CN114528728B - Monte Carlo computer simulation method for canopy reflection of aquatic vegetation in line sowing - Google Patents
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Abstract
The invention relates to a Monte Carlo computer simulation method for reflecting a canopy of a broadcast aquatic vegetation, which comprises the following steps: constructing a three-dimensional scene of aquatic vegetation according to the geometric structure, the row direction and the row spacing characteristics of the row sowing vegetation, and uniformly distributing blades in a ridge; the model is coupled with a spectrum model of the vegetation leaf for calculating the reflectivity and the transmissivity of the leaf; the wave water surface reflection model is coupled to calculate water surface reflection and transmission distribution; the shallow water biological optical model is coupled to calculate the absorption coefficient and the scattering coefficient of the background water body; and randomly generating a large number of incident photons according to illumination distribution, tracking the complete propagation process of each photon in a simulated scene, recording the emergent direction, and calculating the reflectivity of the whole aquatic vegetation canopy according to the statistical distribution of the emergent photons. The invention provides a novel model tool and a novel technical method for the remote sensing monitoring of physiological and biochemical parameters of the aquatic vegetation canopy, and simultaneously provides a scientific basis for the accurate water and fertilizer regulation and control management of the row-sowed crops.
Description
One of the technical fields
The invention relates to a Monte Carlo computer simulation method for canopy reflection of a broadcast aquatic vegetation, belongs to the field of optical remote sensing, can be applied to optical remote sensing imaging simulation or cross verification of other remote sensing models, and has important significance in the aspects of wetland ecological research and crop quantitative monitoring application.
(II) background art
The aquatic vegetation is a common planting structure in agricultural production, and has periodicity in space structure, and the three-dimensional structure can be conveniently represented in a computer. The three-dimensional aquatic vegetation scene has obvious change in radiation transmission process compared with land vegetation due to the existence of water body background, and is mainly due to absorption and scattering effects on light generated by various impurity particles in water-air interfaces and water bodies. Light is reflected or refracted at the water-air interface, and is not perfectly smooth at the natural water surface, so that it cannot be regarded as an ideal scattering or refraction phenomenon. In the water body, light can interact with impurities, so that deflection is caused, and the effect of volume scattering is caused, so that the complexity of remote sensing modeling of aquatic vegetation is increased. Therefore, by exploring the influence of each element in the scene on the light propagation, and further constructing a remote sensing model of the on-going aquatic vegetation, the influence of various biochemical or geometric parameters in the on-going aquatic vegetation scene on the reflection characteristics of the vegetation canopy can be researched, so that corresponding information is more fully extracted from ground measurement or satellite image data, the inversion process is assisted, and a new efficient and accurate way is provided for the monitoring, estimated production, early warning, protection and quantitative inversion of the aquatic vegetation. Meanwhile, a computer simulation model based on the Monte Carlo method is regarded as a more accurate algorithm, and can be used as a verification tool of other simplified models.
The reflection characteristics of an object are typically described precisely by a bi-directional reflection distribution function (Bidirectional Reflectance Distribution Function, BRDF). The invention automatically constructs a three-dimensional scene of the aquatic vegetation on the fly by coupling the plant leaf spectral model, the wave water surface reflection model and the shallow water biological optical model, and further simulates by using a Monte Carlo ray tracing method, thereby developing a Monte Carlo computer simulation method for canopy reflection of the aquatic vegetation on the fly.
(III) summary of the invention
The invention relates to a Monte Carlo computer simulation method for reflecting a canopy of a broadcast aquatic vegetation, which comprises the following steps: constructing a three-dimensional scene of aquatic vegetation according to the geometric structure, the row direction and the row spacing characteristics of the row-sowing vegetation, and assuming that vegetation crowns and water bodies in the direction vertical to the row direction of the ridge are alternately arranged, and uniformly distributing blades in the ridge; the model is coupled with a spectrum model of the vegetation leaf for calculating the reflectivity and the transmissivity of the leaf; the wave water surface reflection model is coupled to calculate water surface reflection and transmission distribution; the shallow water biological optical model is coupled to calculate the absorption coefficient and the scattering coefficient of the water body; randomly generating a large number of incident photons according to illumination distribution, tracking the complete propagation process of each random photon in a simulated scene, and recording the emergent direction if the random photons are not absorbed to form a power-output scene, so that the reflectivity of the whole aquatic vegetation canopy can be calculated according to the statistical distribution of the emergent photons; the method is characterized by exploring the reflection spectrum characteristic and the reflection direction characteristic of the aquatic vegetation canopy, and comprises the following specific steps:
a Monte Carlo computer simulation method for reflecting a canopy of a row-seeding aquatic vegetation is characterized by comprising the following steps:
(1) The line sowing scene is considered as a three-dimensional scene formed by uniformly distributed equilateral triangles, wherein vegetation crowns and water bodies alternately appear in the direction perpendicular to the line direction of a ridge. And automatically constructing a scene to be simulated according to parameters such as geometric structure, orientation, row direction, leaf area index and the like of the broadcasting scene. Proper adjustment is used for representing the side length of the equilateral triangle of the blade, and different side lengths can influence the porosity of a vegetation scene under the same blade area index;
(2) Calculating the reflectivity and the transmissivity of a single blade by using a plant blade spectrum model, obtaining probability distribution of water surface reflection and refraction according to wind speed and wind direction parameters, obtaining the absorption coefficient, the scattering coefficient and a scattering phase function of a water body by using a water body biological optical model according to the water body parameters, setting the attribute of each element in a scene by using the obtained result, and setting all the blades as lambertian bodies;
(3) Sampling the initial position and the initial direction of photons according to the type and the energy distribution of the light source, setting initial weight for the photons, and transmitting the photons into a vegetation scene;
(4) Finding the intersection point of the photon and the element in the scene, sampling a new reflection or refraction direction according to the attribute of the intersection element, modifying the weight of the corresponding photon, re-entering the scene, and tracking the newly generated photon, wherein when the photon weight is smaller than a certain range, the photon is considered to be absorbed;
(5) Taking the distribution space of the whole emergent direction as a hemispherical space, dividing the hemispherical surface into grids, calculating the area of each grid and maintaining an accumulated weight for each grid, obtaining the corresponding grid according to the direction of emergent scene photons, and accumulating the weight of the photons on the accumulated weight;
(6) After tracking a large number of photons, the dichroic reflectivity of the entire aquatic vegetation system is calculated from the weight distribution on the hemispherical grid and the total weight of the incident photons.
A monte carlo computer simulation method of canopy reflection of a row-cast aquatic vegetation according to claim 1, wherein: in the step (1), the row sowing scene is considered as a three-dimensional scene formed by uniformly distributed equilateral triangles in which vegetation crowns and water bodies alternately appear in the direction perpendicular to the row direction of the ridge. "; the method is characterized in that the whole scene is infinitely extended in space and is periodic in the directions perpendicular to the ridge lines and parallel to the ridge lines, in the construction stage, only three-dimensional scenes in one period are constructed, and in the simulation process, the whole scene is regarded as infinitely repeated of a single constructed scene in the directions perpendicular to the ridge lines and parallel to the ridge lines, so that the effect of infinitely extending the scene is simulated; when constructing a scene in a period, the scene to be simulated can be automatically generated according to the input geometric parameters such as period length, ridge length, water depth, vegetation height, area index on water, area index of underwater leaves, side length of triangular leaves and the like.
A method of monte carlo computer simulation of canopy reflection for a row-cast aquatic vegetation according to claim 1, wherein: the method for setting the optical properties of the scene elements, which is described in the step (2), comprises the steps of calculating the reflectivity and the transmissivity of a single blade by using a plant blade spectrum model, obtaining the probability distribution of water surface reflection and refraction according to wind speed and wind direction parameters, obtaining the absorption coefficient, the scattering coefficient and the scattering phase function of a water body by using a water body biological optical model, setting the properties of each element in the scene by using the obtained result, and setting all the blades as lambertian, wherein the specific contents are as follows: all the surfaces of the blades are regarded as lambertian bodies, the reflectivity and the transmissivity of the blades are calculated by coupling PROSPECT, the bidirectional reflection distribution function (Bidirectional Reflectance Distribution Function, BRDF) and the bidirectional transmission distribution function (Bidirectional Transmittance Distribution Function, BTDF) of the water surface are calculated by coupling CoxMunk, the water scattering phase function constructed according to the sea water test measurement result is coupled, and the absorption coefficient and the scattering coefficient of the water are obtained by coupling the water biological optical model 4 the Monte Carlo computer simulation method for the reflection of the canopy of the aquatic vegetation in line according to the claim 1 is characterized in that: the sampling process of the parallel light and sky diffuse reflection light common in remote sensing simulation in the step (3) is as follows:
The parallel light sampling method comprises the following steps:
where (x, y, z) is the starting position of the ray, ζ 1 and ζ 2 are unit random variables, x r is the coordinate range size in the x direction in a periodic scene, y r is the coordinate range size in the y direction in a period, x min、xmax represents the maximum and minimum in the x direction in a period, y min、ymax represents the maximum and minimum in the y direction in a period, z min and z max represent the maximum and minimum in the z direction in a period, Representing the direction vector of incidence of the ray,/>Indicating the direction of incidence of parallel light;
The method for sampling the sky diffuse reflection light comprises the following steps:
Wherein the sampling of the starting point position is the same as that of the parallel light, ω represents the initial direction of the light ray, and (u x,uy,uz) is the coordinate of the starting direction of the light ray
A monte carlo computer simulation method of canopy reflection of a row-cast aquatic vegetation according to claim 1, wherein: the "sampling new reflection or refraction direction, modifying the weight of the corresponding photon according to the attribute of the intersecting element" described in step (4), the process of sampling new scattering direction by each type element is as follows:
For the lambertian body, the scattering of photons satisfies the cosine distribution, and the sampling formula is:
Wherein, xi represents a unit random variable, (u x,uy,uz) represents a coordinate phi of the light starting direction and represents a starting azimuth;
for the calculation of the scattering direction of the water surface, a corresponding cumulative distribution function (Cumulative Distribution Function, CDF) table is calculated according to the Cox-Munk model, and then a new direction is obtained by sampling from the CDF. The formula for calculating BRDF by the Cox-Munk model is as follows:
Where r (ω) is the Fresnel reflectivity of the incident angle ω when the water surface is calm, p (z x,zy) is the probability distribution function of the wave water surface gradient, z x,zy is the micro-bin coordinate system, θ n is the zenith angle direction in which the normal of the specularly reflected wave bin is located, θ s is the solar zenith angle, θ o is the observation zenith angle, and σ is the water surface roughness factor, The BRDF for CoxMunk model is shown;
A method of monte carlo computer simulation of canopy reflection of a row-cast aquatic vegetation according to claim 1, wherein: in the step (5), "consider the distribution space of the whole emergent direction as a hemispherical space, divide the hemispherical surface into grids, calculate the area of each grid and maintain an accumulated weight for each grid", the specific construction steps of the grids are as follows:
Where each theta i represents the maximum zenith angle represented by a cone, K i is the number of blocks of θ i in the corresponding region, a aspect is the aspect ratio of each block, in this application set to 1, r i is the radius corresponding to the area projection of lambertian azimuth angle, etc. of the region corresponding to θ i, so as to satisfy/>
A method of monte carlo computer simulation of canopy reflection of a row-cast aquatic vegetation according to claim 1, wherein: the step (6) of calculating the dichroic reflectivity of the whole aquatic vegetation system according to the weight distribution on the hemispherical grid and the total weight of the incident photons comprises the following specific calculation processes:
In the method, in the process of the invention, The BRF function within the range of the ith block is represented, W i is the total weight accumulated for photons passing through the ith block, ΔΩ i is the corresponding area of the ith block on the unit sphere, i.e., solid angle, θ i c is the zenith angle of a line segment passing through the center of the ith block, and W total is the total weight of all photons incident into the scene.
Compared with the prior art, the invention has the advantages that:
(1) In the vegetation remote sensing field, a general three-dimensional computer simulation model capable of scientifically and effectively describing the bidirectional reflection characteristics of the downstream aquatic vegetation in the water background is lacking at present, and the method provided by the invention has remarkable innovation aiming at the Monte Carlo-based calculation simulation method developed by the downstream aquatic vegetation in the shallow water area, and provides a new way with higher efficiency and accuracy for monitoring, estimating, early warning and protecting the downstream aquatic vegetation and inverting biophysical parameters.
(2) According to the invention, the plant leaf spectrum model, the wave water surface reflection model and the shallow water biological optical model are coupled as the attributes of various elements of the scene, and the three-dimensional scene of the aquatic vegetation is automatically generated, so that all factors influencing radiation transmission in the environment of the aquatic vegetation are considered, and an accurate and efficient computer simulation model is obtained. Compared with the traditional vegetation model, the model disclosed by the invention has the advantages of clear physical concept, strong universality, convenience in calculation, high speed and reliable precision guarantee.
(IV) description of the drawings
FIG. 1 is a technical flow of the present invention. Fig. 2 is a three-dimensional rendering of a multicast aquatic vegetation in three different growth phases (tillering, jointing, booting).
(Fifth) detailed description of the invention
In order to better illustrate the Monte Carlo computer simulation method for the canopy reflection of the aquatic vegetation, the invention adopts the model to test and analyze, and has good effect, and the specific implementation method is as follows:
(1) The line sowing scene is considered as a three-dimensional scene formed by uniformly distributed equilateral triangles, wherein vegetation crowns and water bodies alternately appear in the direction perpendicular to the line direction of a ridge. Automatically constructing a three-dimensional scene of the row-seeding aquatic vegetation in a ridge range through parameters such as geometric structures, orientations, row directions, leaf area indexes, leaf side lengths and the like;
(2) Calculating the reflectivity and the transmissivity of a single blade based on a plant blade spectrum model, calculating the absorption coefficient and the scattering coefficient of a water body by combining the concentration of each substance in the water body, calculating BRDF and BTDF functions of the water surface by using the provided wind speed and wind direction parameters to construct a corresponding CDF table, and finally setting corresponding attributes for all elements in a scene;
(3) Tracking the light source position into the scene by using a large number of photons, recalculating the scattering direction after each scattering event, modifying the corresponding weight, and finally accumulating the weight of photons which are not absorbed to be successfully emitted out of the scene on a corresponding block of the collector;
(4) Traversing each block on the collector, and calculating the bidirectional reflectivity in the corresponding direction of each block according to the position of the block and the accumulated weight combined with the total weight information of the incident scene.
The invention establishes the aquatic vegetation canopy direction reflection model, is helpful for deeper and scientific research on the relationship between the aquatic vegetation canopy reflection spectrum and solar radiation, vegetation structural parameters, water component parameters and water bottom reflection characteristics, and simultaneously supplements the problem that the three-dimensional canopy reflection model lacks of water surface and water body treatment.
Claims (1)
1. A Monte Carlo computer simulation method for reflecting a canopy of a row-seeding aquatic vegetation is characterized by comprising the following steps:
(1) The line sowing scene is a three-dimensional scene formed by uniformly distributed equilateral triangles, wherein vegetation crowns and water bodies alternately appear in the direction perpendicular to the line direction of a ridge; automatically constructing a scene to be simulated according to the geometric structure, the direction, the row direction and the leaf area index of the row broadcasting scene; appropriately adjusting the side length of the equilateral triangle representing the blade; the whole line broadcasting scene is infinitely extended in space and shows periodicity in the directions perpendicular to the ridge lines and parallel to the ridge lines, in the construction stage, only three-dimensional scenes in one period are constructed, and in the simulation process, the whole scene is regarded as infinitely repeated of a constructed single scene in the directions perpendicular to the ridge lines and parallel to the ridge lines, so that the effect of infinitely extending the scene is simulated; when constructing a scene in a period, automatically generating a scene to be simulated according to the input geometric parameters of the period length, the ridge length, the water depth, the vegetation height, the water leaf area index, the underwater leaf area index and the side length of the triangular leaf;
(2) Calculating the reflectivity and the transmissivity of a single blade by using a plant blade spectrum model, and obtaining the probability distribution of water surface reflection and refraction according to wind speed and wind direction parameters; according to the concentration and inherent optical parameters of the water body components, obtaining an absorption coefficient, a scattering coefficient and a scattering phase function of the water body by using a water body biological optical model, setting the attribute of each element in a scene by using the obtained result, and setting all the blades as lambertian bodies; the scene element optical attribute setting method specifically comprises the following steps: all the surfaces of the blades are regarded as lambertian bodies, the reflectivity and the transmissivity of the blades are calculated by coupling PROSPECT, a bidirectional reflection distribution function BRDF and a bidirectional transmission distribution function BTDF of the water surface are calculated by coupling a Cox-Munk model, a water body scattering phase function constructed according to a sea water test measurement result is coupled, and the absorption coefficient and the scattering coefficient of the water body are obtained by coupling a water body biological optical model;
(3) Sampling the initial position and the initial direction of photons according to the type and the energy distribution of the light source, setting initial weight for the photons, and transmitting the photons into the aquatic vegetation scene; for parallel light and sky diffuse reflection light in remote sensing simulation, the sampling process is as follows:
The parallel light sampling method comprises the following steps:
Wherein (x, y, z) is the starting position of the ray; ζ 1 and ζ 2 are unit random variables; x r is the size of the coordinate range in the x direction in a periodic scene; y r is the size of the coordinate range in the y direction in one cycle; x min and x max represent a minimum value and a maximum value in the x direction in one cycle, respectively; y min and y max represent a minimum value and a maximum value in the y direction in one cycle, respectively; z min and z max represent a minimum and a maximum, respectively, in the z direction over a period; a direction vector representing the incidence of the light;
Indicating the direction of incidence of the parallel light;
The method for sampling the sky diffuse reflection light comprises the following steps:
wherein the sampling of the starting point position is the same as the case of parallel light, ω represents the initial direction of the light ray, and (u x,uy,uz) is the coordinate of the starting direction of the light ray;
(4) Finding the intersection point of the photon and the element in the scene in the broadcasting scene, sampling a new reflection or refraction direction according to the attribute of the intersection element, modifying the weight of the corresponding photon, re-entering the scene, tracking the newly generated photon, and considering that the photon is absorbed when the photon weight is smaller than a certain range; the process of sampling the new scattering directions for each type of element is as follows:
For the lambertian body, the scattering of photons satisfies cosine distribution, and the sampling method is as follows:
Wherein (u x,uy,uz) is the coordinates of the ray's starting direction, phi represents the starting azimuth;
calculating a scattering direction of the water surface, namely calculating a corresponding accumulated distribution function CDF table according to the Cox-Munk model, and further sampling from the CDF to obtain a new direction; the method for calculating BRDF by the Cox-Munk model comprises the following steps:
Where r (ω) is the Fresnel reflectivity of the incident angle ω when the water surface is calm, p (z x,zy) is the probability distribution function of the wave water surface gradient, z x,zy is the micro-bin coordinate system, θ n is the zenith angle direction in which the normal of the specularly reflected wave bin is located, θ s is the solar zenith angle, θ o is the observation zenith angle, and σ is the water surface roughness factor, The BRDF of the Cox-Munk model is shown;
(5) Taking the distribution space of the whole emergent direction as a hemispherical space, dividing the hemispherical surface into grids, calculating the area of each grid, setting an accumulated weight for each grid, obtaining the corresponding grid according to the direction of emergent scene photons, and accumulating the weight of the photons on the accumulated weight; the specific construction steps of the grid are as follows:
Where each theta i represents the maximum zenith angle represented by a cone, K i is the number of blocks in the region corresponding to theta i, a aspect is the aspect ratio of each block, 1 is set, r i is the radius corresponding to the area projection of lambertian azimuth angle and the like on the region corresponding to theta i, and the/>
(6) After a large number of photons are tracked, calculating the two-way reflectivity of the whole row-seeding aquatic vegetation system according to the weight distribution on the hemispherical grid and the total weight of the incident photons; the specific calculation method is as follows:
In the method, in the process of the invention, Represents the BRF function in the range of the ith block, W i is the total weight accumulated by photons passing through the ith block, and DeltaOmega i represents the corresponding solid angle of the ith block on the unit sphere,/>To be the zenith angle of a line segment passing through the center of the ith block, W total is the total weight of all photons incident in the scene.
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