1. Introduction
Hyperspectral imaging instruments collect information by exploring the electromagnetic spectrum of a specific geographical area. In contrast to the human eye and traditional camera sensors, which can only perceive visible light (i.e., the wavelengths between 360 to 760 nanometers (nm)), spectral imaging techniques allow to cover a significant portion of wavelengths (i.e., the frequencies of ultraviolet and infrared rays). It is important to note that the spectrum is subdivided into different spectral bands. Therefore, hyperspectral images can be viewed as three-dimensional data (often referred asdatacubes).
For instance, the
Airborne Visible/Infrared Imaging Spectrometer (
AVIRIS) [
1] hyperspectral sensor (NASA Jet Propulsion Laboratory (JPL) [
2]) measures from 380 to 2500 nm of the electromagnetic spectrum. In particular, the spectrum is subdivided into 224 spectral bands.
From the analysis of hyperspectral data, it is possible to identify and/or classify materials, objects,etc. Such capabilities are related to the fact that some objects and materials have a unique signature (a sort offingerprint) in the electromagnetic spectrum, therefore this fingerprint can be used for identification purposes.
Hyperspectral data are widely used in real-life applications including agriculture, mineralogy, physics, surveillance,etc. For instance, in geological applications the capabilities of hyperspectral remote sensing are exploited to identify various types of minerals or to search for minerals and oil.
One of the most important parameters to evaluate the precision of a sensor is the spectral resolution, which is the width between two adjacent bands. For instance, by considering the AVIRIS hyperspectral images, the spectral resolution is 10 nm. The spatial resolution is a relevant aspect too. Informally, the spatial resolution denotes how extensive is the geographical area mapped by the sensor into a pixel. It could be difficult to recognize materials and/or objects from a pixel, if a too wide area is mapped into it.
Many hundreds of gigabytes can be produced every day by a single hyperspectral sensor. Therefore, it is necessary to compress these data, in order to transmit and to store them efficiently. Since such data are often used in delicate tasks and there are high costs involved in the acquisitions, lossless compression is generally required.
This paper focuses on a novel technique for the lossless compression of hyperspectral images. The proposed algorithm is based on the predictive coding model and the proposed predictive structure uses a configurable multiband three-dimensional structure. It is possible to customize our predictor by individuating the number of the previous bands which will be used as references and the wideness of the prediction context. Through appropriate configurations of such parameters, the computational complexity and the memory usage can be optimized depending on the hardware available.
Because of its high configurability, our algorithm is suitable for “on board” implementations on hardware with limited capabilities, as for example on an airplane or on a satellite.
The experimental results we have achieved are comparable and often better, with respect to other state of the art approaches. Our scheme provides a good trade-off between computational complexity/memory usage and compression performances.
The rest of the paper is organized as follows:
Section 2 briefly reviews previous work on lossless and lossy compression of hyperspectral images,
Section 3 outlines the proposed lossless compression approach and
Section 4 focuses on the description of experimental results. Finally,
Section 5 highlights our conclusion and future work directions.
2. Related Works
Lossless compression of hyperspectral images is generally based on the predictive coding model. The predictive-based approaches have different advantages: they use limited resources in terms of computational power and memory usage and achieve good compression performances. Often, these models are suitable for on board implementations.
Spectral-oriented Least SQuares (SLSQ) [
3], Linear Predictor (LP) [
3], Fast Lossless (FL) [
4], CALIC-3D [
5], M-CALIC [
5], and RLS [
6] are among the state-of-art predictive-based techniques.
The
Consultative Committee for Space Data Systems (
CCSDS) has specified the
CCSDS 123 standard, which outlines a method for lossless compression of multispectral and hyperspectral image data and a format for storing the compressed data [
7,
8]. The main objective is to establish a Recommended Standard for a multispectral and hyperspectral images, and to specify the compressed data format. In literature, many proposed approaches implement the recommendations of the CCSDS 123 standard for the lossless compression of hyperspectral images, as for instance, the ones described in [
9,
10,
11].
Other approaches are designed for offline compression, since they use more sophisticated techniques and/or they require the complete availability of the hyperspectral image. These approaches are not suitable for an on board implementation but can achieve better compression performances. Mielikainen, in [
12], proposed an approach for the compression of hyperspectral image through Look-Up Table (LUT). LUT predicts each pixel by using all the pixels in the current and in the previous band, by searching the nearest neighbor, in the previous band, which has the same pixel value as the pixel located in the same spatial coordinates as the current pixel. LUT has high compression performances, but it uses more resources in terms of memory and CPU usage.
Other lossless techniques are based on dimensionality reduction through principal component transform [
13] or they are based on the clustered differential pulse code modulation [
14]. An error-resilient lossless compression technique is proposed in [
15].
For the lossy compression of hyperspectral images, the compression algorithms are generally based on 3D frequency transforms: as for examples 3-D Discrete Wavelet Transform (3D-DWT) [
16], 3-D Discrete Cosine Transform (3D-DCT) [
17], Karhunen–Loève transform (KLT) [
18],
etc. These approaches are easily scalable. On the other hand, they must maintain the entire hyperspectral image at the same time in memory. Locally optimal Partitioned Vector Quantization (LPVQ) [
19,
20] applies a Partitioned Vector Quantization (PVQ) scheme independently to each pixel of the hyperspectral image.
The variable sizes of the partitions are chosen adaptively and the indices are entropy coded. The codebook is included as part of the coded output. This technique can be used also in lossless mode, but the high costs required in terms of CPU and memory do not allow an on board implementation.
3. Lossless Multiband Compression for Hyperspectral Images (LMBHI)
Hyperspectral images present two typologies of correlations:
inter-band correlation;
intra-band correlation.
In particular, contiguous bands are strongly correlated (inter-band correlation) and the pixels are generally correlated, since, for instance, two adjacent pixels map adjacent areas, possibly composed of the same material,etc. (intra-band correlation). Such characterizations are exploited by the compression strategies, in order to optimize the redundancy among the third dimension. The main aim of our approach, which we denoted asLossless MultiBand compression for Hyperspectral Images (LMBHI), is to exploit the correlation with a predictive coding model.
In detail, for each pixel,, of the input hyperspectral image, LMBHI performs the prediction of the current pixel,, by selecting the appropriate prediction context ofX (three-dimensional or a bi-dimensional contexts).
All the pixels that belong to the first band are predicted by using a bi-dimensional predictive structure: the 2-D Linearized Median Predictor (2-D LMP) [
21], which exploits only the intra-band correlation, since the first band has no reference bands. The other pixels are predicted by using a new three-dimensional predictive approach, which uses a prediction context composed of the neighboring pixels of
and its reference pixels in the previous bands.
Once the prediction step is computed, the prediction error
(defined in Equation (1)) is modeled and coded.
3.1. Review of the 2-D Linearized Median Predictor (2D-LMP)
The
2-D Linearized Median Predictor (
2D-LMP) [
21] uses a prediction context that is composed by three neighboring pixels of
, namely,
,
, and
, as shown in
Figure 1. In particular, the predictive structure is derived from the well-established
2-D Median Predictor, which is used in JPEG-LS [
22]. The 2-D Median Predictor has the following predictive structure outlined in the Equation (2).
Basically, Median Predictor is in charge of selecting one of the above three options, depending on the context. By combining all the three options, it is possible to obtain the predictive structure of 2D-LMP, defined as in the Equation (3).
3.2. 3-D MultiBand Linear Predictor (3D-MBLP)
The Multiband Linear Predictor (3D-MBLP) uses a prediction context by considering two parameters:
: number of the previous bands, that are considered for the prediction;
: number of the samples for the current and each previous band, which will be used for the creation of the prediction context.
First of all, we define a bi-dimensional enumeration
, graphically represented in
Figure 2. The main aim of such an enumeration, is to permit the relative indexing of the pixels with respect to the pixel which is currently under analysis (which has
as index in
Figure 2).
In order to define the prediction context of 3D-MBLP, we use the following notations:
: indicates the-th pixel of the-th band, according to the enumeration;
: denotes the pixel that has the same spatial coordinates of, of thej-th band, according to the enumeration.
In the following, we suppose that the current band is the
-th band. In particular, by using our notations, it is possible to observe that
can be also addressed as
In detail, the 3D-MBLP predictor is based on the least squares optimization technique and the prediction is computed by means of the Equation (4).
The coefficients:
are chosen to minimize the energy of the prediction error described by the Equation (5).
can be rewritten in matrix notation by means of the following equation:
where
and
.
Subsequently, by taking the derivate of
and by setting it to zero, we obtain the optimal coefficients by means of the Equation (6).
Once the coefficients, which solve the linear system Equation (6), are computed, the prediction,, of the current pixel, can be calculated.
3.3. Modeling and Coding of Prediction Errors
Starting from the consideration that a prediction error can assume positive or negative values. Similarly to [
23], we use an invertible mapping function (highlighted in the Equation (7)), in order to have only non-negative values. It is important to note that the mapping function does not alter the redundancy among the errors. For the coding of the mapped prediction errors we use the Arithmetic Coder (AC) scheme.
3.4. Computational Complexity
The main computational costs of our approach are due to the resolution of the linear system Equation (6), used to generate the optimal coefficients, which need the computation of the predicted pixel. By using the normal equation method, the linear system can be solved with
floating-point operations [
24].
Figure 3 shows the trend of the computational complexity of our predictive model, in terms of number of operations (
-axis) that are required for the solving of the linear system, by using configurations with different parameters (
-axis).
If we use only the previous band as a reference (
), only about 20 operations are needed to solve the system. Instead, four or nine times more operations are required if we use two previous bands (
) or three previous bands (
). A linear system can have three kinds of solutions: no solutions, one solution, and infinity solutions. In the first and the third scenarios, the proposed predictive structure cannot perform the prediction. In these scenarios, it is desirable to use another low-complexity predictive structure and we have used the
3-D Distances-based Linearized Median Predictor (
3D-DLMP) [
21].
5. Conclusions and Future Works
In this paper, we have investigated on the lossless compression of hyperspectral images by introducing a multiband three-dimensional predictive structure, we named as 3D-MBLP.
Because of its configurability, it is possible to implement the algorithm on different typologies of sensors, by using appropriate configuration for each type of sensors. Moreover, the proposed approach can be also easily scaled for future generation sensors which will have better hardware capabilities. The experimental results we achieved are comparable and often outperform the other state of the art lossless compression techniques.
In future works, we will include a pre-processing stage before the compression of the hyperspectral image, which substantially reorders the bands by considering their correlation. This will possibly improve the compression performance as in [
28,
29].