CN112565718A - Underwater image processing method based on Retinex and high dynamic range image gradient compression - Google Patents

Underwater image processing method based on Retinex and high dynamic range image gradient compression Download PDF

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CN112565718A
CN112565718A CN202011385470.6A CN202011385470A CN112565718A CN 112565718 A CN112565718 A CN 112565718A CN 202011385470 A CN202011385470 A CN 202011385470A CN 112565718 A CN112565718 A CN 112565718A
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祝开艳
苏钒维
宋维波
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Dalian Ocean University
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Abstract

The invention discloses an underwater image processing method based on Retinex and high dynamic range image gradient compression, which comprises the steps of firstly carrying out multi-scale Retinex algorithm processing on an underwater image, then carrying out pixel value compression on the image subjected to Retinex processing by using a gradient domain high dynamic range image compression algorithm, then attenuating low-frequency noise by a high-pass filter, enhancing the edge of the image, and finally recovering the chroma by applying a color saturation adjusting function. The method can inhibit the halo effect caused after the Retinex method processes the underwater image, improve the color cast influence brought by the underwater environment, and effectively process the problems of low illumination, uneven illumination and the like of the underwater image.

Description

Underwater image processing method based on Retinex and high dynamic range image gradient compression
Technical Field
The invention relates to an image processing method, in particular to an underwater image processing method based on Retinex and high dynamic range image gradient compression.
Background
The underwater image is an image shot underwater by using an optical imaging system, is an important carrier of ocean information, and is widely applied to the fields of ocean energy exploration, ocean ecological protection, ocean military and the like at present. However, due to the fact that light rays with different wavelengths have different attenuation characteristics when being transmitted in water and the influence of suspended particles, artificial light sources and the like in an underwater scene, the underwater image often has the quality degradation problems of low contrast, fuzzy texture, color distortion, non-uniform illumination, limited visual range and the like, and great inconvenience is brought to practical application and scientific research. In order to restore clear and real underwater images, various underwater image processing methods are available at present.
Conventional underwater image processing algorithms include histogram equalization, homomorphic filtering, Retinex algorithm and the like, wherein Retinex theory is an image enhancement method established on the basis of scientific experiments and analysis and is proposed by Edwin. Liu Xiao Yang and the like replace Gaussian filtering with bilateral filtering on the basis of Retinex algorithm, so that color distortion and detail loss of images are avoided; fu et al first propose a simple underwater image color deviation correction algorithm, then propose a variable framework separating direct light and reflected light based on retina cortex theory, and finally enhance the separated illumination component by different enhancement strategies, thereby enhancing the contrast of the underwater image; li Hui Fang et al uses variation optimization technique, and adds projection normalization steepest descent method in the technique, and through the combination of the two techniques to remove the illumination component of the remote sensing image, i.e. low frequency component, it can obviously eliminate the gray level nonuniformity of the remote sensing image, and keep the texture information of the image, etc. Although the above method can solve the quality degradation phenomenon of the underwater image to a certain extent, there are still some problems, for example, the multi-scale retinex (msr) algorithm processes each channel of RGB, when processing a single channel, it is easy to cause imbalance after each channel is processed, and further other interference noise is introduced, which causes local color distortion of the image, halo effect ("halo" effect) occurs, that is, a bright boundary appears as a brighter halo at a boundary, and a dark boundary appears as a darker black ring at a bright-dark boundary of a significant edge in the image, so that the overall effect of the image will be deteriorated, and the visual effect is not good.
High Dynamic Range (HDR) images can fully record luminance information that can be perceived by human eyes, and have a larger Dynamic Range and more detailed information than low Dynamic Range images. However, the existing mainstream display device is a low dynamic range display device, and is not capable of directly displaying the HDR image, so the HDR compression algorithm is generated by mapping the dynamic range of the HDR image to the dynamic range that can be displayed by a general display device. The HDR compression algorithm is focused and studied intensively by many researchers due to its superiority in contrast reproduction and halo removal, but the existing HDR compression algorithm also compresses the color gamut in the high dynamic range image compression process, and is prone to color distortion.
Disclosure of Invention
The invention provides an underwater image processing method based on Retinex and high dynamic range image gradient compression, aiming at solving the technical problems in the prior art.
The technical solution of the invention is as follows: an underwater image processing method based on Retinex and high dynamic range image gradient compression is sequentially carried out according to the following steps:
step 1: inputting an underwater image;
step 2: carrying out multi-scale Retinex algorithm processing on the underwater image;
and step 3: high dynamic range image gradient compression is carried out on the image, namely pixel values R of the image are mapped to a compression range Rmin,RmaxThe output pixel value is R through normalization processingnewThe image of (a);
the R ismin=avg(R)-n·var(R)、Rmax=avg(R)+n·var(R)、
Figure BDA0002809556660000021
In the formula: avg (R) represents the average value of the image pixel value R, var (R) represents the variance of the image pixel value R, and n is a compression range adjustment coefficient;
and 4, step 4: and carrying out high-pass filtering processing on the image, wherein the filter function H (u, v) is as follows:
Figure BDA0002809556660000022
in the formula: gamma rayHAnd gammaLFor high-pass filter parameters, gammaH>1,γL<1; c is a constant for controlling the sharpness of the function slope, making the function in gammaHAnd gammaLD (u, v) is the distance of a point in the frequency domain from a central point; d0Is a characteristic value of the attenuation;
and 5: the color saturation adjustment is performed on the image, the color saturation S is adjusted by utilizing an HSL space, and the calculation formula is as follows:
Figure BDA0002809556660000023
Figure BDA0002809556660000024
said max (rgb) is the maximum of the three channel pixel values of image R, G, B, said min (rgb) is the minimum of the three channel pixel values of image R, G, B;
defining a color saturation adjustment value alpha according to the following formula, wherein the value of the auxiliary variable i is-1:
Figure BDA0002809556660000031
after adjustment is calculated as followsRGBnew
Figure BDA0002809556660000032
And 6, outputting the processed underwater image.
The invention uses the gradient domain high dynamic range image compression algorithm to compress the pixel value of the image processed by Retinex, then attenuates low-frequency noise through a high-pass filter, enhances the image edge, and finally recovers the chroma by applying a color saturation adjusting function. The method can inhibit the halo effect caused after the Retinex method processes the underwater image, improve the color cast influence brought by the underwater environment, and effectively process the problems of low illumination, uneven illumination and the like of the underwater image.
Drawings
FIG. 1 is an underwater image artwork used by embodiments of the present invention.
Fig. 2 is a graph comparing the processing results of the entire low-illuminance image according to the present invention and the prior art.
Fig. 3 is a comparison graph of the processing result of the local low-illumination image according to the present invention and the prior art.
FIG. 4 is a graph comparing the results of the present invention and prior art processing of partially reflected underwater images.
Detailed Description
The invention relates to an underwater image processing method based on Retinex and high dynamic range image gradient compression, which is sequentially carried out according to the following steps:
step 1: inputting an underwater image;
step 2: carrying out multi-scale Retinex algorithm (MSR) processing on the underwater image, and attenuating low-frequency components in illumination components in the underwater image, wherein the processed underwater image has a halo effect;
and step 3: high dynamic range image gradient compression is carried out on the image, namely pixel values R of the image are mapped to a compression range Rmin,RmaxThe output pixel value is R through normalization processingnewThe image of (a);
the R ismin=avg(R)-n·var(R)、Rmax=avg(R)+n·var(R)、
Figure BDA0002809556660000033
In the formula: avg (R) represents the average value of the image pixel value R, var (R) represents the variance of the image pixel value R, and n is a compression range adjustment coefficient.
The halo effect is inhibited by compressing the pixel value change rate, namely the compression degree is higher in places with large change rates, and the compression degree is correspondingly reduced in places with small change rates, so that the realization result is effective, the robustness is realized, and better image details can be kept; the compression effect is more obvious when n is larger, but more noise is introduced and color is lost;
and 4, step 4: and carrying out high-pass filtering processing on the image, wherein the filter function H (u, v) is as follows:
Figure BDA0002809556660000041
in the formula: gamma rayHAnd gammaLFor high-pass filter parameters, gammaH>1,γL<1; c is a constant for controlling the sharpness of the function slope, making the function in gammaHAnd gammaLD (u, v) is the distance of a point in the frequency domain from a central point; d0Is a characteristic value of the attenuation, D0The greater the detail enhancement is;
and 5: the color saturation adjustment is performed on the image, the color saturation S is adjusted by utilizing an HSL space, and the calculation formula is as follows:
Figure BDA0002809556660000042
Figure BDA0002809556660000043
said max (rgb) is the maximum of the three channel pixel values of image R, G, B, said min (rgb) is the minimum of the three channel pixel values of image R, G, B;
defining a color saturation adjustment value alpha according to the following formula, wherein the value of the auxiliary variable i is-1:
Figure BDA0002809556660000044
after adjustment is calculated as followsRGBnew
Figure BDA0002809556660000045
And 6, outputting the processed underwater image.
Experiment:
the computer parameter used in the simulation experiment is Intel (R) core (TM) M-5Y10c @0.80GHZ1.00GHZ, the used operating system is 64 bits of Chinese version Windows 8.1, and the used simulation software is MATLAB R2017a 64 bits.
The 3 underwater images shown in fig. 1 are selected, the formats are all jpg, and the problems of the images respectively correspond to three typical problems of the underwater images with uneven illumination, namely, the phenomena of overall low illumination (a), local low illumination (b), reflection (c) in the images and the like. The invention is compared with other four traditional algorithms: HE. HF, MSR, gradient domain HDR compression algorithms were compared.
Subjective evaluation:
fig. 2 is a graph comparing the processing results of the entire low-illuminance image according to the present invention and the prior art. In fig. 2, (a) to (f) are the original image, HE, HF, MSR, gradient domain HDR compression algorithm, and the present invention, respectively.
The parameters of each algorithm are chosen as follows: wherein the HF algorithm parameter is gammaH=2,γL=0.5,c=2,D0256; the MSR algorithm scales are respectively 16, 32 and 64; the scale of the invention is 16, 32 and 64, the three channels of compression adjustment coefficients RGB are 20, 30 and 10 respectively, the color saturation increment i is-0.2, and the parameter of the high-pass filter is gammaH=2,γL=0.9,c=1,D0256. The overall illumination of the original image is very low and has serious blue-green color cast; although the HE algorithm enhances the contrast, the HE algorithm does not improve the color cast influence because the HE method changes the distribution of image gray levels instead of increasing the gray levels, and the color cast phenomenon is also aggravated while the gray levels are homogenized; the HF method is very effective in improving contrast and noise, but has a "Holo" effect and color shift effects; the MSR method has good effect on improving color cast and color degree, but the image has serious noise and a 'Holo' effect; the gradient domain HDR compression method has a poor effect on image processing with a low dynamic range; the invention has satisfactory color cast, contrast, noise processing and color expression effects.
Fig. 3 is a comparison graph of the processing result of the local low-illumination image according to the present invention and the prior art. In fig. 3, (a) to (f) are the original image, HE, HF, MSR, gradient domain HDR compression algorithm, and the present invention, respectively.
The parameters of each algorithm are chosen as follows: HF algorithm parameter is gammaH=2,γL=0.25,c=2,D0256; the MSR algorithm scales are respectively 64, 128 and 256; the invention has the scale of 64, 128 and 256, the three channels of the compression adjustment coefficients RGB are respectively 13, 14 and 19, the color saturation increment i is 0.5, and the parameter of the high-pass filter is gammaH=2,γL=0.99,c=2,D0256. The distant corals in the original image are completely in the dark and difficult to distinguish; the HE method enables the corals in the dark to be completely exposed, the contrast is good, but the color is seriously inconsistent with the reality, and the over-enhancement phenomenon is shown in the bright area; the HF method is better in the details of dark places and the overall brightness, but poor in the processing of color expression and has color cast; the MSR method performs well on the color of the image, but does not handle well on the detail presentation in dark places; the gradient domain HDR compression method has good performance on dark details and image edges, but has poor effect on color performance and overall brightness; the invention has good performance on color presentation, image edge and dark detail enhancement.
FIG. 4 is a graph comparing the results of the present invention and prior art processing of partially reflected underwater images. In fig. 4, (a) to (f) are the original image, HE, HF, MSR, gradient domain HDR compression algorithm, and the present invention, respectively.
The parameters of each algorithm are chosen as follows: HF algorithm parameter is gammaH=2,γL=0.25,c=2,D0256; the MSR algorithm scales are respectively 64, 128 and 256; the invention has the scale of 64, 96 and 256, the three channels of the compression adjustment coefficients RGB are respectively 30, 30 and 40, the color saturation increment i is 0.1, and the parameter of the high-pass filter is gammaH=2,γL=0.99,c=2,D0256. The original image shows a reflection phenomenon on the surface of the fish skin and the whole image shows strong light and shade contrast. The HE processing result has serious over-enhancement phenomenon, and the light reflection phenomenon is intensified; the visual effect of HF treatment is good, the texture on the surface of the fish skin is clearer, but color cast and a 'Holo' effect exist; the MSR method improves the phenomenon of integral illumination nonuniformity of the image, but poor treatment of the phenomena such as color, noise, color cast and the like; gradient domain HDR compression method is expressed in contrast and color densityPoor and accompanied by color bias phenomenon; the invention has good improvement performance in the aspects of blurring, color cast and the like, the detail performance of the light reflecting area is good, the fish skin texture in the light reflecting area can be seen, and the 'Holo' effect is also inhibited.
Experimental results prove that the method has better processing results on the problems of color cast, noise and color attenuation caused by the underwater environment when processing the underwater image with uneven illumination.
Objective evaluation:
at present, image evaluation indexes are generally three types, namely a full reference type, a partial reference type and a no reference type. The full reference type refers to the existence of an ideal image, and a comparison method is carried out by comparing the image to be evaluated with the ideal image; the partial reference type is to compare and analyze the image to be evaluated by taking partial characteristic information of the ideal image as reference so as to obtain an image quality evaluation result; the no-reference type is an image quality evaluation method completely separated from the dependence on an ideal reference image. Because the underwater environment is poor when the underwater image is usually obtained, and an ideal image is difficult to obtain, the invention adopts a non-reference evaluation standard. Currently, common underwater image evaluation standards include methods such as UCIQE (offline color image quality), uiqm (offline image quality measurement), and the like, wherein the evaluation standard of the UCIQE refers to three indexes of color concentration, contrast and saturation of an image, and an image score is obtained through linear weighting; the UIQM selects indices of color density, sharpness, and contrast deteriorated by the characteristics of the aqueous medium, and evaluates them. The invention adopts the two comprehensive evaluation standards to carry out objective evaluation on each image, and also considers the operation speed of each algorithm. HE. The evaluation index values of HF, MSR, gradient domain HDR compression algorithm and the three kinds of uneven illumination image processing results shown in fig. 1 according to the present invention are shown in tables 1 to 3.
Table 1 shows the objective evaluation results of the overall low-illumination image processing results for each algorithm. The results show that: the image processed by the method has satisfactory quality, the scores of UCIQE and UIQM are the highest, and the method has good results in the aspects of color concentration, contrast and definition when processing low-illumination images.
Table 2 shows the objective evaluation results of the local low-illumination image processing results of the algorithms. The results show that: the method has ideal effect when processing the local low-illumination underwater image, has good effect on the aspects of image definition, contrast, color concentration and the like, has the highest score of UCIQE and UIQM, and shows that the image processed by the method has the best quality.
Table 3 shows the objective evaluation results of the local reflection underwater image processing results by each algorithm. The results show that: from the UCIQE score, the HE score is highest because the color density and the average brightness of the HE score are good, but in combination with the subjective impression of the image, the HE processing result is extremely serious, the image details are lost, and the light reflection phenomenon is not improved. Therefore, the underwater image enhancement effect is generally evaluated comprehensively by combining objective evaluation indexes and subjective evaluation indexes. The invention has good performance on clear images and contrast, so the UIQM has the highest score, and the invention combines subjective impression, has no over-enhancement phenomenon on color performance, good detail performance and good improvement on color cast phenomenon.
TABLE 1 evaluation index of overall low-illumination image by each method
Figure BDA0002809556660000071
TABLE 2 evaluation index of local low-illumination image by each method
Figure BDA0002809556660000072
TABLE 3 evaluation indexes of local reflection image
Figure BDA0002809556660000073

Claims (1)

1. An underwater image processing method based on Retinex and high dynamic range image gradient compression is characterized by comprising the following steps in sequence:
step 1: inputting an underwater image;
step 2: carrying out multi-scale Retinex algorithm processing on the underwater image;
and step 3: high dynamic range image gradient compression is carried out on the image, namely pixel values R of the image are mapped to a compression range Rmin,RmaxThe output pixel value is R through normalization processingnewThe image of (a);
the R ismin=avg(R)-n·var(R)、Rmax=avg(R)+n·var(R)、
Figure FDA0002809556650000011
In the formula: avg (R) represents the average value of the image pixel value R, var (R) represents the variance of the image pixel value R, and n is a compression range adjustment coefficient;
and 4, step 4: and carrying out high-pass filtering processing on the image, wherein the filter function H (u, v) is as follows:
Figure FDA0002809556650000012
in the formula: gamma rayHAnd gammaLFor high-pass filter parameters, gammaH>1,γL<1; c is a constant for controlling the sharpness of the function slope, making the function in gammaHAnd gammaLD (u, v) is the distance of a point in the frequency domain from a central point; d0Is a characteristic value of the attenuation;
and 5: the color saturation adjustment is performed on the image, the color saturation S is adjusted by utilizing an HSL space, and the calculation formula is as follows:
Figure FDA0002809556650000013
Figure FDA0002809556650000014
said max (rgb) is the maximum of the three channel pixel values of image R, G, B, said min (rgb) is the minimum of the three channel pixel values of image R, G, B;
defining a color saturation adjustment value alpha according to the following formula, wherein the value of the auxiliary variable i is-1:
Figure FDA0002809556650000015
RGB after adjustment is calculated as followsnew
Figure FDA0002809556650000016
And 6, outputting the processed underwater image.
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