CN113538284A - Transplantation method of image defogging algorithm based on dark channel prior - Google Patents
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Abstract
A transplantation method of an image defogging algorithm based on dark channel prior belongs to the field of image defogging processing. In the existing image defogging algorithm based on dark channel prior, guide filtering used in thinning a light propagation diagram is too complex and the algorithm is too complex. The invention comprises the following steps: acquiring a color fog image with R, G, B three channels as an input image, and inputting the color fog image into the FPGA by using a CameraLink; carrying out normalization; calculating atmospheric light value and transmittance, and performing inversion operation in the simplified foggy image imaging model to obtain three defogged images R, G, BA channel, using t0To limit the lower limit of the ray propagation map for these points; further thinning the transmissivity and recovering a fog-free image; the intermediate result is cached by adopting a DDRSDRAM; and outputs the defogged video using the DVI signal generated by CH 7301C. The method realizes the improved transplantation of the defogging algorithm based on the dark channel prior image in the FPGA and has the advantage of simple calculation process.
Description
Technical Field
The invention relates to a transplantation method of an image defogging algorithm based on dark channel prior.
Background
At present, in all image defogging algorithms, the physical degradation process of an image in a foggy day is not considered in an image enhancement-based method, and the degraded image in the foggy weather is enhanced only by using the existing and mature digital image processing technology, so that the purposes of highlighting the edge details of the image and improving the overall contrast of the image are achieved. Although the method can improve the influence of haze weather on the image to a certain extent, the method does not consider the physical degradation mechanism of the image in the haze weather, so that the result often has the problem of color distortion, and is not suitable for image defogging under the dense fog condition, so that the application range is not wide. The image defogging method based on the atmospheric scattering model is based on the principle of image imaging, and the atmospheric light intensity and the medium transmissivity are estimated by combining certain hypothesis or priori knowledge, so that a clear image is recovered. Because the physical degradation mechanism of the foggy image is considered and modeling solution is carried out on the foggy image imaging, the image recovered by the method is natural and reasonable, can be applied to the dense fog condition and is the key research direction in the field of image defogging at present.
Different from the image defogging technology, the research is quite extensive and mature day by day, and the research on the video defogging technology is still in the initial stage at present, so that a plurality of problems still need to be solved. However, with the continuous improvement of the technology level and the increasing importance of the society on intelligent transportation, more and more industry experts and researchers are believed to pay attention to and put into the research of the video defogging technology. Therefore, video defogging and related technologies will be a focus of research in the next period of time.
In the current image defogging algorithm based on dark channel prior, guiding filtering used when a light propagation image is thinned is too complex, a filter template coefficient of the guiding filtering needs to be recalculated once every filtering is performed, the algorithm is too complex, and the method is not suitable for being transplanted to an embedded system composed of a DSP, an FPGA and the like. Therefore, the invention researches how to improve the image defogging algorithm based on dark channel prior, and transplants the image defogging algorithm into the FPGA after being cut.
Disclosure of Invention
The invention aims to solve the problems that guide filtering used in the process of thinning a light propagation diagram is too complex and an algorithm is too complex in the existing image defogging algorithm based on dark channel prior, and provides a transplantation method of the image defogging algorithm based on the dark channel prior.
A transplantation method of image defogging algorithm based on dark channel prior comprises the following steps,
step one, acquiring a color fog image with R, G, B three channels as an input image, and inputting the color fog image into an FPGA by using a CameraLink;
step two, normalizing the color image with R, G, B three channels;
first, assuming that the atmospheric light value a is known, for R, G, B three channels of the simplified foggy day image imaging model, the atmospheric light value a is divided by two sides of the equation at the same time, and the result is expressed as:
step three, calculating dark channel values at two sides of the formula:
set in an arbitrary local region, i.e., in the window Ω (x), the transmittance t (x) is a constant and is expressed as
Then, calculating the dark channel values on both sides of the equation to obtain:
since j (y) represents a fog-free image, j (y) dark channel tends towards 0, according to the nature of dark channel, i.e.:
since Ac is a positive number, we deduce:
this can result in:
the sky in the foggy image with sky is close to the atmospheric light value, so that at the sky of the foggy image with sky, the following results are obtained:
further obtain
The transmissivity of the sky area at infinity is also approximate to 0, so that the sky area and the non-sky area of the foggy image do not need to be separately processed during defogging processing;
step four, calculating an atmospheric light value A: firstly, selecting the brightest first 0.1% pixels of a dark channel image of a foggy image, wherein the pixel points are often the places with the most fog, and corresponding the pixel points to corresponding points of the foggy image, wherein the maximum values of three channels of the corresponding points R, G, B of the foggy image are respectively regarded as atmospheric light values of the three channels;
and step five, calculating to obtain an atmospheric light value and transmittance through the previous steps, and performing inversion operation in a simplified foggy day image imaging model to recover and obtain a defogged image R, G, B through the following calculation by respectively using the following formulas:
using t0To limit the lower limit of the ray propagation map for these points;
sixthly, further thinning the transmissivity, and recovering the fog-free image by using the thinned transmissivity, wherein the method specifically comprises the following steps:
first, the foggy image is subjected to edge detection to generate a mask image BWmask(ii) a Then, dark channels are respectively obtained for the foggy images by using 3 × 3 and 15 × 15 filter templates; thereafter, mask image BW is usedmaskSynthesizing dark channel inspection, and obtaining a light propagation diagram t (x) by using a newly generated dark channel;
step seven, caching the intermediate result by adopting a DDRSDRAM; and a DVI signal generated by CH7301C is adopted to output defogged video, so that the transplantation of an image defogging algorithm based on dark channel prior on a defogging system is realized.
Based on the embodiment of the invention, preferably, the FPGA of the FPGA-based vehicle-mounted real-time defogging system selects EP4CE10F17C8 type as a main control chip, the input adopts CameraLink input, the output adopts DVI signal output generated by CH7301C, and the intermediate result adopts DDRSDRAM for caching;
wherein the content of the first and second substances,
(1) the transmitter receives a 28-bit single-ended data signal and a 1-bit single-ended clock signal, then the data signal and the clock signal are converted into a group of 4-bit differential data stream and 1-bit differential clock signal by the transmitter in a ratio of 7:1, after the converted data signal and clock signal are sent out, the receiver receives the data signal and the differential clock signal and restores the differential data signal and the differential clock signal into a 28-bit single-ended data signal and a 1-bit single-ended clock signal, and data transmission is completed;
the CameraLink continues to use the bottom layer architecture of the ChannelLink, and the transmission rate of the CameraLink is 2.38 Gbits/s; the CameraLink protocol is divided into five configurations, namely simplified configuration, basic configuration, middle-end configuration, high-end configuration and 80-bit configuration.
(2) The DVI protocol is called DigitalVisualInterface, namely a digital video interface; the FPGA controls CH7301C, and then CH7301C generates DVI signal;
(3) DDRSDRAM is called double DateRateSDRAM, Chinese is called double-rate synchronous dynamic random access memory; the ddr sdram latches data on both the rising and falling edges of the clock signal.
Preferably, according to the embodiment of the present invention, the method further comprises the steps of,
the method for correcting transmission by setting a fixed threshold value is used for correcting color distortion of a fog-containing image containing a sky or white area, and specifically comprises the following steps:
recording sky or white and other areas of the fog-containing image, presenting high peak characteristics on the gray level histogram, and obtaining an adaptive threshold value by utilizing the characteristics to partition a sky or white and other bright areas and non-bright areas; and then, designing a self-adaptive correction function, improving an image atmospheric dissipation function, and performing defogging treatment on the regions respectively.
The invention has the beneficial effects that:
the invention mainly solves the problems that:
(1) and (3) realizing the improved image defogging algorithm on the FPGA.
(2) And the defogging treatment is carried out on the video image to meet the requirement of real-time property.
Aiming at the structure and the characteristics of an FPGA hardware platform and having the characteristic of strong parallel processing capability, the real-time defogging processing of a video image is realized on the FPGA hardware platform through the improvement and the simplification of an algorithm based on the algorithm, and the method is applied to a vehicle-mounted system, so that the problem of traffic accidents caused by the blocked vision of a driver in a foggy day is solved. In order to achieve the expected effect, the original dark channel prior theory is firstly learned and known, a filter template needs to be recalculated every time guiding filtering for soft matting is carried out when a ray propagation diagram is obtained, time is consumed, the refined dark channel is directly used as the depth of field for estimating the ray propagation diagram, the ray propagation diagram does not need to be subjected to soft matting, the algorithm running time is greatly reduced, the expected effect can be achieved after defogging, a Camera Link input control module and a DVI output module of CH7301C are designed in an FPGA based on Verilog language, and a DDRSDRAM controller is achieved by using a memory interface solution provided by Altera. The implementation of these modules lays a good foundation for implementing an improved dark channel prior-based image defogging algorithm in an FPGA. And finally, the improved dark channel prior image defogging algorithm based transplantation is realized in the FPGA.
The invention relates to a vehicle-mounted real-time defogging system based on an FPGA framework. Aiming at the strong parallel computing processing capacity of FPGA, the real-time processing of video images can be completely realized, because the image defogging algorithm based on the dark channel prior theory is too complex when the transmissivity is refined, the processing speed can be influenced, the real-time performance cannot be achieved, and the image defogging algorithm is not suitable for being realized on hardware, in order to enable the transmissivity calculation and the atmospheric light estimation to be realized on hardware more quickly and conveniently, the calculation process is simplified, and finally, an optimization method realized on the basis of FPGA is used for realizing the hardware logics of a plurality of modules such as dark channel acquisition, transmissivity calculation, atmospheric light estimation, final image restoration and the like. The vehicle-mounted real-time defogging system can acquire front road information in real time in a foggy weather, helps a driver to accurately judge road conditions, avoids traffic accidents caused by the problem of blocked vision, and meanwhile, most of the current advanced auxiliary driving systems based on vision in the market cannot process foggy day images, so that the vehicle-mounted real-time defogging system has very important significance in research.
Drawings
FIG. 1 is a flow chart of the method of the present invention;
fig. 2 is a data conversion and transmission process of the ChannelLink according to the present invention;
fig. 3 is a schematic diagram of DVI signals output by the FPGA controlling CH7301C according to the present invention.
Detailed Description
The first embodiment is as follows:
the transplantation method of the image defogging algorithm based on the dark channel prior in the embodiment is shown in a method flow chart in fig. 1, and the method comprises the following steps,
step one, acquiring a color fog image with R, G, B three channels as an input image, and inputting the color fog image into an FPGA by using a CameraLink;
step two, normalizing the color image with R, G, B three channels;
first, assuming that the atmospheric light value a is known, for R, G, B three channels of the simplified foggy day image imaging model, the atmospheric light value a is divided by two sides of the equation at the same time, and the result is expressed as:
step three, calculating dark channel values at two sides of the formula:
set in an arbitrary local region, i.e., in the window Ω (x), the transmittance t (x) is a constant and is expressed as
Calculating the dark channel values on both sides of the equation, we can obtain:
since j (y) represents a fog-free image, j (y) dark channel tends towards 0, according to the nature of dark channel, i.e.:
since Ac is a positive number, we deduce:
this can result in:
the sky in the foggy image with sky is close to the atmospheric light value, so that at the sky of the foggy image with sky, the following results are obtained:
further obtain
The transmissivity of the sky area at infinity is also approximate to 0, so that the sky area and the non-sky area of the foggy image do not need to be separately processed during defogging processing;
step four, calculating an atmospheric light value A: firstly, selecting the brightest first 0.1% pixels of a dark channel image of a foggy image, wherein the pixel points are often the places with the most fog, the pixel points are corresponding to corresponding points of the foggy image, and the respective maximum values of three channels of the corresponding points R, G, B of the foggy image are respectively regarded as atmospheric light values of the three channels;
and step five, calculating to obtain an atmospheric light value and transmittance through the previous steps, and performing inversion operation in a simplified foggy day image imaging model to recover and obtain a defogged image R, G, B through the following calculation by respectively using the following formulas:
due to calculated ray propagation diagram
Very close to 0 at some points, if the light propagation pattern of these points is directly used to restore the defogged image, the defogged image may be very noisy, so t is used0To limit thisThe lower limit of the ray propagation map for points;
step six, when the depth of the adjacent objects in the scene is discontinuous, a circle of obvious white thin band appears in the edge zone generally, and the phenomenon is called Halo effect (Halo).
This phenomenon arises because, when the image is defogged based on the dark channel prior theory, it is in the local window region
Within Ω (x), we assume a coarse transmittance
Is a constant, and the transmittance at the non-edge area with gentle image depth can be correctly estimated. However, at the abrupt change of the depth of the image scene, the transmittance t (x) is not constant within the range of the current window, so that the transmittance estimation of the foggy area at the edge of the image is deviated, and the foggy area at the edge is treated as a clear part during processing, so that the foggy area is not subjected to image enhancement and is directly transmitted out together with a near object, and a halo is generated. Therefore, the rough transmittance needs to be further refined, the transmittance is further refined, and the refined transmittance is used to recover the haze-free image, which specifically comprises the following steps:
first, the foggy image is subjected to edge detection to generate a mask image BWmask(ii) a Then, dark channels are respectively obtained for the foggy images by using 3 × 3 and 15 × 15 filter templates; thereafter, mask image BW is usedmaskAnd (4) synthesizing a dark channel test, and obtaining a light propagation diagram t (x) by using the newly generated dark channel. The method uses a smaller filter template to carry out filtering on the edge of an object in the foggy image to obtain the dark channel, not only can the edge be reserved, but also the steps designed by the invention are utilized to carry out the obtaining of t (x) of the light propagation diagram, so that the step of soft image matting can be omitted, and the operation time is greatly saved;
step seven, caching the intermediate result by adopting a DDRSDRAM; and a DVI signal generated by CH7301C is adopted to output defogged video, so that the transplantation of an image defogging algorithm based on dark channel prior on a defogging system is realized.
The second embodiment is as follows:
different from the specific embodiment, in the transplantation method of the image defogging algorithm based on the dark channel prior, the FPGA of the vehicle-mounted real-time defogging system based on the FPGA selects EP4CE10F17C8 model of cycleiv of Altera company as a main control chip, the input adopts CameraLink input, the output adopts DVI signal output generated by CH7301C, and the intermediate result adopts DDRSDRAM for caching;
wherein the content of the first and second substances,
(1) the CameraLink protocol is a standard protocol developed on the basis of the ChannelLink protocol proposed by national semiconductor corporation of america and now becomes image video data transmission. The channeliLink protocol comprises a transmitter and a receiver, wherein the transmitter receives a 28-bit single-ended data signal and a 1-bit single-ended clock signal, then the data signal and the clock signal are converted into a group of 4-bit differential data stream and 1-bit differential clock signal by the transmitter in a ratio of 7:1, the converted data signal and clock signal are transmitted out, the receiver receives the data signal and differential clock signal and then restores the data signal and differential clock signal into a 28-bit single-ended data signal and a 1-bit single-ended clock signal, data transmission is completed, and the data transmission and conversion process is as shown in FIG. 2;
the CameraLink continues to use the bottom layer architecture of the ChannelLink, and the transmission rate of the CameraLink is 2.38 Gbits/s; the CameraLink protocol is divided into five configurations, namely a reduced (Lite) configuration, a Base (Base) configuration, a Medium (Medium) configuration, a high (Full) configuration, and an 80bit configuration.
(2) The DVI protocol is called DigitalVisualInterface, namely a digital video interface; it is a video interface standard proposed by DDWG (digital display working group) made by intel, compaq, IBM, etc., and controls CH7301C by FPGA, and then CH7301C generates DVI signals as shown in fig. 3:
(3) DDRSDRAM is called double DateRateSDRAM, Chinese is called double-rate synchronous dynamic random access memory; in terms of RAM, ddr sdram latches data on both the rising and falling edges of the clock signal, which is equivalent to doubling the data throughput rate.
The third concrete implementation mode:
unlike the first or second embodiment, the transplantation method based on the dark channel prior image defogging algorithm of the present embodiment further includes the following steps,
the method for correcting transmission by setting a fixed threshold value is used for correcting color distortion of a fog-containing image containing a sky or white area, and specifically comprises the following steps:
the method comprises the steps that as areas such as sky or white of a fog-containing image are bright, the areas such as sky or white of the fog-containing image are recorded, a high-peak characteristic is presented on a gray level histogram, and an adaptive threshold value is obtained by utilizing the characteristic to partition a bright area such as sky or white and a non-bright area; and then, designing a self-adaptive correction function, improving an image atmospheric dissipation function, and performing defogging treatment on the regions respectively. Therefore, the problem that after the fog-containing image containing bright areas such as sky and white is defogged, the color of the bright areas is distorted, and the contrast of the whole image is reduced is solved.
The fourth concrete implementation mode:
different from the third specific embodiment, in the transplantation method of the image defogging algorithm based on the dark channel prior, the traditional image defogging algorithm is directed at a single image, the atmospheric light intensity of each frame of image is independently estimated, and the flicker and jitter phenomenon of the restored video image can be caused finally due to the jump of the atmospheric light intensity estimated values of the previous and next frames. In order to improve the flicker jitter of the defogged video, a proper filtering mode is found to be processed when the atmospheric light intensity estimated value is calculated
The above is only a preferred embodiment of the present invention, and is not intended to limit the present invention, and various modifications and changes may be made to the present invention by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (3)
1. A transplantation method of an image defogging algorithm based on dark channel prior is characterized by comprising the following steps: the method comprises the following steps of,
step one, acquiring a color fog image with R, G, B three channels as an input image, and inputting the color fog image into an FPGA by using a CameraLink;
step two, normalizing the color image with R, G, B three channels;
first, assuming that the atmospheric light value a is known, for R, G, B three channels of the simplified foggy day image imaging model, the atmospheric light value a is divided by two sides of the equation at the same time, and the result is expressed as:
step three, calculating dark channel values at two sides of the formula:
set in an arbitrary local region, i.e., in the window Ω (x), the transmittance t (x) is a constant and is expressed as
Calculating the dark channel values on both sides of the equation, we can obtain:
since j (y) represents a fog-free image, j (y) dark channel tends towards 0, according to the nature of dark channel, i.e.:
since Ac is a positive number, we deduce:
this can result in:
the sky in the foggy image with sky is close to the atmospheric light value, so that at the sky of the foggy image with sky, the following results are obtained:
further obtain
The transmissivity of the sky area at infinity is also approximate to 0, so that the sky area and the non-sky area of the foggy image do not need to be separately processed during defogging processing;
step four, calculating an atmospheric light value A: firstly, selecting the brightest first 0.1% pixels of a dark channel image of a foggy image, wherein the pixel points are often the places with the most fog, and corresponding the pixel points to corresponding points of the foggy image, wherein the maximum values of three channels of the corresponding points R, G, B of the foggy image are respectively regarded as atmospheric light values of the three channels;
and step five, calculating to obtain an atmospheric light value and transmittance through the previous steps, and performing inversion operation in a simplified foggy day image imaging model to recover and obtain a defogged image R, G, B through the following calculation by respectively using the following formulas:
using t0To limit the lower limit of the ray propagation map for these points;
sixthly, further thinning the transmissivity, and recovering the fog-free image by using the thinned transmissivity, wherein the method specifically comprises the following steps:
first, the foggy image is subjected to edge detection to generate a mask image BWmask(ii) a Then, dark channels are respectively obtained for the foggy images by using 3 × 3 and 15 × 15 filter templates; thereafter, mask image BW is usedmaskSynthesizing dark channel inspection, and obtaining a light propagation diagram t (x) by using a newly generated dark channel;
step seven, caching the intermediate result by adopting a DDRSDRAM; and a DVI signal generated by CH7301C is adopted to output defogged video, so that the transplantation of an image defogging algorithm based on dark channel prior on a defogging system is realized.
2. The transplantation method of image defogging algorithm based on dark channel prior according to claim 1, wherein: the transplanting method is that the FPGA is transplanted on a vehicle-mounted real-time defogging system based on the FPGA, the FPGA of the vehicle-mounted real-time defogging system based on the FPGA selects an EP4CE10F17C8 type as a main control chip, the input adopts CameraLink input, the output adopts DVI signal output generated by CH7301C, and the intermediate result adopts DDRSDRAM for caching;
wherein the content of the first and second substances,
(1) the transmitter receives a 28-bit single-ended data signal and a 1-bit single-ended clock signal, then the data signal and the clock signal are converted into a group of 4-bit differential data stream and 1-bit differential clock signal by the transmitter in a ratio of 7:1, after the converted data signal and clock signal are sent out, the receiver receives the data signal and the differential clock signal and restores the differential data signal and the differential clock signal into a 28-bit single-ended data signal and a 1-bit single-ended clock signal, and data transmission is completed;
the CameraLink continues to use the bottom layer architecture of the ChannelLink, and the transmission rate of the CameraLink is 2.38 Gbits/s; the CameraLink protocol is divided into five configurations, namely simplified configuration, basic configuration, middle-end configuration, high-end configuration and 80-bit configuration.
(2) The DVI protocol is called DigitalVisualInterface, namely a digital video interface; the FPGA controls CH7301C, and then CH7301C generates DVI signal;
(3) DDRSDRAM is called double DateRateSDRAM, Chinese is called double-rate synchronous dynamic random access memory; the ddr sdram latches data on both the rising and falling edges of the clock signal.
3. A dark channel prior based image defogging algorithm transplanting method according to claim 1 or 2, wherein: the method further comprises the step of,
the method for correcting transmission by setting a fixed threshold value is used for correcting color distortion of a fog-containing image containing a sky or white area, and specifically comprises the following steps:
recording sky or white and other areas of the fog-containing image, presenting high peak characteristics on the gray level histogram, and obtaining an adaptive threshold value by utilizing the characteristics to partition a sky or white and other bright areas and non-bright areas; and then, designing a self-adaptive correction function, improving an image atmospheric dissipation function, and performing defogging treatment on the regions respectively.
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