CN108829906B - Secondary scattering calculation method for background light radiation by complex target - Google Patents

Secondary scattering calculation method for background light radiation by complex target Download PDF

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CN108829906B
CN108829906B CN201810315511.0A CN201810315511A CN108829906B CN 108829906 B CN108829906 B CN 108829906B CN 201810315511 A CN201810315511 A CN 201810315511A CN 108829906 B CN108829906 B CN 108829906B
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李良超
李奎
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Xidian University
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Abstract

The invention provides a method for calculating secondary scattering of background light radiation by a complex target, which solves the problems of judgment of secondary scattering between bin pairs of the complex target, calculation of scattering area between the bin pairs and calculation of secondary scattering of the complex target. The realization is as follows: establishing a target model; inputting MOD model information of the established target and carrying out shielding treatment; performing primary judgment on secondary scattering between the shielded target surface element pairs; converting a coordinate system and calculating the scattering area between the surface element pairs; secondary scattering of the complex target is accomplished. The method firstly finds out the surface element pairs which are likely to generate secondary scattering, projects the surface element pairs in the space into an XOY plane of a conversion coordinate system through conversion coordinates, converts the three-dimensional problem into two dimensions, simplifies the operation, reduces the program complexity, obtains the secondary scattering brightness between the surface element pairs and the secondary scattering brightness of the whole complex target on background light radiation by using BRDF according to the obtained scattering area, and is used for accurately detecting the complex targets on the sea surface, the space and the ground.

Description

Secondary scattering calculation method for background light radiation by complex target
Technical Field
The invention belongs to the field of light scattering of a target in a complex background on background radiation, and mainly relates to the aspects of space complex target modeling, background optical radiation calculation, single and secondary light scattering discrimination and calculation among target plates, and calculation results can be applied to the aspects of scattering characteristic calculation of the target in the sea surface and the space, tracking and identification of the target and the like.
Background
The method for calculating the secondary scattering of the complex target on the background light radiation has important application value in the detection, identification and tracking of the optical information of the complex target in the backgrounds of the sea surface, the sun, the sky and the like. The method is an important means for researching the characteristics of the target, is an important component for detecting the optical characteristics of the complex target, researches the secondary scattering characteristics of the complex target on the background radiation, and provides a more accurate calculation result for the calculation of the characteristics of the target.
The current research on the secondary scattering of targets is mostly on radar scattering interfaces (RCS). However, the estimation of the secondary scattering RCS by the target and the secondary scattering algorithm of the target on the optical wave bands of the sun, the sky and the sea surface have fundamental differences, so that only a specific method can be proposed in the research of calculating the secondary scattering of the target on the sun, the sky and the sea surface.
When the secondary scattering of the target is studied, the target needs to be calculated separately. In the article of research and implementation of a ray tracing algorithm in target light scattering characteristic calculation, Wu Kangfeng et al provide a relatively effective ray testing and calculating method by using a ray tracing idea, apply the testing method to theoretical modeling of the light scattering characteristic of a complex target, and provide a light scattering brightness graph of a secondary scattering effect of the complex target under sunlight. The algorithm does not give a specific calculation of the scattering area between pairs of calculation bins. In the text of 'high-frequency hybrid method research of complex target multiple scattering calculation', Zhaoweijiang et al, on the basis of a geometric optical method, a method capable of effectively calculating multiple scattering of a complex target, namely a regional projection physical optical method, is derived by combining the geometric optical method and a ray tracing method. The method is specially applied to the secondary scattering calculation of the target on the radar, so that when the secondary scattering of the complex target on the background is calculated, reference values are provided only when secondary scattering occurrence conditions are judged and the scattering area is calculated, and a new algorithm is needed to calculate the secondary scattering of the target on the sun, the sky and the sea surface.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a secondary scattering calculation method of a complex target with low complexity and easy programming.
The invention relates to a secondary scattering calculation method of a complex target for background light radiation, which is characterized by comprising the following steps of:
the method comprises the following steps: establishing a target model: establishing a 3dmax model of the target through 3dmax, and then converting the established 3dmax target model into an MOD model; the MOD model is divided into five parts, wherein the first part is model format identification information, surface element number and top point number; the second part is a surface element index value, a component index value, a material index value, a color index value and a vertex index value; the third part is a vertex index and a coordinate value of a point; the fourth part is a component index value and a component type name; the fifth part is the material index value and the material name.
Step two: MOD model information of the input target: firstly, carrying out self-occlusion processing on a model by using a geometric method, then aiming at all surface elements in a target MOD model after the self-occlusion processing, processing the occlusion between the surface elements by using a Z-BUFFER blanking algorithm, wherein the Z-BUFFER blanking algorithm establishes a Cartesian three-dimensional coordinate system by taking an incident direction or a scattering direction as a Z axis, projecting the target MOD model into the coordinate system, dividing the projected target MOD model into a plurality of pixels in an XOY plane, wherein the number of the pixels is far larger than the number of the surface elements of the model, storing pixel information which is not occluded by other pixels in all the pixels according to the position information of the pixels in the coordinate system, finally obtaining the area size of the surface element which is not occluded in all the surface elements of the target MOD model through the stored pixel information, and considering that the surface element is not occluded if the area of the surface element is smaller than half, otherwise, the surface element is considered to be blocked; finally, removing all shielding surface elements in the target MOD model to obtain an unblocked target MOD model;
performing secondary scattering calculation on the target MOD model surface element which is not shielded under the background light radiation;
step three: primary judgment of secondary scattering between target surface element pairs: judging all surface elements of the target MOD model which are not shielded, wherein the surface elements are used as initial surface elements, namely the surface element 1 and the residual surface elements in a traversing mode in sequence, if secondary scattering possibly occurs, the two surface elements form a surface element pair, executing the step IV, converting a coordinate system, calculating the secondary scattering between the surface element pair, and if the secondary scattering does not occur, repeatedly executing the step III, and performing primary judgment on the secondary scattering between the next surface element pair;
step four: and (3) converting a coordinate system: taking the sun as an incident light as an example, calculating the secondary scattering of a target to the sun, calculating the mirror image scattering direction of sunlight irradiating the surface element 1 through the sun incident direction, establishing a new coordinate system by taking the mirror image scattering direction as a Z axis, defining the new coordinate system as an S coordinate system, still taking the S coordinate system as a Cartesian three-dimensional coordinate system, and projecting the surface element pair into an XOY plane of the S coordinate system;
step five: the scattering area between bin pairs is calculated: in the S coordinate system established in the step four, the position relation among the projection surface elements in the X0Y plane is intersected, separated and contained, all intersection points and mutually contained points among surface element line segments are stored, then repeated points are removed, and if the number of the remaining points is less than 3, the closed area cannot be formed, namely secondary scattering cannot occur; otherwise, finding out the point with the minimum Y value from the rest points, setting the point as MinP, sequentially calculating the opening angle of other points relative to the MinP, sequencing the points according to the opening angle, and finally performing polygon area calculation on the sequenced points;
step six: calculating and completing the secondary scattering of the complex target on the background light radiation: and (3) solving the mirror image scattering brightness on the surface element 1 according to a Bidirectional Reflection Distribution Function (BRDF), obtaining the irradiance received by the surface element 2 according to the conversion relation between the radiance and the irradiance, then solving the scattering area of the surface element on the surface element 1 and the surface element 2 according to the intersection area solved in the step five, and finally calculating the secondary scattering between the surface elements according to the Bidirectional Reflection Distribution Function (BRDF) on the surface element 2 so as to complete the secondary scattering calculation of the complex target.
According to the invention, secondary scattering calculation of the complex target on background light radiation is realized by judging whether secondary scattering can occur between the complex target surface element pairs and calculating the scattering area of the secondary scattering surface element.
Compared with the prior art, the invention has the advantages that:
1. the complexity is low: when calculating the second scattering between bin pairs, it is difficult to calculate the size of the scattering area between the bins. The invention provides an algorithm with lower complexity for calculating the scattering area when calculating the secondary scattering of a complex target. Establishing a new coordinate system by taking the mirror image scattering direction of one surface element as a z-axis, projecting the surface element pair capable of generating secondary scattering into the new coordinate system, and storing the intersected and contained points by judging the position relationship of two projection surface elements; and (3) finding out one point with the minimum y value from the stored points, then calculating the opening angle of the point of each remaining point, sequencing the opening angles, and calculating the area of a polygon formed by the points which are sequenced, thereby finishing the secondary scattering brightness between the upper elements of one surface element pair. And finally, calculating all surface element pairs to obtain the secondary scattering size of the whole target.
2. Easy programming: the method has the characteristic of easy programming, the scattering area size between the surface elements needs to be calculated in the process of calculating the secondary scattering calculation of the complex target, wherein the problem of polygon intersection is involved. When the secondary scattering of the sea surface target or the secondary scattering of the space target is calculated, the method can use a relatively convenient programming language to calculate the secondary scattering of the background light radiation by the target.
Drawings
FIG. 1 is a flow chart of the method of the present invention;
FIG. 2 is a schematic diagram of a bin secondary scattering primary judgment model;
FIG. 3 is a schematic diagram of a bin pair intersection;
FIG. 4 is a schematic representation of a bin pair;
FIG. 5 is a plot of the flare angle of each point relative to the MinP point;
FIG. 6 is a schematic diagram of bin versus secondary scattering;
FIG. 7 is a model of an example of the calculation of the present invention;
FIG. 8 is a BRDF model of aluminum honeycomb material used in the calculation of the present invention;
FIG. 9 is a graph of solar spectral irradiance at a solar zenith angle of 45 degrees under visible light conditions, as calculated by the present invention;
fig. 10 shows the scattering brightness of the target for solar radiation calculated by the present invention, which is the scattering brightness calculated for the secondary scattering and the scattering brightness not calculated for the secondary scattering, respectively.
Detailed Description
Example 1
When the secondary scattering of a complex target is calculated, in the prior art, people calculate the secondary scattering of the complex target by using a regional projection physical optics method, although the size of a scattering area can be effectively calculated, the method is complex when programming is realized, the method is specially used for calculating a radar scattering cross section (RCS), and the method has no general significance for the calculation of the complex target under different backgrounds. The invention provides a secondary scattering calculation method of a complex target under the sunlight, sky and sea surface light radiation background by research aiming at the problems, and the method is shown in figure 1 and comprises the following steps:
the method comprises the following steps: establishing a complex target model: establishing a 3dmax model of the target through 3dmax, exporting a file with a 3DS format from the established 3dmax target model, modifying the file into a target model with a MOD file type through the exported 3DS file, wherein the target MOD model bin type is a triangular bin; the MOD model is divided into five parts, wherein the first part is model identification information, surface element number and vertex number; the second part is a surface element index value, a component index value, a material index value, a color index value and a vertex index value; the third part is a vertex index and coordinate values of points, and one vertex index corresponds to three coordinate values; the fourth part is a component index value and a component type name; the fifth part is the material index value and the material name. The information content of the MOD file model after modification is simpler, the surface element number and the vertex number of the model can be clearly and visually seen, and programming is easier to realize when the surface element information and the vertex information of the file are programmed and read. When the interface of the model is realized by utilizing OpenGL, the model can be adjusted more easily according to the own needs by the established MOD model, and the file information can be read more easily, so the MOD model is adopted for calculation in the invention, as shown in FIG. 7, FIG. 7 is an example model used by the invention, the model consists of a square flat plate with the side length of 10 meters and a cube with the side length of 2.5 meters, the cube is positioned at the center of the flat plate, the sides of the upper bottom surface and the lower bottom surface of the cube are parallel to the sides of the flat plate, for the convenience of understanding, the sun incidence direction and the detector receiving direction are marked obviously in the drawing, the sun incidence direction in the drawing points to the center of the model, the Z axis is deviated to the X axis by 45 degrees, and the detector direction is from the Z axis to the X axis along the clockwise direction. The complex target is a three-dimensional model, which is a three-dimensional complex target consisting of thousands of surface elements, and all the surface elements of the model need to be calculated in the calculation process. In the following description, the objects mentioned all refer to complex objects, unless specified otherwise.
Step two: MOD model information of the input target: in the established MOD model, firstly, the MOD model is subjected to external self-shielding treatment by a geometric method, the self-shielding treatment is to judge whether a surface element is shielded by the self by using the included angle between the reverse direction of the incident direction and the normal line of the surface element, if the included angle is larger than 90 degrees, the surface element is considered to be shielded by the self, if the included angle is smaller than 90 degrees, the surface element is considered not to be shielded by the self, and for the shielding problem between the surface elements of the complex target, the simple geometric method cannot carry out correct judgment. Because one surface element of the complex target may be shielded by other surface elements in an incident direction, but the shielded surface element conforms to the geometric relationship and cannot be judged as shielding, the mutual shielding problem between the surface elements cannot be eliminated in the target MOD model after being blanked by the geometric method, and the problem of the mutual shielding of the surface elements is solved by using a Z-BUFFER blanking algorithm. The basic principle of the Z-BUFFER blanking algorithm is as follows: and establishing a Cartesian three-dimensional coordinate system for the Z axis of the new coordinate system through the incident direction or the scattering direction, and taking the projection of the Z axis in the ship coordinate system on an XOY plane of the new coordinate system as the Y axis of the new coordinate system, namely the new coordinate system established based on the Z-BUFFER algorithm. The method projects a target MOD model into the new coordinate system, the projected target MOD model is a planar model in an XOY plane of the new coordinate system, the planar model is divided into a plurality of pixels in the XOY plane, the number of the pixels is far larger than that of the surface elements of the target MOD model, pixel information which is not shielded by other pixels in all the pixels is stored according to the position information of the pixels in the new coordinate system, and finally the area of each surface element in the target MOD model which is not shielded by other surface elements in a ship coordinate system can be obtained through the stored pixel information. In the invention, if the blocked area of a surface element is less than half, the surface element is considered to be not blocked, otherwise, if the blocked area of the surface element is more than or equal to half, the surface element is considered to be blocked. And finally, removing all shielding surface elements in the target MOD model to obtain the target MOD model with the complex target not shielded.
Performing secondary scattering calculation on the target MOD model which is not shielded;
step three: primary judgment of secondary scattering between target surface element pairs: all surface elements of the target MOD model which are not shielded are traversed and sequentially used as initial surface elements, namely a surface element 1 and residual surface elements for judgment, as shown in FIG. 2, a direction vector from the center of the surface element 1 to the center of the surface element 2 is
Figure BDA0001623645700000061
It can be seen from the positional relationship between bin 1 and bin 2 that if the direction vector is
Figure BDA0001623645700000062
The included angle between the normal direction of the surface element 1 and the normal direction of the surface element is less than 90 degrees and the direction vector
Figure BDA0001623645700000063
If the angle between the two surface elements 2 is greater than 90 degrees, it is considered that the two surface elements may be inclined to each otherPerforming secondary scattering, namely forming the two surface elements into a surface element pair, executing the step four, performing coordinate system conversion, and calculating the secondary scattering between the surface element pair, wherein the surface element pair comprises a surface element 1 and a surface element 2; and if the secondary scattering does not occur, repeating the step three, and performing primary judgment on the secondary scattering of the next surface element pair.
Step four: and (3) converting a coordinate system: taking sunlight as an incident light as an example, calculating the secondary scattering of a target to the sun, calculating the mirror scattering direction of the sunlight on a surface element 1 through the incident direction of the sun, establishing a new coordinate system by taking the mirror scattering direction as a Z axis, naming the new coordinate system as an S coordinate system, projecting the Z axis in an XOY plane of an S coordinate system in a ship coordinate system as a Y axis of the new coordinate system, wherein the S coordinate system is still a Cartesian three-dimensional coordinate system, projecting the surface element pair into the XOY plane of the S coordinate system, compared with the original coordinate system, establishing the S coordinate system is used for conveniently calculating the scattering area of the surface element pair, because the target model is a three-dimensional model, if the S coordinate system is not established, the scattering area of two surface elements in the space needs to be calculated, the scattering area is more complicated, and when the surface element pair is projected into the S coordinate system, the invention only needs to calculate the size of the intersecting area of the projected surface element pair in the XOY plane in the S coordinate system, and then the scattering area size of the surface element to the surface element 1 and the surface element 2 can be calculated through inverse transformation of a coordinate system, so that the calculation process is greatly simplified, and the programming is easy to realize.
Step five: the scattering area between bin pairs is calculated: in the S coordinate system, the projection bins intersect, separate and contain the positional relationship in the lower X0Y plane. FIG. 3 is a schematic diagram of the intersection of bin pairs, where the projection bin of bin 1 in the XOY plane of the S coordinate system is bin 1 as shown in FIG. 31The projection bin of bin 2 in the plane of the S coordinate system XOY is bin 21Surface element 11Three edges of (2) are aligned with the surface element1And performing intersection test on the three edges, and storing the intersection points. If the two line segments have an overlapped part in the calculation process, two points of the overlapped part are taken as contained points. If two line segments do not intersect, then no intersection is considered. FIG. 4 is a schematic view of a complete mutual inclusion of pairs of bins in the XOY plane of the S coordinate systemThe projection surface element pair is surface element 11Sum bin 21From fig. 4, it can be seen that bin 21Is completely contained in bin 11If there are three contained vertexes, the coordinates of the three points are stored. The inclusion is divided into full inclusion and partial inclusion, and if some of the vertices of one bin of the object are also included in another bin, all included vertex coordinates are saved. There are no intersection points if the pairs of projection bins are separated from each other in the XOY plane in the S coordinate system. And (3) storing all intersection points and mutually contained vertexes among the surface element line segments, removing repeated points, if the number of the stored points is less than 3, proving that the stored points cannot form a closed area, namely, secondary scattering cannot occur, returning to the step three, and continuing to perform primary judgment on the secondary scattering between the next surface element pair of the target. Otherwise, if the number of the stored points is more than or equal to 3, finding out the point with the minimum Y value in the XOY plane in the S coordinate system from the stored points, setting the point as MinP, sequentially calculating the opening angle size of other points relative to the MinP point, sequentially sequencing the points according to the opening angle size, and finally performing polygon area calculation on the sequenced points.
Step six: calculating and completing the secondary scattering of the complex target on the background light radiation: the mirror image scattering brightness of sunlight on the surface element 1 is calculated according to a Bidirectional Reflection Distribution Function (BRDF), the spectral irradiance received on the surface element 2 is obtained through the conversion relation between the radiance and the irradiance, then the scattering area of the surface element 1 and the surface element 2 on the surface element pair is calculated according to the intersection area size obtained in the fifth step and the included angle between the Z axis and the surface element pair on the two surface elements in the S coordinate system, and finally the secondary scattering of the surface element on the surface element 1 on the surface element 2 is calculated according to the Bidirectional Reflection Distribution Function (BRDF) on the surface element 2, so that the secondary scattering calculation of the complex target is completed. The invention takes the sunlight as a radiation source for calculation, the sunlight incidence direction is single, the calculation is easy, and the calculation result is a comparison graph of the brightness value of the calculated secondary scattering and the brightness value of the non-calculated secondary scattering of the target under the sunlight radiation as shown in figure 10.
The research target of the invention has feasibility in calculating the secondary scattering of the sky background and the sea background, and only the surface element is calculated in each direction according to the method and integrated in all angles. Taking the secondary scattering calculation of the ship target on the background light radiation as an example, when the ship is subjected to not only solar light radiation but also sky background light radiation and sea surface background light radiation, and the secondary scattering of the ship on the solar light radiation is calculated, because the incident direction of the sun is single, the calculation is simple, and the secondary scattering calculation of the target on the solar light radiation can be completed according to the routine; when calculating the secondary scattering of the target on the marine background light radiation, each direction in the space needs to be calculated, 180x360 directions in which the target surface element can receive the marine background light radiation in the space are total, and when calculating the secondary scattering of the target on the marine background light radiation, each direction needs to be calculated. When the secondary scattering of a target on the sea-sky background light radiation is calculated, the target surface element receives the sea-surface background light radiation and the sky background light radiation at the same time, whether the surface element receives the sky background light radiation or the sea-surface background light radiation in one direction is judged in the calculation, namely if the incident angle of incident light in a ship coordinate system is smaller than 90 degrees, the surface element is considered to receive the sky background radiation, if the incident angle of incident light in the ship coordinate system is larger than 90 degrees, the surface element is considered to receive the sea-surface background radiation, finally, each direction received by the target surface element is taken as the incident direction of the routine to be calculated, and finally, the integral summation is carried out, so that the secondary scattering brightness of the complex target on the sea-sky background light radiation can be obtained. Therefore, during calculation, the difference between the secondary scattering calculation of the target marine background light radiation and the secondary scattering calculation under the target solar background light radiation is that the secondary scattering of the target by the sky and sea background radiation is calculated by taking each direction which can be received by a surface element as an incident direction, and the secondary scattering of the target under the solar background light radiation only needs to take the sunlight irradiation direction as the incident direction. Therefore, the method has feasibility for calculating complex targets on sea background light radiation and sky background light radiation. The method has higher engineering application value for calculating the secondary scattering of airplanes, satellites, vehicles and the like.
When calculating the secondary scattering of a complex target, it is difficult to calculate the size of the scattering area between the bin pairs. The invention provides a new algorithm for calculating the scattering area between surface elements, which is low in complexity and easy to program. Establishing a new coordinate system by taking the mirror image scattering direction of one surface element as a z-axis, projecting the surface element pair capable of generating secondary scattering into the new coordinate system, and storing the intersected and contained points by judging the position relationship of two projection surface elements; and (3) finding out one point with the minimum y value from the stored points, then calculating the opening angle of the point of each remaining point, sequencing the opening angle, then calculating the area of a polygon, and finally solving the scattering area of the surface element on the upper element 1 and the surface element 2 according to the inverse transformation of a coordinate system.
The invention provides a more effective algorithm aiming at the secondary scattering of the target to the sun, thereby having engineering application value.
Example 2
The method for calculating the secondary scattering of the complex target is the same as the embodiment 1, and the coordinate system conversion described in the fourth step comprises the following steps:
4.1 let the direction of solar incidence be
Figure BDA0001623645700000081
Then the incident light is
Figure BDA0001623645700000082
Normal to the sum bin 1
Figure BDA0001623645700000083
The incident ray direction can be obtained by the following geometric formula
Figure BDA0001623645700000084
Direction of specular reflection on surface element 1
Figure BDA0001623645700000085
Figure BDA0001623645700000086
4.2 in the reflection direction
Figure BDA0001623645700000087
Z as a new coordinate1Axis, new coordinate system x1o1y1The plane being perpendicular to the direction of the reflected wave, y1The axis is taken as the z axis of the coordinate system of the ship and is in x1o1z1The new coordinate system of the projection in the plane is the aforementioned S coordinate system, and the ship coordinate system and the S coordinate system have the following transformation relationship:
Figure BDA0001623645700000091
Figure BDA0001623645700000092
Figure BDA0001623645700000093
wherein theta is the incident angle of the sunlight incident direction on the coordinate system of the land ship,
Figure BDA0001623645700000094
azimuth angle, o, of sunlight incident direction on a yield-reduced coordinate system1x1y1z1Representing the S coordinate system, the oxyz being the ship coordinate system,
Figure BDA0001623645700000095
is x in the S coordinate system1In the direction of the axis of the shaft,
Figure BDA0001623645700000096
is y of S coordinate system1In the direction of the axis of the shaft,
Figure BDA0001623645700000097
z being the S coordinate system1The axial direction.
The purpose of the coordinate system conversion is to convert the scattering area of two triangles in the three-dimensional coordinate into the intersection area of two triangles in the two-dimensional plane, thereby simplifying the calculation process. The invention establishes an S coordinate system by taking the mirror image scattering direction on the surface element 1 as a Z axis, and the projection of the Z axis in the ship coordinate system in an XOY plane in the S coordinate system is a Y axis, so that the purpose is to enable the calculated surface element pair to keep upright in a new coordinate system and conveniently calculate the x axis of the surface element pair in the S coordinate system1o1y1The intersection area in the plane is convenient for calculating the secondary scattering of the target finally, so the invention designs coordinate transformation and calculates the secondary scattering of the complex target, and the whole calculation process is easier to realize and calculate in a programmed way.
Example 3
The method for calculating the secondary scattering of the complex target is the same as that in embodiment 1, and the scattering area between the bin pairs is calculated in the step five under the S coordinate system, and the method comprises the following steps:
5.1 projection bins of bin 1 and bin 2 capable of secondary scattering in the XOY plane in the S coordinate system are respectively the bin 11Sum bin 21The relationship between these two projection bins is three in the XOY plane in the S coordinate system: intersecting, containing, and separating. Intersection is whether the opposite side of the projection surface element intersects with the side, see fig. 3. Inclusion refers to whether the projection bin is included for the vertices of two triangles, see fig. 4. The separation is that the projection surface element pair does not have any intersection point and inclusion point in the S coordinate system, so only two cases of intersection and inclusion need to be discussed when calculating the scattering area.
5.2 bin 11Sum bin 21Sequentially performing intersection test on three edges in an XOY plane under an S coordinate system, if an intersection point exists, storing the coordinate of the intersection point, and displaying a surface element 1 as shown in FIG. 31Three side-to-side elements 2 of1If the intersection test is performed on the three sides in sequence, four intersection points are obviously seen in the figure, so that the four intersection points are stored. If in the course of the determination,if the two edges where the judgment has occurred have a portion that overlaps, the two points of the overlapping portion are treated as including points. If the two determined edges are determined not to intersect, the two edges are considered to have no intersection point.
5.3 judgment of Panel 11Sum bin 21Whether or not there is mutual inclusion in the XOY plane in the S coordinate system, as shown in FIG. 4, bin 21Complete surface element 11Contains, so these three points are saved. The method for judging the mutual inclusion of the surface element pairs sequentially judges the surface element 1 in an XOY plane in an S coordinate system1Is in bin 21Inner and then judge bin 21Whether three vertices of (1) are in bin 11And (4) the following steps. To judge bin 11Is in bin 21Taking an example, bin 1 is selected in turn1The three vertexes are used as vertexes to be judged, and the vertex participating in the judgment and the surface element 21May form three triangles if this vertex participating in the determination is in bin 21Inner, the sum of the areas of the three triangles and bin 21Are equal in size. If bin 11Vertex holding surface element 2 participating in judgment1The area of the divided three triangles and the bin 21If the areas are not equal, then surface element 1 is proved1Vertex not in bin 2 participating in judgment1Inner, sequential opposite surface element 11The three vertexes of the bin 2 are judged according to the method after all the points meeting the conditions are stored1Whether three vertices of (1) are in bin 11Then all points that meet the conditions are saved.
5.4, carrying out duplicate checking treatment on all the stored point coordinates, and deleting the points with the same XY coordinate value in the stored point coordinates by a specific method if the X coordinate values of the two points have an absolute value difference of not more than 10-4And the difference between the Y coordinate values is not more than 10 in absolute value-4And (4) considering that the coordinates of the two points are coincident, if the number of the remaining points after the duplicate points are deleted is less than 3, the two triangular surface elements cannot generate secondary scattering, and executing the step three to perform primary judgment on the secondary scattering of the next surface element pair. Otherwise, the duplicate point is deletedAnd if the number of the remaining points is more than or equal to 3, the two surface elements are considered to generate secondary scattering, and the step 5.5 is executed to sequence the stored points.
5.5 sorting the saved points: finding out the point with the minimum Y value projected to the XOY plane in the S coordinate system from the stored points as the MinP, as shown in fig. 5, four points and the MinP point in fig. 5 are the points stored in step 5.4, and then sequentially sorting the four points according to the opening angle sizes of the four points relative to the MinP point, so as to obtain four sorted points in the graph, namely, point 1, point 2, point 3, and point 4. The opening angles of the point 1, the point 2, the point 3 and the point 4 relative to the MinP are increased in sequence, so that the sorted points are saved again according to the sequence, the MinP point is saved finally, and the saved points are connected in sequence to form a polygon capable of being calculated.
5.6 the polygon area calculation is carried out on all saved points: the polygon formed by storing each point is regarded as the sum of areas of a plurality of triangles for calculation, the number of the stored points is n (n is more than or equal to 3), the n points are numbered from 1 to n in sequence, the polygons formed by the n points are combined into n-2 triangles according to the following combination mode, namely the point (1,2,3), the point (1,3,4) … point (1, n-1, n), each bracket represents a triangle, so that the sum of the areas of the polygons is the sum of the areas of the n-2 triangles. And finally, calculating the size of the area of a polygon enclosed by the stored points by utilizing the Z axis of the S coordinate system and the included angle between the two surface elements of the surface element pair in the ship coordinate system, and calculating the scattering areas of the two surface elements on the surface element pair.
Example 4
The method for calculating the secondary scattering of the complex target is the same as the method for calculating the secondary scattering of the complex target in the sixth step of the embodiment 1 and finishing the secondary scattering of the complex target, and comprises the following steps of:
6.1, according to the mirror image scattering direction on the surface element 1 and the solar incidence direction obtained in the step 4.1, the incidence angle and the azimuth angle of the solar incidence direction on the surface element 1 and the scattering angle and the azimuth angle of the mirror image scattering direction on the surface element 1 are obtained. The BRDF model used by the invention is a five-parameter model, and the scattering brightness in any scattering direction on the surface element 1 is calculated according to the incident angle and the azimuth angle of sunlight on the surface element 1, so that the BRDF is used for calculatingBrightness L of the mirror scattering on the bin 1i：
Li＝f1Esun(λ)cosθi
In the formula f1BRDF size of bin 1, Esun(λ) is the solar irradiance corresponding to a fixed wavelength, θiIs the angle between the incident ray of the sunlight and the normal of the surface element 1.
6.2 according to the size of the intersection area obtained in the XOY plane in the S coordinate system and the size of the included angle between the Z axis of the S coordinate system and the surface element centering surface element 1 and the surface element 2, obtaining the size of the scattering area of the surface element centering surface element 1 and the surface element 2: scattering area of bin 1 is S1The scattering area of the surface element 2 is S2。
6.3 conversion between radiance and irradiance: as shown in fig. 6, bin 1 scattering area S1Emitting and falling onto the scattering area S of the surface element 22The radiation flux Φ above is:
Φ＝LicosθrS1Ω
in the formula [ theta ]rIs the angle between the normal of the surface element 1 and the connecting line of the centers of the two surface elements, and omega is the scattering area S of the surface element 22Scattering area S of the opposite surface element 11Extended cube corner, i.e.
Figure BDA0001623645700000111
The distance between the centers of the surface element 1 and the surface element 2 is R, thetadThe included angle between the normal of the surface element 2 and the connecting line of the two surface elements is defined as the scattering area S of the surface element 11Scattering area S in surface element 22The spectral irradiance E (λ) produced above is:
E(λ)＝f1Esun(λ)cosθi·cosθr·cosθd·S1/R2
6.4 the second scattering brightness dL of the face element 1 and the face element 2 can be obtained by using the BRDF on the face element 2r：
dLr＝f2(f1Esun(λ)cosθi)S2cosθscosθrcosθddλ/R2
Finally by using asThe secondary scattering brightness L of the complex target to the sun can be obtained by the above formular，
Figure BDA0001623645700000121
And completing the calculation of the secondary scattering of the complex target, wherein n represents all the surface element logarithms capable of generating the secondary scattering.
On the basis of researching the conditions and judgment rules for forming the secondary scattering of the target, the invention provides an algorithm for judging the surface element pair of the target surface capable of generating the secondary scattering, calculating the effective area of the generated secondary scattering surface element and finally finishing the calculation of the secondary scattering of the complex target.
The research on the light scattering property of the target is one of the important components of the optical property of the target, and the key point and difficulty in the calculation of the light scattering property of the complex target are to solve the shielding relation between different parts of the target and the problem of multiple scattering. When calculating the secondary scattering of a complex target, firstly, the surface element pair of the target capable of generating the secondary scattering needs to be judged, a dihedral angle structure capable of generating the secondary scattering is found out, the secondary scattering between the surface element pair is obtained by finding out the surface element pair capable of generating the secondary scattering, and then the secondary scattering brightness of the whole target is calculated. The calculation of the secondary scattering has important application significance for accurate estimation of target detection. In the invention, firstly, the surface element pairs which are likely to generate secondary scattering are found out, so that the scattering area between the surface element pairs is calculated conveniently, the conversion coordinate system projects the surface element pairs in the space into an XOY plane of the conversion coordinate system, the three-dimensional problem is converted into a two-dimensional problem, the operation process is simplified, the complexity of the program is reduced, the secondary scattering brightness between the surface element pairs can be obtained by utilizing BRDF through the calculated scattering area, and the secondary scattering brightness of the whole target is further obtained. Therefore, the method has the advantages of low complexity, easiness in programming and the like when the secondary scattering of the complex target is calculated, is suitable for accurate detection of the sea surface target, the space target and the ground target, and has high engineering application value.
A more detailed example is given below to further illustrate the invention
Example 5
The calculation of the secondary scattering of complex targets is the same as in examples 1-4,
in the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention, but the present invention may be practiced in other ways than those specifically described and will be readily apparent to those of ordinary skill in the art without departing from the spirit of the present invention, and it is therefore not limited to the specific examples disclosed below.
FIG. 1 is a flow chart of the invention. Referring to fig. 1, the method for calculating the secondary scattering of the complex target of the present invention includes the following steps:
the method comprises the following steps: establishing a complex target model: establishing a 3dmax model of a target through 3dmax, then exporting the model into a 3DS type file, and simplifying the 3DS model modification into an MOD model used by the invention, wherein the MOD model is divided into five parts, and the first part comprises model identification information, surface element number and vertex number; the second part is a surface element index value, a component index value, a material index value, a color index value and a vertex index value; the third part is a vertex index and coordinate values of points, and one vertex index corresponds to the coordinates of three points; the fourth part is a component index value and a component type name; the fifth part is the material index value and the material name. Compared with 3DS, the MOD model simplifies the content of the model, the 3DS file is stored by binary system, and the texture, the mapping and the like of the model are contained in the 3DS file, which is useless for the calculation of the invention, so that the calculation by the MOD model is easier to realize by programming in the calculation process. FIG. 7 is a computational model used in the present invention, which consists of a flat plate with a side length of 10m and a cube with a side length of 2.5 m;
step two: inputting model information, firstly, utilizing a geometric method to carry out external self-shielding processing on the model, namely judging whether a target surface element faces away from an incident light direction, and setting a reverse vector of the incident light direction as
Figure BDA0001623645700000131
Normal to the target surface element is
Figure BDA0001623645700000132
Then, as can be seen from the geometric relationship, if
Figure BDA0001623645700000133
The surface element is proved to be back to the incident light, namely belonging to the surface element which is shielded by the surface element; if it is not
Figure BDA0001623645700000134
It is proven that the bins are not occluded by themselves and then the occlusion problem between bins is handled using the Z-BUFFER algorithm. The basic principle of the Z-BUFFER blanking algorithm is as follows: establishing a Cartesian three-dimensional coordinate system by taking an incident direction or a scattering direction as a Z axis, taking a Y axis of a new coordinate system as the projection of a Z axis of a ship coordinate system in an XOY plane of the new coordinate system, projecting a target MOD model into the coordinate system, dividing the projected target MOD model into a plurality of pixels in the XOY plane of the coordinate system, wherein the number of the pixels is far greater than that of the pixels of the target MOD model, storing pixel information which is not shielded by other pixels in all the pixels according to the position information of the pixels in the new coordinate system, and finally obtaining the area of each surface element of the target MOD model which is not shielded through the stored pixel information; and finally, removing all shielding surface elements in the target MOD model to obtain the target MOD model which is not shielded. And performing secondary scattering calculation on the processed target MOD model.
Step three: primary judgment of secondary scattering between target surface element pairs: traversing all surface elements of the target MOD model which are not shielded, sequentially judging the surface elements as initial surface elements, namely the surface element 1 and the rest surface elements, and finding out a surface element pair which is possibly subjected to secondary scattering, wherein the surface element pair comprises the surface element 1 and the surface element 2. As shown in fig. 2: n is1And n2The outside normal vectors of bin 1 and bin 2 respectively,
Figure BDA0001623645700000141
is the center o of bin 11Center o of the surface element 22If the three satisfy the geometric relationship, the vector of (A) and (B) can be obtained
Figure BDA0001623645700000142
And is
Figure BDA0001623645700000143
If the two surface elements are considered to be possible to generate secondary scattering, executing a fourth step and carrying out coordinate system transformation; otherwise, the third step is repeatedly executed if the secondary scattering does not exist between the surface element pair, and the primary judgment of the secondary scattering between the next surface element pair is carried out.
Step four: and (3) converting a coordinate system: taking the sun as the incident light, the incident light is assumed to be
Figure BDA0001623645700000144
Normal to bin 1 is
Figure BDA0001623645700000145
Then the incident light is
Figure BDA0001623645700000146
The incident ray can be obtained by the following geometric formula
Figure BDA0001623645700000147
Mirror reflection direction on bin 1
Figure BDA0001623645700000148
Figure BDA0001623645700000149
In the direction of reflection
Figure BDA00016236457000001410
Z as a new coordinate system, i.e. S coordinate system1Axis, x1o1y1Plane andthe reflection direction is vertical, y in S coordinate system1The axis is taken as the z axis of the coordinate system of the ship and is in x1o1z1And (3) projection in a plane, converting a ship coordinate system and an S coordinate system as follows:
Figure BDA00016236457000001411
Figure BDA00016236457000001412
Figure BDA00016236457000001413
wherein theta is the incident angle of the sunlight incident direction in the ship coordinate system,
Figure BDA00016236457000001414
is the azimuth angle, o, of the incident direction of sunlight on the ship coordinate system1x1y1z1Representing the S coordinate system, the oxyz being the ship coordinate system,
Figure BDA00016236457000001415
is x in the S coordinate system1In the direction of the axis of the shaft,
Figure BDA00016236457000001416
is y of S coordinate system1In the direction of the axis of the shaft,
Figure BDA00016236457000001417
z being the S coordinate system1The axial direction.
When the scattering areas of the upper element 1 and the lower element 2 of the surface element pair are calculated, the surface element pair is also located at two different positions in space because the model is a three-dimensional model, and the difficulty is relatively high if the scattering areas of the surface element pair of the surface element 1 and the surface element 2 are directly solved. Therefore, an S coordinate system is established by taking the mirror image scattering direction of the surface element 1 as a Z axis, the surface element pair is projected into the S coordinate system, the upright of the surface element pair body can be kept, the size of the intersection area of two projection triangles in the plane is only needed to be calculated in the XOY plane of the S coordinate system, and then the scattering areas of the surface element pair surface element 1 and the surface element 2 can be obtained according to the inverse transformation of the coordinate system, so that the calculation process can be simplified by establishing the S coordinate system, and the S coordinate system is easy to understand.
Step five: and (3) calculating scattering areas of the bin 1 and the bin 2:
5.1 projection bin of bin 1 in S coordinate system is bin 11The projection bin of the bin 2 in the S coordinate system is the bin 21Bin 1 in S coordinate system1Sum surface element 21The positional relationship between these two bins is three in the XOY plane: intersecting, containing and separating;
5.2 bins 11Three edges are sequentially opposite to the surface element 21Intersection tests are carried out on three edges: if there is an intersection, the coordinates of the intersection are saved. As shown in fig. 3, bin 1 is utilized1Three edges of (2) are aligned with the surface element1After intersection tests are carried out on the three edges, four intersection points can be obviously seen in the figure, so that coordinates of the four points are stored; if the two line segments are partially overlapped, two intersection points of the overlapped part are stored as contained points; if the two line segments are separated from each other, no intersection point exists;
5.3 judgment of Panel 11Sum bin 21Whether there is a mutual inclusion in the XOY plane: the inclusion is divided into full inclusion and partial inclusion, and if some of the vertices of one bin of the object are also included in another bin, all included vertex coordinates are saved. Bin 2 is shown in two triangular positions in fig. 41Quilt panel 11All the included points are included, so that the total number of the included points meeting the requirement is 3, and the three included points are stored;
and 5.4, judging the stored point coordinates, deleting repeated points, if the number of the remaining points after deleting the repeated points is less than 3, enabling the two triangular surface elements not to generate secondary scattering, returning to execute the step three, and performing primary judgment on the secondary scattering of the next surface element pair. Otherwise, if the number of the remaining points after deleting the repeated points is more than or equal to 3, determining that the secondary scattering occurs, executing the step 5.5, and sequencing the stored points;
5.5 sorting the saved points: finding out the point with the minimum Y value in the XOY plane under the new coordinate system from the stored points as MinP, sequentially calculating the opening angle size relative to the MinP points for the rest points, and sequencing the points according to the opening angle size, wherein as shown in FIG. 5, the point 1, the point 2, the point 3 and the point 4 are respectively the rest four points, and the opening angle of the point 1, the point 2, the point 3 and the point 4 relative to the MinP point is clearly increased in the figure, so that the four points of the point 1, the point 2, the point 3 and the point 4 are sequentially stored, and finally the MinP point is stored, and at the moment, the stored points form a polygon according to the sequence.
5.6 enclose all the stored points into a polygon, the area of the polygon can be regarded as the sum of the areas of a plurality of triangles for calculation, if n (n is more than or equal to 3) points are stored, the stored points are numbered as 1-n in sequence, the n points can be divided into n-2 triangles such as (1,2,3), (1,3,4) … (1, n-1, n) and the like, each bracket represents a triangle, finally the areas of the n-2 triangles are added to obtain the size of the intersection area, and finally the size of the scattering area of the two surface elements of the surface element pair is obtained by utilizing the size of the included angle between the Z axis of the new coordinate system and the two surface elements of the surface element pair.
Step six: calculating and completing the secondary scattering of the complex target:
6.1 according to the mirror image scattering direction on the surface element 1 and the solar incidence direction obtained in the fourth step, the incident angle and the azimuth angle of the solar incidence direction on the surface element 1 are obtained, the scattering angle and the azimuth angle of the mirror image scattering direction on the surface element 1 of the surface element 1 are obtained, and the mirror image scattering brightness L on the surface element 1 is obtained according to the BRDF on the surface element 1i：
Li＝f1Esun(λ)cosθi
In the formula f1BRDF size of bin 1, Esun(λ) is the solar irradiance corresponding to a fixed wavelength, θiThe included angle between the incident ray of the sunlight and the normal line of the surface element 1 is formed;
6.2 determining the size of the intersection area determined in the S coordinate system for the upper element 1 and the surface element 2 in the surface element pairScattering area size: as shown in fig. 6, the scattering area on the bin 1 is S1Scattering area S on surface element 22；
6.3 conversion between radiance and irradiance: as shown in fig. 6, the scattering area on the bin 1 is S1Scattering area on surface element 2 is S2The scattering area S is then obtained from the bin 11The intensity of spectral irradiation generated on oo' is Iθr：
Iθr＝Li·S1·cosθr＝f1·Esun(λ)·cosθi·cosθr
Wherein theta isrThe included angle between the normal line of the surface element 1 and the connecting line of the centers of the two surface elements is shown;
so by S1Sent out and falls to S2The radiant flux above is Φ:
Φ＝LicosθrS1Ω
wherein Ω is the scattering area S of bin 22Scattering area S of the opposite surface element 11Extended cube corner, i.e.
Figure BDA0001623645700000161
The distance between the centers of bin 1 and bin 2 is R, as shown in fig. 6, the distance between the centers of the two bins, θdThe included angle between the normal of the surface element 2 and the connecting line of the centers of the two surface elements is defined as the scattering area S of the surface element 11Scattering area S in surface element 22The spectral irradiance E (λ) produced above is:
E(λ)＝f1Esun(λ)cosθi·cosθrcosθdS1/R2
6.4 utilization of BRDF, i.e. f, on bin 22The secondary scattering brightness dL of the bin 1 and the bin 2 can be determinedr：
dLr＝f2·E(λ)＝f2(f1Esun(λ)cosθi)S2cosθscosθrcosθd/R2
Finally, the secondary scattering brightness L of the complex target to the sun is obtained by using the formular，
Figure BDA0001623645700000171
The calculation of the secondary scattering of a complex target is completed, and fig. 10 shows the scattering brightness of the target under the background condition of solar radiation calculated by the process. The model used for calculation is shown in fig. 7 and consists of a cube with the side length of 2.5 meters and a square flat plate with the side length of 10 meters, wherein the cube is positioned at the center of the flat plate, and the sides of the upper bottom surface and the lower bottom surface of the cube are parallel to the sides of the square flat plate. The BRDF model used for calculation is shown in fig. 8, which shows the magnitude of the BRDF at the scattering angle from-90 degrees to 90 degrees when the incident angle is 45 degrees and the azimuth angle is 0 degrees, and it is apparent from the figure that when the scattering angle is 45 degrees, an obvious peak appears because the five-parameter BRDF model used in the present invention is an aluminum honeycomb model, which has a strong mirror image effect, and therefore a peak appears at 45 degrees. The solar spectral irradiance used for calculation is shown in fig. 9, wherein the abscissa in the graph is the wavelength range of visible light, the ordinate is the spectral irradiance under the corresponding visible light wavelength, and 0.4-0.75 is the wavelength range of the visible light spectrum. Fig. 10 is a graph of the intensity of secondary scattering calculated using the present invention, in which the dotted line indicates the result of calculation of the intensity of scattering by calculating only primary scattering, and the solid line indicates the intensity of scattering by adding the results of calculation of primary and secondary scattering. It is evident from the figure that at a scattering angle of 45 degrees, the result of calculating the second scattering has a distinct peak. The reason is that the BRDF model adopted by the invention has obvious mirror image scattering effect, so that when the secondary scattering is calculated, the secondary scattering brightness of the detector has the maximum value when the detector is positioned in the same direction with the sun.
When the scattering of the background radiation by the complex target is calculated, calculation of secondary scattering between target surface elements is a difficult point, the method is suitable for judging the surface elements which can generate the secondary scattering of the target, calculating the size of the scattering area of the surface elements which can generate the secondary scattering, and finally finishing the calculation of the secondary scattering of the background radiation by the complex target.
According to the method for calculating the secondary scattering of the complex target on the background light radiation, firstly, model information is input, and the model is shielded. Judging whether secondary scattering is possible to occur to the surface element pairs or not by using the geometrical relationship among the surface elements, wherein the surface element pairs are marked as a surface element 1 and a surface element 2 according to the sequence of reading the surface elements in the model; if the situation is possible, establishing a new coordinate system by taking the mirror image scattering direction of the sunlight on the surface element 1 as a Z axis, obtaining the mirror image scattering brightness on the surface element 1 by utilizing the BRDF of the surface element 1, projecting the surface element pair into an XOY plane of the new coordinate system, judging the position relation of the projection surface element pair in the XOY plane of the new coordinate system, storing the intersection points and the included points, and removing the repeated points; if the number of the stored points is less than 3, the bin pair is considered to be incapable of generating secondary scattering, namely the bin pair searched by the user is not; if the number of the saved points is more than 3, finding out the point with the minimum Y value in the new coordinate system from the points, then sequentially calculating the opening angle size of the rest points relative to the point, and finally sequentially saving the points according to the opening angle size; calculating the area of the intersection surface by using the stored points, wherein the polygon can be regarded as the sum of the areas of a plurality of triangles for calculation, and the scattering areas of the upper two surface elements of the surface element pair are respectively obtained; converting the mirror scattering brightness on the surface element 1 into irradiance by using the conversion relation between irradiance and radiance, and then solving the secondary scattering brightness of the surface element 1 and the surface element 2 in the surface element 2 by using BRDF of the surface element 2; and finally, calculating all surface element pairs capable of generating secondary scattering of the target model to obtain the secondary scattering brightness of the complex target to the background light radiation.
In short, the method for calculating the secondary scattering of the complex target on the background light radiation solves the problems of judgment of secondary scattering between bin pairs of the complex target, calculation of scattering area between the bin pairs and calculation of the secondary scattering of the complex target. The realization is as follows: establishing a target model; inputting MOD model information of the established target and carrying out shielding treatment; performing primary judgment on secondary scattering between the shielded target surface element pairs; converting a coordinate system and calculating the scattering area between the surface element pairs; and calculating and completing the secondary scattering of the background light radiation by the complex target.
The method and the device complete the calculation of the secondary scattering of the whole target by finding out the surface element pair with the secondary scattering in the target model to calculate the secondary scattering. Firstly, the surface element pairs which are likely to generate secondary scattering need to be found out, then in order to calculate the scattering area between the surface element pairs conveniently, the conversion coordinate system projects the surface element pairs in the space into an XOY plane of the conversion coordinate system, the three-dimensional problem is converted into the two-dimensional problem, the operation process is simplified, the program complexity is reduced, the secondary scattering brightness between the surface element pairs can be obtained by the calculated scattering area through BRDF, and the secondary scattering brightness of the whole target is obtained.

Claims (4)

1. A method for calculating the secondary scattering of background light radiation by a complex target is characterized by comprising the following steps:
the method comprises the following steps: establishing a target model: establishing a 3dmax model of the target through 3dmax, and then converting the established 3dmax target model into an MOD model; the MOD model is divided into five parts, wherein the first part is model format identification information, surface element number and top point number; the second part is a surface element index value, a component index value, a material index value, a color index value and a vertex index value; the third part is a vertex index and a coordinate value of a point; the fourth part is a component index value and a component type name; the fifth part is a material index value and a material name;
step two: MOD model information of the input target: firstly, carrying out self-occlusion processing on a model by using a geometric method, then aiming at all surface elements in a target MOD model after the self-occlusion processing, processing the occlusion between the surface elements by using a Z-BUFFER blanking algorithm, wherein the Z-BUFFER blanking algorithm establishes a Cartesian three-dimensional coordinate system by taking an incident direction or a scattering direction as a Z axis, projecting the target MOD model into the coordinate system, dividing the projected target MOD model into a plurality of pixels in an XOY plane, storing pixel information which is not occluded by other pixels in all the pixels according to the position information of the pixels in the coordinate system, and finally obtaining the area size of the surface element which is not occluded by each surface element in all the surface elements of the target MOD model through the stored pixel information; finally, removing all shielding surface elements in the target MOD model to obtain an unblocked target MOD model;
performing secondary scattering calculation on the target MOD model surface element which is not shielded;
step three: primary judgment of secondary scattering between target surface element pairs: judging all surface elements of the target MOD model which are not shielded, wherein the surface elements are used as initial surface elements, namely the surface element 1 and the rest surface elements in a traversing mode, if secondary scattering possibly occurs, the two surface elements form a surface element pair, the step IV is executed, coordinate system conversion is carried out, secondary scattering between the surface element pair is calculated, and the surface element pair comprises the surface element 1 and the surface element 2; if the secondary scattering does not occur, the third step is repeatedly executed, and the secondary scattering primary judgment of the next surface element pair is carried out;
step four: and (3) converting a coordinate system: calculating the secondary scattering of the target on incident light, calculating the mirror reflection direction of the incident light on the surface element 1 through the incident direction, establishing a new coordinate system by taking the mirror reflection direction as a Z axis, defining the new coordinate system as an S coordinate system, still taking the S coordinate system as a Cartesian three-dimensional coordinate system, and projecting the surface element pair into an XOY plane of the S coordinate system;
step five: the scattering area between bin pairs is calculated: in the S coordinate system established in the step four, the position relation among the projection surface elements in the X0Y plane is intersected, separated and contained, all intersection points and mutually contained points among surface element line segments are stored, then repeated points are removed, and if the number of the remaining points is less than 3, the closed area cannot be formed, namely secondary scattering cannot occur; otherwise, finding out the point with the minimum Y value from the rest points to be set as MinP, sequentially calculating the opening angles of the other points relative to the MinP, sequencing the points according to the opening angles, and finally performing polygon area calculation on the sequenced points;
step six: calculating and completing the secondary scattering of the complex target: and finally, calculating the secondary scattering between the surface elements according to the bidirectional reflection distribution function BRDF on the surface element 2, and further completing the secondary scattering calculation of the complex target.
2. A method for calculating the secondary scattering of background light radiation by a complex object as defined in claim 1, wherein the coordinate system transformation in step four comprises the following steps:
4.1 let the incident direction of the incident light be
Figure FDA0003581727060000021
Then the following geometric formula is used to obtain the incident ray direction
Figure FDA0003581727060000022
Mirror reflection direction on bin 1
Figure FDA0003581727060000023
Figure FDA0003581727060000024
Figure FDA0003581727060000025
Represents the normal of bin 1;
4.2 in the reflection direction
Figure FDA0003581727060000026
As z1Axis, x1o1y1Plane perpendicular to the direction of reflection, y1The axis is taken as the z axis of the coordinate system of the ship in the x direction1o1z1The projection in the plane, the ship coordinate system and the S coordinate system are converted as follows:
Figure FDA0003581727060000027
Figure FDA0003581727060000028
Figure FDA0003581727060000029
wherein o is1x1y1z1Representing an S coordinate system, and oxyz is a ship coordinate system, wherein theta is an incident angle of an incident light incidence direction on the ship coordinate system,
Figure FDA00035817270600000210
is the azimuth angle of the incident light direction on the ship coordinate system,
Figure FDA00035817270600000211
is x in the S coordinate system1In the direction of the axis of the shaft,
Figure FDA00035817270600000212
is y of S coordinate system1In the direction of the axis of the shaft,
Figure FDA00035817270600000213
z being the S coordinate system1The axial direction.
3. A method for calculating the secondary scattering of background light radiation by a complex target according to claim 1, wherein the scattering area between the bin pairs is calculated in step five under the S coordinate system, comprising the following steps:
5.1 projection bin pairs of bin 1 and bin 2 in the S coordinate system are respectively bin 11Sum bin 21In the S coordinate system, the relationship between the two bins is three in the XOY plane:intersecting, containing and separating;
5.2 bin 11Sum bin 21Sequentially carrying out intersection test on three edges in an XOY plane under an S coordinate system, and if an intersection point exists, storing the coordinate of the intersection point;
5.3 judgment of Panel 11Sum bin 21If partial vertexes of one surface element of the target belong to the inclusion condition in another surface element, all included vertex coordinates are stored;
5.4, judging the stored point coordinates, deleting repeated points, if the number of the remaining points after deleting the repeated points is less than 3, the two triangular surface elements cannot generate secondary scattering, executing the step three, otherwise, considering that the secondary scattering occurs, and executing the step 5.5;
5.5 sorting the saved points: finding out the point with the minimum Y value in the XOY plane under the S coordinate system from the stored points as MinP, sequentially calculating the opening angle size relative to the MinP point in the XOY plane for the rest points, sequencing the points according to the opening angle size, sequentially storing the points according to the opening angle size, and connecting the stored points to form a polygon;
and 5.6, calculating the area of the polygon formed by enclosing all the stored points, taking the enclosed polygon as the sum of the areas of a plurality of triangles, calculating the area of the polygon formed by enclosing all the stored points, and finally calculating the scattering area of the surface element pair by using the size of an included angle between the Z axis of the S coordinate system and the two surface elements of the surface element pair.
4. The method of claim 2, wherein the step of calculating and completing the second scattering of the complex object comprises the steps of:
6.1 according to the mirror reflection direction and the incident light direction on the surface element 1 obtained in the step 4.1, obtaining the incident light incident angle and the azimuth angle on the coordinate system of the surface element 1, the scattering angle and the azimuth angle in the mirror reflection direction, and obtaining the surface element according to the BRDFMirror reflection luminance L at 1i：
Li＝f1Esun(λ)cosθi
In the formula f1BRDF size of bin 1, Esun(λ) is incident light irradiance corresponding to a fixed wavelength, θiIs the included angle between the incident light and the normal of the surface element 1;
6.2, according to the size of the intersection area obtained in the S coordinate system and the size of an included angle between the new coordinate system and the old coordinate system, the size of the scattering area of the surface element 1 and the surface element 2 in the surface element pair is obtained;
6.3 conversion between radiance and irradiance: scattering area on surface element 1 is S1Scattering area on surface element 2 is S2Scattering area S by surface element 11Emitting and falling onto the scattering area S of the surface element 22The radiation flux Φ above is:
Φ＝LicosθrS1Ω
wherein Ω is the scattering area S of bin 22Scattering area S of the opposite surface element 11The solid angle being laid out, i.e.
Figure FDA0003581727060000041
θrThe included angle between the normal line of the surface element 1 and the connecting line of the centers of the two surface elements is shown; the distance between the centers of the surface element 1 and the surface element 2 is R, thetadThe included angle between the normal of the surface element 2 and the connecting line of the two surface elements is defined as the scattering area S of the surface element 11Scattering area S in surface element 22The spectral irradiance E produced above is:
E＝f1Esun(λ)cosθi·cosθrcosθdS1/R2
6.4 the second scattering brightness dL of the face element 1 and the face element 2 can be obtained by using the BRDF on the face element 2r：
dLr＝f2(f1Esun(λ)cosθi)S2cosθscosθrcosθddλ/R2
Finally, the secondary scattering brightness L of the complex target to the incident light is obtained by using the formular，
Figure FDA0003581727060000042
The calculation of the secondary scattering of a complex target is completed, wherein n represents all the bin logarithms where the secondary scattering can occur.
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