CN108961178B - SAR single-shot image brightness compensation method based on local histogram compression - Google Patents

SAR single-shot image brightness compensation method based on local histogram compression Download PDF

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CN108961178B
CN108961178B CN201810634614.3A CN201810634614A CN108961178B CN 108961178 B CN108961178 B CN 108961178B CN 201810634614 A CN201810634614 A CN 201810634614A CN 108961178 B CN108961178 B CN 108961178B
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孙增国
黄海超
蔡畅
吴杉
李琦伟
张祎彬
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Shaanxi Baota Xingkong Aerospace Technology Co ltd
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Abstract

The invention relates to a local histogram compression-based SAR single-shot image brightness compensation method, which comprises the following steps: s1, pairSelecting a region with uneven brightness as a selected region from an SAR monoscopic image to be processed; s2, traversing the selected area to obtain the lowest pixel gray value and the highest pixel gray value in the selected area; s3, making a histogram of the selected area, selecting the compression probability p of the histogram to obtain the maximum gray value x after the selected area is compressed, wherein the gray level range of the selected area is divided into [ A, x]And [ x +1, B]Two parts; s4, obtaining a proper compression factor m through an iteration methodfSo that the gray level of the selected region is within an initial range [ A, B ]]Compressed to [ A, x](ii) a S5, traversing the selected area and aiming at the gray value I of any pixelkIf I isk∈[A,x]Then output Ik(ii) a Otherwise, output
Figure DDA0001701112800000011
And obtaining an image after brightness compensation. The invention can effectively avoid the black spot phenomenon, retain the detail information of the image and obtain the satisfactory brightness compensation effect.

Description

SAR single-shot image brightness compensation method based on local histogram compression
Technical Field
The invention belongs to the technical field of SAR image brightness compensation in the field of image processing, and particularly relates to an SAR single-shot image brightness compensation method based on local histogram compression.
Background
The specific coherent imaging mode of the Synthetic Aperture Radar (SAR) enables the SAR to penetrate cloud, fog, rain and snow and generate image products with higher resolution and contrast, so that the SAR is widely applied to various fields of agriculture, disaster monitoring, geology, lake, ocean monitoring, navigation and the like. The extensive application and the deep research of the SAR image usually require the image to be uniform in brightness, moderate in contrast and consistent in color, so that various DOM mosaic products can be manufactured by the SAR image and can be fused with a geographic information system. However, the SAR is a microwave slant range imaging radar, the brightness information of the SAR image actually reflects the backscattering intensity of the ground target to the radar wave, and the intensity of the backscattering intensity depends on many factors such as the side view angle of the radar, the radar wavelength, the polarization mode of the radar, the surface orientation, the surface roughness, the target medium and the water content. Therefore, the actual complex feature of the ground feature makes the brightness information of the SAR image fluctuate in a large dynamic range. In fact, the inherent characteristics of the SAR imaging system often cause radiation distortion, which causes the phenomenon of uneven brightness on the image, and seriously affects the subsequent application of the image, so that a specific method is required for brightness compensation processing.
The royal nation of professor wuhan university proposes a frequency domain low-pass filtering method of two-step compensation of a monoscopic image (royal nation pine. SAR image automatic registration and embedding method research [ D ]. wuhan university 2015): the frequency domain low-pass filtering method utilizes Gaussian low-pass filtering to obtain a background image, and then difference is conducted on the original image and the background image to obtain an image after brightness compensation.
The above luminance compensation methods all have some problems: the frequency domain low-pass filtering method cannot ensure that the pixel gray value of the background image does not exceed the original image, if some pixel values of the background image are greater than the original image, a negative value appears in the corresponding pixel gray value of the difference image, so that corresponding detail information is lost, and a black spot phenomenon appears on the brightness compensation image.
Disclosure of Invention
The invention aims to provide a local histogram compression-based SAR single-shot image brightness compensation method, which can compensate the uneven brightness part of the SAR single-shot image, can retain the detail information of the SAR single-shot image and effectively avoid the black spot phenomenon. The technical problem to be solved by the invention is realized by the following technical scheme:
the SAR single-shot image brightness compensation method based on local histogram compression comprises the following steps:
step one, selecting an area with uneven brightness as a selected area for an image to be processed;
traversing the selected area to obtain a lowest pixel gray value A and a highest pixel gray value B in the selected area;
step three, making a histogram of the selected region, and selecting the compression probability p of the histogram to obtain the maximum gray value x of the compressed selected region, so that the gray level range of the selected region is divided into two parts of [ A, x ] and [ x +1, B ], wherein:
x is B.p, wherein A < x < B;
step four, obtaining a proper compression factor m through an iteration methodfMaking the gray level range of the selected region from initial [ A, B ]]Compressed to [ A, x]Namely:
Figure GDA0002586661680000021
step five, traversing the selected area and aiming at the gray value I of any pixelkIf I isk∈[min,x]Then directly output Ik；
Otherwise, then output
Figure GDA0002586661680000031
And obtaining an image after brightness compensation.
In the method for compensating the brightness of the SAR monoscopic image based on the local histogram compression, the mode of selecting the area with uneven brightness in the first step is manual selection.
In the second step, a double for loop is utilized to traverse the rows and columns of the selected region to obtain the lowest pixel gray value a and the highest pixel gray value B.
In the above method for compensating brightness of an SAR single-view image based on local histogram compression, the step of selecting the compression probability p of the histogram in the third step is:
1) traversing the histogram of the selected area, and finding out the gray level x of the pixel point corresponding to the maximum vertical coordinate in the histogram0；
2) Calculating a gray level range [ A, x ] in the histogram]Each thereinThe sum n of the number of the pixel points corresponding to each gray level and the total number S of the pixel points in the histogram are obtained, and p is obtained0：
Figure GDA0002586661680000032
And 0 < p0＜1
p∈[p0,1)
In the formula, p0Is the lower probability limit of histogram compression.
In the above method for compensating brightness of SAR single-view image based on local histogram compression, a suitable compression factor m is selected in the fourth stepfThe method comprises the following specific steps: setting an initial value m of a compression factor0Step of compression, for a compression factor m at the i-th iteration step, 1iAnd judging whether the following conditions are met:
Figure GDA0002586661680000033
if miSatisfy the above two formulae at the same time, then miI.e. the selected compression factor mf(ii) a Otherwise, judging the compression factor m in the iteration of the (i + 1) th stepi+1＝miWhether step satisfies the above two formulas at the same time until miWhen the two formulas are simultaneously satisfied, a proper compression factor m is outputf＝mi。
Compared with the prior art, the invention has the beneficial effects that:
the invention carries out iterative compression on the gray level of the histogram in the selected area by analyzing the relationship between the distribution of the gray level of the histogram in the selected uneven brightness area and the uneven brightness value, so that the gray level of the histogram in the selected area is in a proper range, and finally the aim of compensating the uneven brightness value of the SAR image is fulfilled; compared with a frequency domain low-pass filtering method, the method does not involve difference image processing, and can effectively avoid the phenomenon of black spots, thereby better retaining the detail information of the image and obtaining a more satisfactory brightness compensation effect.
Drawings
Fig. 1 is a flowchart of a method for compensating brightness of an SAR single-shot image.
Fig. 2 is an SAR single-view image a and an SAR single-view image b used in the experiment of the embodiment of the present invention.
Fig. 3 is a graph sequentially showing the result of the brightness compensation of the SAR single-view image based on local histogram compression by applying frequency domain low-pass filtering to the SAR single-view image a from left to right.
Fig. 4 is a graph sequentially showing the result of the luminance compensation of the SAR single-view image based on local histogram compression by applying frequency domain low-pass filtering to the SAR single-view image b from left to right.
Detailed Description
The present invention will be described in further detail with reference to specific examples, but the embodiments of the present invention are not limited thereto.
Referring to fig. 1, the present embodiment discloses a method for compensating brightness of an SAR single-shot image based on local histogram compression, which includes: selecting an area with uneven brightness as a selected area for an SAR monoscopic image to be processed; traversing the selected area to obtain a lowest pixel gray value A and a highest pixel gray value B in the selected area; step three, making a histogram of the selected region, selecting the compression probability p of the histogram to obtain the maximum gray value x of the compressed selected region, and dividing the gray level range of the selected region into [ A, x ]]And [ x +1, B]Two parts; wherein x is B.p (A < x < B); step four, obtaining a proper compression factor m through an iterative methodfMaking the gray level range of the selected region from initial [ A, B ]]Compressed to [ A, x]Namely:
Figure GDA0002586661680000051
step five, traversing the selected area and aiming at the gray value I of any pixelkIf I isk∈[A,x]Then directly output Ik(ii) a Otherwise, then output
Figure GDA0002586661680000052
Thereby obtaining a brightness compensated image.
Before describing the method of the present embodiment in detail, the following description is made on the schematic diagram:
the histogram mentioned in the invention is the gray level histogram of the image, the gray level histogram is the function about gray level distribution, it expresses the number of the corresponding pixel of each gray level in the image, the frequency that each gray level appears in the reaction image; in other words, the gray histogram is a statistic of the frequency of occurrence of all pixels in the digital image according to the size of the gray value. The gray level histogram only represents the number of pixel points corresponding to each gray level of the image and does not represent the distribution positions of the pixel points in the image.
The SAR single-shot image brightness compensation method based on local histogram compression comprises the following steps:
selecting an area with uneven brightness as a selected area for an SAR monoscopic image to be processed;
specifically, the method comprises the following steps: the target to be processed is to perform brightness compensation on the area with uneven brightness in the SAR single-scene image, and in the SAR image, the area with uneven brightness is only in a certain range of the image, so the area with uneven brightness is manually selected, and the selected area with uneven brightness is used as the selected area. And the selected area should be as large as possible, including the area of uneven brightness within the selected area.
Traversing the selected area to obtain a lowest pixel gray value A and a highest pixel gray value B in the selected area;
specifically, the method comprises the following steps: and traversing the rows and the columns of the selected area by adopting double for circulation to obtain the lowest pixel gray value A and the highest pixel gray value B when the SAR single-scene image is a two-dimensional matrix.
Step three, making a histogram of the selected region, and selecting the compression probability p of the histogram to obtain the maximum gray value x of the compressed selected region, so that the gray level range of the selected region is divided into two parts of [ A, x ] and [ x +1, B ]; wherein the content of the first and second substances,
x is B.p, wherein A < x < B;
specifically, the method comprises the following steps: the step of selecting the compression probability p of the histogram is:
1) traversing the histogram of the selected area, and finding out the gray level x of the pixel point corresponding to the maximum vertical coordinate in the histogram0；
2) Calculating a gray level range [ A, x ] in the histogram]The sum n of the number of the pixel points corresponding to each gray level in the histogram and the total number S of the pixel points in the histogram are obtained to obtain the lower limit p of the compression probability0，p0The expression of (a) is:
Figure GDA0002586661680000061
and 0 < p0＜1
In order to ensure the effect of gray level brightness compensation and avoid the reduction of image contrast caused by too small range of gray level after compression, the compression probability p must be [ p ]0And 1) taking the value. I.e. in [ p ]01), i.e., p ∈ [ p ]0,1)。
Step four, obtaining a proper compression factor m through an iteration methodfMaking the gray level range of the selected region from initial [ A, B ]]Compressed to [ A, x]Namely:
Figure GDA0002586661680000062
specifically, the compression factor m is obtained by an iterative methodfThe method comprises the following specific steps:
setting an initial value m of a compression factor0Step of compression, for a compression factor m at the i-th iteration step, 1iAnd judging whether the following conditions are met:
Figure GDA0002586661680000071
if miSatisfy the above two formulae at the same time, then miI.e. the selected compression factor mf(ii) a Otherwise, judging the compression factor m in the iteration of the (i + 1) th stepi+1＝miWhether step satisfies the above two formulas at the same time until miWhen the two formulas are simultaneously satisfied, a proper compression factor m is outputf＝mi。
Step five, traversing the selected area and aiming at the gray value I of any pixelkIf I isk∈[A,x]Then output Ik(ii) a Otherwise, output
Figure GDA0002586661680000072
Thereby obtaining a brightness compensated image.
In this embodiment, the compressed histogram is compressed to obtain the maximum gray value x by selecting the compression probability p, and the gray level range of the selected region is divided into [ a, x [ ]]And [ x +1, B]Two parts, and [ x +1, B ]]Compressing the pixel points in the range to make the gray scale range of the selected region from the initial [ A, B ]]Compressed to [ A, x]The method ensures that the compression factor is really suitable through an iteration method
Figure GDA0002586661680000073
Finally, the compression of the gray level of the selected area is completed.
In the present embodiment, two images, that is, an SAR monoscopic image a and an SAR monoscopic image b shown in fig. 2 are used in the experiment.
As shown in fig. 3, in the luminance compensation result graph of the SAR single-view image a, where the cutoff frequency using the frequency domain low-pass filtering method is 6, the present embodiment proposes that the compression probability p of the luminance compensation method based on the local histogram compression is 0.89, and step is 0.5.
As shown in fig. 4, in the luminance compensation result graph of the SAR monoscopic image b, where the cutoff frequency using the frequency domain low-pass filtering method is 6, the present embodiment proposes that the compression probability p of the luminance compensation method based on the local histogram compression is 0.86, and step is 0.3.
As is apparent from fig. 3 and 4, the frequency domain low-pass filtering method can reduce the bright tracks of the image to a certain extent, but does not effectively eliminate the bright track phenomenon, the gray levels of the processed result image still show obvious differences, and the detail information on the image is lost, and many black point regions appear on the result image.
In summary, the SAR single-view image brightness compensation method based on local histogram compression provided in this embodiment only needs to select the compression probability p and the iteration step size, and can iteratively obtain the appropriate compression factor mfAccording to a suitable compression factor mfAnd the image after brightness compensation is automatically obtained, and the realization is convenient. Meanwhile, because the difference operation of pixel gray levels is not involved, the detail information of the SAR image can be effectively reserved, the black spot phenomenon is avoided, and the high brightness compensation precision is obtained.
The foregoing is a more detailed description of the invention in connection with specific preferred embodiments and it is not intended that the invention be limited to these specific details. For those skilled in the art to which the invention pertains, several simple deductions or substitutions can be made without departing from the spirit of the invention, and all shall be considered as belonging to the protection scope of the invention.

Claims (4)

1. The SAR monoscopic image brightness compensation method based on local histogram compression is characterized by comprising the following steps of:
selecting an area with uneven brightness as a selected area for an SAR monoscopic image to be processed;
traversing the selected area to obtain a lowest pixel gray value A and a highest pixel gray value B in the selected area;
step three, making a histogram of the selected region, and selecting the compression probability p of the histogram to obtain the maximum gray value x of the compressed selected region, so that the gray level range of the selected region is divided into two parts of [ A, x ] and [ x +1, B ]; wherein the content of the first and second substances,
x is B.p, wherein A < x < B;
step four, obtaining a proper compression factor m through an iteration methodfSo that the gray level of the selected region is within an initial range [ A, B ]]Compressed to [ A, x]Namely:
Figure FDA0002578926330000011
step five, traversing the selected area and aiming at the gray value I of any pixelkIf I isk∈[A,x]Then directly output Ik；
Otherwise, then output
Figure FDA0002578926330000012
Obtaining an image after brightness compensation;
the step of selecting the compression probability p of the histogram in the third step is as follows:
1) traversing the histogram of the selected area, and finding out the gray level x of the pixel point corresponding to the maximum vertical coordinate in the histogram0；
2) Calculating a gray level range [ A, x ] in the histogram]The sum n of the number of the pixel points corresponding to each gray level in the histogram and the total number S of the pixel points in the histogram are obtained, and p is obtained0：
Figure FDA0002578926330000021
And 0 < p0＜1
p∈[p0,1)
Wherein p is0Is the lower probability limit of histogram compression.
2. The SAR monoscopic image brightness compensation method based on local histogram compression as claimed in claim 1, wherein the mode of selecting the non-uniform brightness area in the first step is manual selection.
3. The SAR monoscopic image brightness compensation method according to claim 1, wherein in the second step, the selected area is traversed by using double for loop to obtain the lowest pixel gray value A and the highest pixel gray value B.
4. The SAR monoscopic image brightness compensation method based on local histogram compression as claimed in claim 1, characterized in that in the fourth step, a suitable compression factor m is selectedfThe method comprises the following specific steps: setting an initial value m of a compression factor0Step of compression, for a compression factor m at the i-th iteration step, 1iAnd judging whether the following conditions are met:
Figure FDA0002578926330000022
if miSatisfy the above two formulae at the same time, then miI.e. the selected compression factor mf(ii) a Otherwise, judging the compression factor m in the iteration of the (i + 1) th stepi+1＝miWhether step satisfies the above two formulas at the same time until miWhen the two formulas are simultaneously satisfied, a proper compression factor m is outputf＝mi。
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