CN112070683B - Underwater polarized image restoration method based on polarization and wavelength attenuation combined optimization - Google Patents
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Abstract
The invention discloses an underwater polarized image restoration method based on polarization and wavelength attenuation combined optimization, which comprises the following steps: s1, acquiring a color image and 4 polarized images corresponding to an underwater scene through an industrial camera and a linear polaroid, and estimating back scattered light and target information light by combining an underwater imaging model, so as to estimate a scene depth map based on polarized information according to the back scattered light; s2, estimating a local spatial average color and realizing rough estimation of a target information light attenuation coefficient; s3, deriving scene depth information based on wavelength attenuation information by combining an underwater imaging model, and realizing joint optimization of local neighborhood by combining the wavelength attenuation information and polarization information so as to finish accurate estimation of attenuation coefficient and realize recovery of a final clear underwater image. The problems of contrast reduction and color distortion existing in the existing underwater imaging technology are solved.
Description
Technical Field
The invention belongs to the technical field of polarized image processing, and particularly relates to an underwater polarized image restoration method based on polarization and wavelength attenuation combined optimization.
Background
At present, the image in the underwater scene has serious degradation problems: the absorbance of the water body to the light with different wavelengths is different, and only the light with specific wavelength can be transmitted in the water body with lower attenuation rate, so that the color distortion of the underwater image is caused; particles and other substances contained in the water body not only can make the underwater image have larger noise, but also can generate scattering effect on light, and can transmit particle information in the water body to the detector to cause valance effect, so that image contrast is reduced.
The backward scattered light causing the contrast reduction of the underwater image has polarization correlation, and the underwater polarization technology can effectively inhibit the backward scattered light and improve the contrast of the image. In addition, compared with other underwater imaging technologies, the underwater polarization imaging technology has the advantages of simple and portable system, low power consumption and low cost. Although the underwater polarization imaging method solves the problem of contrast reduction in the figure, the recovery result of the method has serious color distortion problem because the wavelength dependence of light transmitted in a water body is not considered, the recovery effect is limited, and the accuracy of the subsequent recognition and positioning of the underwater target is further limited.
Disclosure of Invention
The invention aims to provide an underwater polarized image restoration method based on polarization and wavelength attenuation combined optimization, so as to solve the problems of contrast reduction in the existing underwater imaging and color distortion in the underwater polarized imaging technology.
The invention adopts the following technical scheme: the underwater polarized image restoration method based on polarization and wavelength attenuation combined optimization comprises the following steps:
s1, acquiring a color image and 4 polarization images corresponding to an underwater scene through an industrial camera and a linear polaroid, and estimating back scattered light and target information light by combining an underwater imaging model, so as to estimate a scene depth map based on polarization information according to the back scattered light;
s2, restraining a local neighborhood according to the depth information of the scene depth map, so as to estimate local space average color and realize rough estimation of a target information light attenuation coefficient;
s3, combining an underwater imaging model, feeding back the primarily estimated attenuation coefficient to the model, deducing scene depth information based on wavelength attenuation information, combining the wavelength attenuation information and polarization information to realize joint optimization of local neighborhood so as to finish accurate estimation of the attenuation coefficient, and finally recovering a clear underwater image.
Further, in step S1, the underwater imaging model is expressed as:
I c ＝D c +B c (1)，
I c ＝J c e -βz +B ∞ (1-e -βz ),c∈{r,g,b} (2)，
i in formula (1) c Representing degraded underwater image, D c Representing the target information light, B c Represents backscattered light, J in equation (2) c Representing target unattenuated irradiance in a scene, i.e. no irradiance to be solvedA clear image of color distortion, β representing the attenuation coefficient, z representing the distance of the target from the camera, e -βz Representing the attenuation of the target information light, B ∞ Representing backscattered light at infinity, c representing three color channels of the image, r representing the red channel, g representing the green channel, and b representing the blue channel.
Further, in step S1, the method for removing the back scattered light includes:
acquiring an image I of the brightest underwater scene max And image I at darkest min At the same time consider the degree of polarization p of the backscattered light and the target scat And p obj Then:
b in the formula (3) 1 Representing the back-scattered light obtained by the polarization method, (x, y) representing the coordinate position of the pixel within the field of view;
the back scattered light in the scene can be estimated by utilizing the above method after the back scattered light and the target polarization degree are estimated by selecting a specific region in the scene;
after the back scattered light is obtained, further calculating a depth map of the scene:
z in 1 Represents a depth map estimated via polarization information.
Further, in step S2, when calculating the local spatial average color, the depth information is used to screen the pixel points in the local neighborhood;
setting a threshold delta, selecting connected pixels within the local neighborhood having a distance to the center pixel not greater than the threshold delta, and estimating a local spatial average color from the pixels:
in the formula (5) of the present invention,representing a set of pixels having a difference from the center pixel depth less than a threshold delta, (x ', y') representing other pixel points having a difference from the center pixel depth less than the threshold delta within the local neighborhood, |·|| representing calculating a difference between the two pixel depths, z 1 (x, y) represents the distance of the center pixel from the camera, z 1 (x ', y') represents the distance from the camera of the pixel point of the non-central pixel in the local neighborhood;
in the formula (6) of the present invention,representing the local spatial average color estimated by each iterative calculation +.>Representation set->The local spatial average color of the inner pixel points. In formula (7)>Representing the local spatial average color estimated by iterative calculation, the parameter p describes the area of calculation of the average, the size of which depends on the size of the image;
estimating a local spatial average colorThe entire illuminance in the scene can then be +.>And (3) estimating:
in equation (8), the parameter f is a scaling factor;
attenuation coefficient of the target information lightIt can be expressed as:
further, in step S3, according toScene depth information z by underwater imaging model 2 Estimation is performed again:
when calculating the local space average color, carrying out joint optimization treatment on the local neighborhood of the local space average color:
in the formula (11), the color of the sample is,representing a pixel set, wherein the difference between the depths of the pixel points and the center pixel is smaller than a threshold delta in the depth map calculated by two times, and (x ', y') represents the pixel points in the set, z 2 (x, y) represents the distance of the center pixel from the camera, z, in the second estimated depth map 2 (x ', y') represents the distance between the camera and the pixel point of the non-central pixel in the local neighborhood;
obtaining a local space average color on the optimized local neighborhood through iterative calculation:
in the formula (12) of the present invention,representing the local spatial average color at each iterative calculation,/->Representing the set +.>Local spatial average color for each pixel point within. In formula (13)>Representing the local spatial average color obtained by iterative calculation;
and then, the scene illumination and the attenuation coefficient are accurately estimated:
in the formula (15), β represents the attenuation coefficient of the target information light estimated finally, and is combined with the image D from which the back scattered light has been removed c Finally, the unattenuated underwater image J can be obtained c ：
The beneficial effects of the invention are as follows: the invention aims to provide a method capable of effectively improving the quality of an underwater image by combining a traditional polarization imaging method with a color constancy theorem. According to the characteristic points of the traditional underwater imaging method, the acquired scene depth map is utilized to restrict local space average color, the scene illumination is estimated, the primary estimation of the wavelength attenuation coefficient is realized, the primary result is fed back to the underwater imaging model to carry out secondary estimation on depth information, and the polarization information and the wavelength attenuation information are utilized to carry out joint optimization on local neighborhood so as to achieve the aim of carrying out accurate compensation on the wavelength attenuation. The method can effectively solve the color distortion problem of the traditional underwater polarization method, and is beneficial to subsequent image recognition and other processing.
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FIG. 1 is a flow chart of the method for restoring an underwater polarized image based on the combined optimization of polarization and wavelength attenuation.
Detailed Description
The invention will be described in detail below with reference to the drawings and the detailed description.
The technical scheme adopted by the invention is that a traditional polarization imaging method is adopted to process back scattered light in an underwater degradation image and obtain a scene depth map, the local neighborhood is restrained by the depth map, the primary estimation of the target information light attenuation coefficient is realized by estimating the local space average color and scene illumination, the attenuation coefficient is fed back to an imaging model to carry out secondary estimation on the depth information, the polarization information and wavelength attenuation information are utilized to carry out joint optimization on the local neighborhood, and finally, the accurate estimation of the target information light attenuation coefficient is realized, and the unattenuated clear underwater image is recovered.
Traditional underwater polarization imaging methods are used for descattering and depth map estimation:
color images and 4 polarized images corresponding to the underwater scene are acquired through an industrial camera and a linear polaroid, and the backward scattered light and the target information light are estimated by combining an underwater imaging model, so that a scene depth map based on polarized information is estimated according to the backward scattered light.
The underwater imaging model is expressed as:
I c ＝D c +B c (17)，
I c ＝J c e -βz +B ∞ (1-e -βz ),c∈{r,g,b} (18)，
i in formula (1) c Representing degraded underwater image, D c Representing the target information light, B c Represents backscattered light, J in equation (2) c Representing unattenuated irradiance of the object in the scene, i.e. a clear image of achromatic distortion to be solved, β representing the attenuation coefficient, z representing the distance of the object from the camera, e -βz Representing the attenuation of the target information light, B ∞ Representing backscattered light at infinity, c representing three color channels of the image, r representing the red channel, g representing the green channel, and b representing the blue channel. The back-scattered light was removed using the following method: acquiring an image I of a "brightest" underwater scene max And image I at "darkest min At the same time consider the degree of polarization p of the backscattered light and the target scat And p obj Then:
b in the formula (3) 1 The back-scattered light obtained by the polarization method is represented, and (x, y) represents the coordinate position of the pixel within the field of view. The back-scattered light in the scene can be estimated by using equation (3) after the back-scattered light and the target polarization degree are estimated by selecting a specific region in the scene. After the back scattered light is obtained, further calculating a depth map of the scene:
z in 1 Represents a depth map estimated via polarization information.
And (3) carrying out constraint on the neighborhood by combining a depth map based on polarization information, and roughly estimating attenuation coefficients:
the method utilizes the depth map based on polarization information to restrict local neighborhood, estimates local space average color and carries out preliminary estimation on the attenuation coefficient of target information light.
Constraining the local neighborhood using the acquired depth map:
in the formula (5)Representing a set of pixels having a difference from the center pixel depth less than a threshold delta, (x ', y') representing other pixel points having a difference from the center pixel depth less than the threshold delta within the local neighborhood, and |·| representing calculating a difference between the two pixel depths, z 1 (x, y) represents the distance of the center pixel from the camera, z 1 (x ', y') represents the distance from the camera of the pixel point of the non-center pixel in the local neighborhood. Obtaining the local space average color by iterative calculation according to the screened local neighborhood>
In the formula (6)Representing the local spatial average color estimated by each iterative calculation +.>Representation set->The local spatial average color of the inner pixel points. Male (Male)In formula (7)>The parameter p describes the area of calculation of the average, the size of which depends on the size of the image, representing the local spatial average color estimated by iterative calculation. After the local space average color is estimated through iterative calculation, the whole illumination in the scene can be subjected to +.>And (3) estimating:
the parameter f in equation (8) is a scaling factor. Attenuation coefficient of the target information lightIt can be expressed as:
and combining wavelength attenuation information and polarization information to perform joint optimization on the local neighborhood, so as to realize accurate estimation of attenuation coefficients:
and feeding back the attenuation coefficient of the primarily estimated target information light to an underwater imaging model, and carrying out joint optimization on the local space neighborhood by combining the wavelength attenuation information and the polarization information, so as to further restrict the local neighborhood, thereby realizing accurate estimation of the local space color, finally realizing accurate estimation of the attenuation coefficient, and achieving the purpose of accurately carrying out color correction. Feeding back the attenuation coefficient obtained preliminarily to an underwater imaging model for depth information z 2 Estimation is performed again:
combining polarization information and wavelength attenuation information to perform joint optimization on the local neighborhood:
in the formula (11)Representing a pixel set, wherein the difference between the depths of the pixel points and the central pixel is smaller than a threshold delta in the depth map calculated twice successively, (x ', y') represents the pixel points in the set, and z 2 (x, y) represents the distance of the center pixel from the camera, z, in the second estimated depth map 2 (x ', y') represents the distance between the camera and the pixel point of the non-center pixel in the local neighborhood.
Accurately estimating the local spatial average color by using the optimized local neighborhood:
in the formula (12)Representing the local spatial average color at each iterative calculation,/->Representation set->Local spatial average color for each pixel point within. In formula (13)>Representing the local spatial average color obtained by iterative calculation. And then, the scene illumination and the attenuation coefficient are accurately estimated:
in the formula (15), beta represents the attenuation coefficient of the target information light finally estimated, and is combined with the image D from which the back scattered light has been removed c Finally, the unattenuated underwater image J can be obtained c ：
Embodiment one:
the working flow chart of the invention is shown in fig. 1, and the embodiment of the invention provides an underwater polarized image restoration method based on polarization and wavelength attenuation combined optimization, which comprises the following steps:
the traditional underwater polarization imaging method is used for scattering and estimating a depth map based on polarization information;
constraint is carried out on the neighborhood by combining a depth map based on polarization information, and attenuation coefficients are roughly estimated;
and combining wavelength attenuation information and polarization information to perform joint optimization on the local neighborhood, so as to realize accurate estimation of attenuation coefficients and restore clear underwater images.
The underwater polarization image restoration method based on polarization and wavelength attenuation combined optimization solves the problem of color distortion existing in the traditional underwater polarization imaging method, improves the image contrast, simultaneously displays the real color information of the target in the underwater scene, and effectively expands the application range of the underwater polarization imaging method.
Embodiment two:
the embodiment of the invention provides an underwater polarized image restoration method based on polarization and wavelength attenuation combined optimization, which is specifically introduced on the basis of the embodiment, and specifically comprises the following steps:
step one: the traditional underwater polarization imaging method removes back scattered light and estimates a depth map based on polarization information;
in the embodiment, the acquisition device of the common industrial camera and the linear polaroid is adopted to shoot the underwater target scene, the color image of the scene and 4 corresponding polarized images are acquired, and the brightest image I corresponding to the underwater target scene can be deduced from the 4 polarized images max And "darkest" image I min Descattering is performed in combination with an underwater imaging model and a scene depth map based on polarization information is estimated.
The underwater imaging model is expressed as:
I c ＝D c +B c (1)，
I c ＝J c e -βz +B ∞ (1-e -βz ),c∈{r,g,b} (2)，
i according to formula (1) max 、I min The relationship of the target information light and the back-scattered light is shown in the following formula:
I max ＝D max +B max (3)，
I min ＝D min +B min (4)，
assuming that the target information light and the back-scattered light are both polarized, selecting a specific background area and the polarization degree p of the target area to the back-scattered light scat Polarization degree p of target information light obj And (3) estimating:
the back-scattered light of the target scene can be deduced by combining equations (1), (3), (4), (5), (6):
according to equation (1), there is no scattering effect but still attenuated waterLower scene image D c The method comprises the following steps:
D c ＝I c -B c (8)，
the depth map of the scene based on polarization information is further estimated by the estimated back-scattered light at the same time. It is generally considered that the back-scattered light increases with distance under natural illumination, and in the present invention, it is considered that the condition is similar to that of natural illumination when the light source is close to the camera, whereby the calculation formula of the depth map is derived from the integral calculation formula of the back-scattered light:
z in 1 Represents a depth map estimated via polarization information.
Step two: constraint is carried out on the neighborhood by combining a depth map based on polarization information, and attenuation coefficients are roughly estimated;
the back scattered light is removed through a traditional underwater polarization imaging method, a depth map of a scene based on polarization information is estimated according to the back scattered light, and the local space average color is constrained by the depth map to estimate the scene illuminance so as to complete the preliminary estimation of the attenuation coefficient.
Constraining the local neighborhood with a depth map based on polarization information:
performing iterative calculation on the screened local neighborhood to estimate the local spatial average color:
estimating local spatial average color by iterative computationThe entire illuminance in the scene can then be +.>Estimation is performed:
f is a factor of the scale and is the same for all color channels f. In this embodiment, f=2 is used. The attenuation coefficient of the target information light can then be expressed as:
step three: and combining wavelength attenuation information and polarization information to perform joint optimization on the local neighborhood, so as to realize accurate estimation of attenuation coefficients.
And feeding back the attenuation coefficient obtained by the primary estimation to an underwater imaging model to carry out secondary estimation on depth information, carrying out joint optimization on local space neighborhood by utilizing polarization information and wavelength attenuation information, and further accurately screening pixels in the local neighborhood so as to realize accurate estimation on local space average color, thereby accurately estimating the attenuation coefficient of target information light and finally realizing underwater clear imaging without color distortion.
And feeding back the attenuation coefficient estimated preliminarily to the underwater imaging model, and estimating scene depth information again:
and carrying out joint optimization treatment on the local neighborhood by combining polarization information and wavelength attenuation information:
more accurate local neighborhood can be obtained through the joint optimization of the formula (16), and accurate estimation of local space average color is completed on the neighborhood through iterative calculation:
and then, the scene illumination and the attenuation coefficient are accurately estimated:
for the target information light, the attenuation model is as follows:
D c ＝J c e -βz (21)，
according to this formula, the unattenuated image can then be calculated by the following formula:
the embodiment realizes the improvement of the traditional underwater polarization imaging by considering the wavelength attenuation problem existing in the underwater imaging process on the basis of the traditional underwater polarization imaging, and solves the color distortion problem existing in the existing underwater polarization imaging. When solving the color distortion problem, it is critical to accurately estimate the wavelength attenuation coefficient. According to the method, the attenuation coefficient deduced through the polarization information is fed back to the imaging model again, the depth map based on the wavelength attenuation information is deduced, the local neighborhood is subjected to more accurate joint constraint by combining the two depth maps, the accuracy of the depth information under the dual conditions of the polarization information and the wavelength attenuation information is ensured, and the finally estimated attenuation coefficient is accurate enough, so that the accurate correction of the color information is realized.
The above is a preferred embodiment of the present invention, and those skilled in the art to which the present invention pertains can also make variations and modifications to the above-described embodiment. Therefore, the present invention is not limited to the above-described embodiments, but is intended to be capable of modification, substitution and variation in light of the above-described embodiments, which will be apparent to those skilled in the art.
Claims (4)
1. The underwater polarized image restoration method based on polarization and wavelength attenuation combined optimization is characterized by comprising the following steps of:
s1, acquiring a color image and 4 polarized images corresponding to an underwater scene through an industrial camera and a linear polaroid, and estimating back scattered light and target information light by combining an underwater imaging model, so as to estimate a scene depth map based on polarized information according to the back scattered light;
the underwater imaging model is expressed as:
I c ＝D c +B c (1)，
i in formula (1) c Representing degraded underwater image, D c Representing the target information light, B c Represents backscattered light, J in equation (2) c Represents the unattenuated irradiance of the object in the scene, i.e., a clear image of achromatic distortion to be solved, β represents the attenuation coefficient, z represents the object-to-camera distance,representing the attenuation of the target information light, B ∞ Represents the back-scattered light at infinity, c represents the three color channels of the image, r represents the red channel, g represents the green channel, b representing a blue channel;
s2, restraining a local neighborhood according to the depth information of the scene depth map, so as to estimate local space average color and realize rough estimation of a target information light attenuation coefficient;
s3, combining an underwater imaging model, feeding back the primarily estimated attenuation coefficient to the model, deducing scene depth information based on wavelength attenuation information, combining the wavelength attenuation information and polarization information to realize joint optimization of local neighborhood so as to finish accurate estimation of the attenuation coefficient, and finally recovering a clear underwater image.
2. The underwater polarized image restoration method based on the combined optimization of polarization and wavelength attenuation as claimed in claim 1, wherein in the step S1, the method for obtaining the backscattered light is as follows:
acquiring an image I of the brightest underwater scene max And image I at darkest min At the same time consider the degree of polarization p of the backscattered light and the target scat And p obj Then:
b in the formula (3) 1 Representing the back-scattered light obtained by the polarization method, (x, y) representing the coordinate position of the pixel within the field of view;
the back scattered light in the scene can be estimated by selecting a specific area in the scene to estimate the back scattered light and the target polarization degree and then utilizing the above method;
after the back scattered light is obtained, further calculating a depth map of the scene:
z in 1 Represents a depth map estimated via polarization information.
3. The underwater polarization image restoration method based on polarization and wavelength attenuation combined optimization according to claim 1 or 2, wherein in the step S2, when calculating the local spatial average color, the depth information is used to screen the pixels in the local neighborhood;
setting a threshold delta, selecting connected pixels within the local neighborhood with a distance from the center pixel not greater than the threshold delta, and estimating a local spatial average color from the pixels:
in the formula (5) of the present invention,representing a set of pixels having a difference from the center pixel depth of less than a threshold delta, (x ', y') represents other pixel points where the difference between the depth in the local neighborhood and the depth of the center pixel is smaller than the threshold delta, and z represents calculating the difference between the depths of the two pixels 1 (x, y) represents the distance of the center pixel from the camera, z 1 (x ', y') represents the distance from the camera of the pixel point of the non-central pixel in the local neighborhood;
in the formula (6) of the present invention,representing the local spatial average color estimated by each iterative calculation +.>Representation set->Local spatial average color of the inner pixel, equation (7) Middle->Representing the local spatial average color estimated by iterative calculation, the parameter p describes the area of calculation of the average, the size of which depends on the size of the image;
estimating the local spatial average color a c 1 And then can be used for the whole illumination in the sceneAnd (3) estimating:
in equation (8), the parameter f is a scaling factor;
attenuation coefficient of the target information lightIt can be expressed as:
4. the underwater polarized image restoration method based on the combined optimization of polarization and wavelength attenuation as claimed in claim 1 or 2, wherein in the step S3, according to the followingScene depth information z by underwater imaging model 2 Estimation is performed again:
when calculating the local space average color, carrying out joint optimization treatment on the local neighborhood of the local space average color:
in the formula (11), the color of the sample is,representing a pixel set, wherein the difference between the depths of the pixel points and the center pixel is smaller than a threshold delta in the depth map calculated by two times, and (x ', y') represents the pixel points in the set, z 2 (x, y) represents the distance of the center pixel from the camera, z, in the second estimated depth map 2 (x ', y') represents the distance between the camera and the pixel point of the non-central pixel in the local neighborhood;
obtaining a local space average color on the optimized local neighborhood through iterative calculation:
in the formula (12) of the present invention,representing the local spatial average color at each iterative calculation,/->Representing a collectionLocal spatial average color of each pixel in (13)/(x)>Representing the local spatial average color obtained by iterative calculation;
and then, the scene illumination and the attenuation coefficient are accurately estimated:
in the formula (15), β represents the attenuation coefficient of the target information light estimated finally, and is combined with the image D from which the back scattered light has been removed c Finally, the unattenuated underwater image J can be obtained c ：
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