Underwater imagery and transmission possess numerous challenges like lower signal bandwidth, slower data transmission bit rates, Noise, underwater blue/green light haze etc. These factors distort the estimation of Region of Interest and are prime hurdles in deploying efficient compression techniques. Due to the presence of blue/green light in underwater imagery, shape adaptive or block-wise compression techniques faces failures as it becomes very difficult to estimate the compression levels/coefficients for a particular region. This method is proposed to efficiently deploy an Extreme Learning Machine (ELM) model-based shape adaptive Discrete Cosine Transformation (DCT) for underwater images. Underwater color image restoration techniques based on veiling light estimation and restoration of images followed by Saliency map estimation based on Gray Level Co-occurrence Matrix (GLCM) features are explained. An ELM network is modeled which takes two parameters, signal strength and saliency value of the region to be compressed and level of compression (DCT coefficients and compression steps) are predicted by ELM. This method ensures lesser errors in the Region of Interest and a better trade-off between available signal strength and compression level.

Performance of underwater imagery system is influenced by channel characteristics like, limited bandwidth, blue/green lighting haze and noisy distortions due to lengthy cables between imaging device and storage device. Similarly, the transmission of underwater images under lower bandwidth and signal strength is a challenge and requires efficient compression techniques with minimum errors for reconstructed images or requires Spatial Modulation Schemes [

Another Adaptive method of Block Compression Sensing (BCS) was proposed [

The above compression methods are block-wise compression techniques with fixed compression ratios or non-block-wise methods with different compression levels applied on the whole image. In recent years, there has been a lot of progress in several areas of learning technology, particularly image processing and computer vision. However, video compression learning is still in its infancy. This article looks at the collaborative work on learning for image and video codecs that has been ongoing for several years. For underwater images, a method of deriving saliency maps post veiling light removal and block-wise compression of regions by finding compression coefficients using an ELM-based network.

DCT is a straightforward transform technique. Some information is lost to compress an image. As a result, DCT classifies the spectral sub-bands of an image into the most significant frequency coefficients and the least important frequency coefficients. Environmental variables such as optical Noise, wave disturbances, light stability and equality, temperature changes, and other factors may all impact underwater photography quality, making underwater domain recording one of the most difficult undertakings. Various can use image processing techniques to process data and discover the best items and crucial features in pictures. Optical scattering is one of the primary difficulties in the underwater environment that can produce different distortions, including morphological change in the underwater image.

The benefit of getting DCT instead of Discrete Fourier Transform (DFT) is that it requires fewer total multiplications. When truncating frequency coefficients, DFT loses the form of the signal. Due to its continuous periodic characteristic, DCT preserves the form of the signal even after frequency coefficients are shortened. A concept for underwater photography, whether done by divers or with other specialist equipment However, most of the time, the image looked fuzzy. As a result, the image falls short of the viewers’ expectations. Certain processes must be carried out to get the image’s maximum perfection.

And the only multiplications involved in DCT are actual multiplications. Another benefit is that the lower frequency components store the majority of the signal energy compression. This characteristic of DCT arose from the image compression standards JPEG and MPEG. The undersea surface is a complex concept of image processes analysis. The surface has numerous aspects that make it more appealing than a terrestrial surface.

The Discrete Cosine Transform (DCT) breaks down a signal into its basic frequency components. It’s a popular image compression method. We create some basic methods to compute the DCT and compress pictures in this section. In all fields of digital signal processing, the Discrete Fourier Transform (DFT) is critical. It’s used in compression, filtering, and feature extraction to get a signal’s frequency-domain (spectral) representation. An image may be decomposed into its constituent parts using DCT.

Underwater imagery is a vital tool for documenting and reconstructing ecologically or historically significant locations that are inaccessible to the general public and scientific community. A range of technological and advanced equipment is utilized to get underwater imagery, including sonar, thermal imaging, an optical camera, and a laser. The discrete cosines transformation is widely used in image compression and video frame compression. This type of transformation will transfer a signal from the spatial domain to the time domain. DCT is suited for real-time applications because of its increased throughput. All Joint Photographic Experts Group (JPEG) image compression and Moving Picture Experts Group (MPEG) video compression formats. High-quality optical cameras are the preferred method for various computer vision tasks in contemporary underwater photography, covering from scene reconstruction and navigation to intervention tasks. However, processing underwater pictures and maintaining high levels of image quality might have some significant downsides.

Underwater images suffer from veiling light which makes it very difficult to determine the ROI. Adding to veiling light, other factors that affect underwater image quality are color distortion, low contrast, blurriness, scattering effects and white color disbalancing, propagated light attenuation, and light scattering due to depth requires multiple repeated shots of the same object. The resulting underwater images have poor object boundaries making it difficult to recognize ROIs. This deeply affects the shape adaptive compression algorithms. As most shape adaptive compression techniques work on the saliency maps to detect the ROI, underwater image distortions give wrong saliency maps. This leads to the wrong estimation of the compression levels of objects of interest.

The first objective of this work is to restore the underwater color image of the veiling light to estimate the correct saliency values of the ROI. Another challenge in underwater sea picture imagery is the limited bandwidth of satellite/data uplinks which varies from place to place in the sea and weather. Hence compression levels to be applied on the ROI and rest of the region is dependent on the available bandwidth requires the computation of compression coefficient vis a vis input signal strength/bandwidth of channel transmitting the image and saliency value of the region. Several learning methods are available for the computation of compression coefficients. Still, most of them suffer from a typical problem like unsuitability when the target values overlap for given inputs, slow speed of learning network development, inability to expand the network when larger datasets arrive for training/testing, and the utmost prime issues like initializing the weights. To overcome this an ELM network is proposed to estimate the DCT coefficients. This reduces the training time and also, the hidden layers can be randomly assigned and is free from problems like finding local minima.

Assuming an underwater camera, capturing the images in the deep sea with artificial lights. The use of artificial lights added to the veiling light components causes’ background scattering and also underwater floating particles generate unwanted noises and cause dimming of the captured images [

Any pixel coordinate (m, n) captured by the camera can thus be represented by a superposition model of direct energy transmission, forward scattering, and background scattering components represented by the equation below.

Here

Image restoration techniques must recover the original image

Here * is the convolutional operator.

This can be represented in frequency domain by

An underwater color image is represented by three channels c ∈ {R, G, B}, and the intensity of each pixel is given by two components namely attenuated signal and veiling light Mao et al. 2014 [

Here

The transmission depends on the objects distance d(x) and the water attenuation coefficient for each channel

Underwater imagery performed with the camera has three attenuation coefficients namely

For transmission estimation of a particular channel, scene values can be used. The generalized scene values are given by

Using

A lower bound on the transmission of the blue channel is given by

A soft matting method was used for calculating the transmission.

Overall the method involves the following steps:

Detection of veiling light pixels using structured edges and calculation of veiling light A.

The ratio of attenuation coefficients of Blue/Red and Blue/Green water type calculates the image intensity at a particular pixel for each value.

Cluster the pixels into 500 Haze lines and estimate the initial transmission.

Apply soft matting.

Regularization using guided image filter with contrast-enhanced input as guidance.

Calculate the Restored image.

Return the image that best adheres to the Gray world assumptions.

Saliency maps efficiently provide the Regions where most of the activity or objects of importance are present. Saliency maps effectively find the Region of Interest and are known to be used in several image segmentation problems to detect the salient visual regions. Several methods were proposed earlier for detecting saliency maps. Yang et al. [

Saliency maps can be derived from single scale (properties) or multiple scales by adding two or more feature characteristics in deriving the saliency maps. There are various methods to determine saliency based on color, contrast, illumination etc. Most of the methods use the local contrast as one of the parameters for saliency map calculation. Feature vectors of a group of pixels for a particular region can be compared with feature vectors of the group of pixels for another region.

Split the image into 8 × 8 regions and calculate the GLCM features such as Regional contrast, Regional correlation and Entropy. GLCM features are normalized so that some of its elements are 1.

Let (i, j) be the pixel coordinate of an image region R1. Assume

Contrast [

Correlation measures how correlated a pixel is to its neigh boring pixel over the whole image and ranges between −1 to 1.

The GLCM feature [

Weighted Euclidean Distance can be used to determine if any GLCM feature of Region 1 and Region 2 represents ROI or is not part of the ROI. The Euclidean distance between the jth variable of a feature between Region 1 and Region 2 can be given as

Here

ELM models can be solved analytically comprising of a closed-form involving matrix multiplication and inversion. Hence learning process can be started without any iterative methods and is considered faster compared to other algorithm. A classical neural network regression and prediction algorithm use fixed activation functions and dynamic changes are very difficult to implement. ELMs structure is simple and learning can be faster with good generalized performance. The Single hidden Layer Feed forward Neural Network (SLFN) architecture is shown in

Assuming N number of training samples

Here

The inner product of

H is the output matrix of the hidden layer. And illustratively represented as [

The network cost function given as ||O-T|| can be minimized by assigning hidden node learning parameters without considering the input data, resulting in Hβ = O becoming a linear system and the output weights

Here Dim is the set of the positive integer representing the dimension of the function used. Function

For unbalanced learning, considering an N × N diagonal matrix W associated with every training sample

Here

Subject to

Feature vector mapping in the hidden layer is represented by

ELMs come with their risks such as empirical and structural ones. A generalized ELM model must be able to balance and keep the empirical and structural risks at a minimum. But in cases where the input data results in overlapping of the target causes overfitting. Assuming input and output sample dataset for regression analysis as

Here

Or

Subject to

As per the maximum margin theory t

Thus the overall steps involved in the ELM can be summarised as

Providing input training set, activation function f (.) and the number of hidden nodes.

Randomly assigning input weight w, input weight w and biases b.

Calculation of hidden layer output matrix H.

Calculation of output weight w.

To test the ELM network input Test Region Saliency values are given with the Test signal strength values. ELM network then estimates the correct output DCT coefficients/Compression Ratios. The coefficient is then selected for the region and the image is compressed for the region R1. The test process is done on the whole image while inputting image blocks to compress the whole image.

Although ELM is fast and gives good generalized performance since the output weights w and hidden biases are assigned randomly, some corner cases where ELM suffers from overfitting due to non-optimal input weights and hidden biases are present. To test the performance of the ELM RMSE is calculated.

Here

To test the algorithm, underwater marine images were taken.

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After training the ELM for determining the DCT coefficients for the input of Saliency map value and signal strength for image transmission, the ELM network is tested.

The

With the decrease in the signal strength, the compression ratio has to be high, increasing the underground image’s processing timemage.

To handle the underwater image compression problem, this paper proposed a method that restores the underwater color image from the blue/green light rays and uses the saliency maps and signal strength to decide region level compression coefficients. Using ELM to find the compression coefficients proves itself efficient in providing optimal compression levels and PSNR values with minimized errors, minimized processing time and better structural content. The use of ELM resulted in a smaller processing time to set up the network as ELM has faster learning. The proposed method gives good results since the ELM reaches the solutions quickly without problems like local minima and learning rate. The PSNR and RMSE value of proposed system is 28.52 dB and 0.33 dB.