands and thus limit the spectral
resolution. When the number of sampled wavelengths becomes
large, bandpass filters, push-room, and other strategies may be
desirable. These systems usually have limited temporal resolu-
tion due to the inherent scanning procedure. Advances in
photonics and 2D materials give rise to compact solutions
to single-shot spectrometers at a high spectral resolution
[2–5]. More recently, it has been applied for spectral imaging
via combining stacking [6], optical parallelization [7], and com-
pressive sampling [8] strategies, where the trade-off between the
spatial pixel and spectral resolution still remains a challenge.
Thanks to compressive sensing (CS) [9 –11] and the advent
of decompressive inference algorithms over the past couple
of decades, there is substantial interest in hyperspectral color
filter arrays [12–14]. Such sampling strategies capture localized
coded image features and are well-matched to sparsity-based
inference algorithms [15–17]. With these advanced algorithms,
this technique has led to single-shot imaging for hyperspectral
images (HSIs), and we dub it snapshot compressive imaging
(SCI) [16,18]. In this paper, we focus on the s pectral SCI,
which aims to measure the x, y, λ data cube.
Spectral SCI is a hardware encoder plus software decoder
system, where the hardware encoder denotes the optical system,
which compresses the 3D x, y, λ data cube to a snapshot mea-
surement on the 2D detector, and the software decoder denotes
the reconstruction algorithms used to recover the 3D data cube
from the snapshot measurement.
The underlying principle of the spectral SCI hardware is to
modulate different bands (corresponding to different wave-
lengths) in the spectral data cube by different weights and then
integrate the light to the sensor. To perform the modulation,
which should be different for different spectral bands, various
techniques have been used. The pioneer work of coded aperture
snapshot spectral imaging (CASSI) [12] used a fixed mask (coded
aperture) and two dispersers to implement the band-wise
B18
Vol. 9, No. 2 / February 2021 / Photonics Research
Research Article
2327-9125/21/020B18-12 Journal © 2021 Chinese Laser Press
modulation, termed DD-CASSI; here DD means dual disperser.
Following this, the single-disperser (SD) CASSI was developed
[19], which achieves modulation by removing a disperser.
Following CASSI, various spectral SCI systems have been built
using disperser/prism and masks [20–24]. Recently, motivated
by the spectral variant responses of other media, spatial light mod-
ulators [25], ground-glass-based light field modulation [26], and
scatters [27] have also been employed for spectral SCI. In addi-
tion, some compact systems have also been built [28,29].
The software decoder, i.e., the reconstruction algorithm,
plays a pivotal role in spectral SCI as it outputs the desired data
cube. At the beginning, optimization-based algorithms devel-
oped for inverse problems such as CS were employed. Since
spectral SCI is an ill-posed problem, regularizers or priors
are generally used, such as the sparsity [30] and total variation
[15]. Later, the patch-based methods such as dictionary learn-
ing [25,31] and Gaussian mixture models [32] were developed
for the reconstruction of spectral SCI. Recently, by utilizing the
nonlocal similarity in the spectral data cube, group sparsity [17]
and low-rank models [16] have been developed to achieve state-
of-the-art results. The main bottleneck of these high perfor-
mance iterative optimization-based algorithms is the low
reconstruction speed. Since the spectral data cube is usually
large-scale, sometimes it needs hours to reconstruct a spectral
data cube from a snapshot measurement. This precludes the
real applications of spectral SCI systems.
To address the above speed issue in optimization algorithms,
and inspired by the performance of deep-learning approaches
for other inverse problems [33,34], convolutional neural net-
works (CNNs) have been used to solve the inverse problem of
spectral SCI for the sake of high speed [35–39]. These net-
works have led to better results than their optimization counter-
parts, given sufficient training data and time, which usually take
days or weeks. After training, the network can output the
reconstruction instantaneously and thus lead to end-to-end
spectral SCI sampling and reconstruction [39]. However, these
networks are usually system-specific. For example, different
numbers of spectral bands exist in different spectral SCI sys-
tems. Further, due to the different designs of masks, the trained
CNNs cannot be used in other systems, while retraining a new
network from scratch would take a long time.
Bearing the above concerns in mind, i.e., optimization-
based and deep-learning-based algorithms each have their
own pros and cons, it is desirable to develop a fast, flexible,
and high accuracy algorithm for spectral SCI. Fortunately,
the plug-and-play (PnP) framework [40,41] has been proposed
for inverse problems with provable convergence [42,43]. The
idea of PnP is intuitive, since the goal is to use the state-of-the-
art denoiser as a simple plug-in for recovery. The rationale here
is to employ recent advanced deep denoisers [44–
46] in the
iterative optimization algo rithm to speed up the reconstruction
process. Since these denoisers are pretrained with a wide range
of noise levels, the PnP algorithm is very efficient and usually
only tens or hundreds of iterations would provide promising
results [18]. More importantly, no training is required for dif-
ferent tasks and thus the same denoising network can be di-
rectly used in different systems. Therefore, PnP is a good
trade-off for reconstruction quality, speed, and flexibility.
However, since most existing flexible denoising networks are
designed for natural images, i.e., the gray-scale or RGB images,
directly using these networks into spectral SCI systems would
not lead to good results. To address this issue, in this paper, we
propose training a flexible denoising network for multispectral/
HSIs and then apply it to the PnP framework to solve the
reconstruction problem of spectral SCI.
Our proposed approach enjoys the advantages of speed, flex-
ibility, and high accuracy. We apply the proposed method in five
different real systems (three SD-CASSI systems [39,47,48], one
mutispectral endomicroscopy system [36], and one ghost imag-
ing spectral system [26]) and all of them have achieved promising
results. To compare with other state-of-the-art algorithms, sim-
ulations are also conducted to provide quantitative analysis.
Spectral sensor design and fabrication [2,4–8]maybenefitfrom
our method by taking inspiration from the coding mechanisms
and the simple plug-in for recovery.
Note that the PnP framework has been used in other inverse
problems such as video CS [18], which emphasized the theo-
retical analysis of PnP for SCI problems in general and used an
off-the-shelf denoiser (FFDNet) [46] to demonstrate its
capability in video SCI. No spectral SCI results have been
shown therein because spectral SCI is more challenging in
terms of its various coding mechanisms and no off-the-shelf
denoiser could provide a fast, flexible, and high-accuracy sol-
ution. As a matter of fact, this observation serves as the initial
motivation for this paper. Towards this end, the novelty of this
paper is twofold. First, we propose a CNN-based deep spectral
denoising network as the spatio-spectral prior, which is flexible
in terms of data size and the input noise levels. Second, we
summarize the image-plane and aperture-plane coding mech-
anisms for spectral SCI and use the PnP method combined
with our proposed deep spectral denoising prior for both
simulations and five different spectral SCI systems (including
image-plane and aperture-plane coding-based ones).
The paper is organized as follows. Section 2 introduces dif-
ferent spe ctral SCI systems. The proposed PnP method is de-
rived in Section 3. Extensive results are shown in Section 4, and
Section 5 concludes the entire paper.
2. SPECTRAL SCI
The basic idea of SCI is to encode 3D or multidimensional
visual information onto 2D sensor measurement. For spectral
SCI, a 3D spatio-spectral data cube is encoded to form a snap-
shot 2D measurement on the charge coupled device (CCD) or
complementary metal oxide semiconductor (CMOS) sensor, as
shown in Fig. 1.
A. SCI Forward Model
The forward model of SCI is linear. For spectral SCI, the spec-
tral data cube of the scene X ∈ R
W ×H×B
, where W , H , and B
denote the width, height, and the number of spectral bands,
respectively, is encoded onto a single 2D measurement
Y ∈ R
W ×H
(or similar size) via spectrally variant coding. By
vectorizing the scene’s spectral cube and measurement, that
is, x vecX ∈ R
WHB
and y vecY ∈ R
WH
, we can
form a linear system for spectral SCI,
y Ax ε, (1)
Research Article
Vol. 9, No. 2 / February 2021 / Photonics Research B19
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