A method for extracting tree trunk volume based on triangular surface model and tetrahedron model

Abstract

The application provides a kind of trunk volume calculation method based on triangular surface model and tetrahedron model, comprising: step 1, obtaining the initial point of trunk triangular surface model construction;Step 2, constructing the initial triangular surface model of trunk;Step 3, subdividing the triangular surface model of trunk;Step 4, trunk tetrahedron model construction and trunk volume calculation.The application subdivides the initial triangular surface model of trunk to obtain the geometric characteristics that can truly reflect the unevenness of trunk surface, then constructs the tetrahedron model of trunk by triangular surface model, obtains the trunk volume by the accumulation of each tetrahedron volume, the triangular surface model constructed can effectively reflect the geometric characteristics of trunk surface, the tetrahedron model constructed is a solid geometric model, which can accurately calculate the trunk volume, and can provide basic data for estimating tree volume, tree biomass, carbon storage and trunk volume optimization.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of tree trunk surface three-dimensional reconstruction and tree trunk tetrahedron model reconstruction, and particularly relates to a tree trunk volume extraction method based on triangular surface model and tetrahedron model. BACKGROUND

[0002] The tree trunk is the main component of the tree, and has the largest proportion of tree volume and the highest economic value. The accurate calculation of the tree trunk volume is the basis for accurately calculating the tree trunk biomass and the basis for accurately calculating the stand volume, forest volume and carbon storage. The existing volume calculation methods are mostly based on the assumption that the tree trunk cross section is circular. However, the tree trunk cross section is not circular, and the tree trunk is curved and twisted, which leads to a deviation between the calculated tree trunk volume based on the circular tree trunk cross section and the true value. At present, there is a tree trunk volume calculation method based on a continuous curved surface model of the tree trunk surface. However, the tree trunk surface is uneven and rough, and the continuous curved surface model cannot effectively reflect the true geometric characteristics of the tree trunk surface, which leads to a deviation between the calculated tree trunk volume based on the continuous curved surface model of the tree trunk surface and the true value. SUMMARY

[0003] Therefore, the present application aims to provide a tree trunk volume extraction method based on triangular surface model and tetrahedron model, which accurately reflects the geometric characteristics of the tree trunk surface by constructing a triangular surface model of the tree trunk, and accurately extracts the tree trunk volume based on the triangular mesh surface model.

[0004] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0005] A tree trunk volume extraction method based on triangular surface model and tetrahedron model, comprising the following steps:

[0006] Step 1: obtaining initial points for constructing a triangular surface model of the tree trunk;

[0007] Step 2: constructing an initial triangular surface model of the tree trunk;

[0008] Step 3: subdividing the triangular surface model of the tree trunk;

[0009] Step 4: constructing a tetrahedron model of the tree trunk and calculating the tree trunk volume.

[0010] Preferably, the step 1 of obtaining initial points for constructing a triangular surface model of the tree trunk comprises:

[0011] Step 101: vertically segmenting the tree trunk point cloud, comprising: taking the tree trunk point cloud P obtained by scanning a tree with a ground three-dimensional laser scanner as input, calculating the maximum value Z max and the minimum value Z min, and the Z min coordinates of the points in the first vertical segment are set as Z max , and the Z min coordinates of the points in the last vertical segment are set as Z max , and the Z L coordinates of the points in the first vertical segment are set as Z L , and the Z L coordinates of the points in the last vertical segment are set as Z L ; and the Z L coordinates of the points in the first vertical segment are set as Z U , and the Z U coordinates of the points in the last vertical segment are set as Z U ;

[0012] Step 102, vertical segment angle partitioning, comprising: partitioning each vertical segment into angle partitions with a selected angle parameter a, and the average coordinates of all points in each angle partition is the gravity point of the angle partition;

[0013] Step 103, angle partition sampling point, comprising: setting the Z min axis coordinate value of the gravity point of each angle partition in the first vertical segment as Z max , and taking it as the sampling point of each angle partition; setting the Z L axis coordinate value of the gravity point of each angle partition in the last vertical segment as Z L , and taking it as the sampling point of each angle partition; for each angle partition of other vertical segments, searching for the point with the minimum distance to the gravity point of the angle partition in the angle partition, and taking it as the sampling point of the angle partition, and the collection of all angle partition sampling points of all vertical segments constitutes the initial point set P0.

[0014] Preferably, the step 2, constructing the initial trunk triangular surface model, comprises:

[0015] Step 201, trunk upper and lower surface reconstruction, comprising: taking all sampling points of the first vertical segment to form a point set P L , and arranging the point set in a counterclockwise order, then each two adjacent sampling points form a directed edge arranged in a counterclockwise order, and these directed edges constitute a boundary edge set B L of the lower surface, then taking P L and B L as inputs, and constructing a two-dimensional Delaunay triangular net of the sampling points of the first vertical segment by using the Delaunay triangular net construction algorithm of the two-dimensional plane point set, and the triangular net is a lower surface triangular net T L of the trunk; similarly, the collection of all sampling points of the last vertical segment P U and the boundary edge set B U construct a Delaunay triangular net of the upper surface T U of the trunk; in the construction process, the counterclockwise order of the points represents a triangle, that is, if there are three points A, B and C, and the counterclockwise order is A, C, B, then the triangle formed by the three points should be marked as ΔACB;

[0016] Step 202, trunk outer surface reconstruction, comprising: taking the initial point set P0as input, and taking the reverse edges of the boundary edges of the lower surface of the trunk and the reverse edges of the boundary edges of the upper surface of the trunk as the boundary edge set BO Reconstruct the triangulated mesh of the trunk's outer surface O ; Take any edge from the boundary edge set as the initial base edge AB, and perform the operation of finding the third point to maximize the angle between the base edge AB: Find a point C in the point set P0 that does not belong to P L With P U , determine whether the triangle formed by the base side and point C satisfies the triangle non-intersection rule, that is, ΔABC and T O Do the existing triangles with three points A, B and C as nodes intersect with each other, including direct intersection in three-dimensional space and intersection in projection space? If there is no intersection, select a node D from the candidate node set as one of the candidate nodes to maximize the angle ∠ADB, and a new triangle ΔABD can be formed. At this time, since the base edge AB forms a new triangle ΔABD, AB is no longer a boundary edge, so it is removed from the boundary edge set B. O Delete; for the other two directed edges BD and DA of ΔABD, if edge BD is in the boundary edge set B O If exists in the set, remove edge BD from the boundary edge set B. O Otherwise, add edge BD to the boundary edge set B O Similarly, perform similar operations on the directed edge DA, and continue to select any edge from the boundary edge set as the base edge to perform the above-mentioned operation of maximizing the angle of the base edge until the boundary edge set is empty. If the directed edge BD and DA are in the boundary edge set B O If there are two edges in the tree trunk, these two edges are prioritized as base edges. If there is no candidate node for a base edge during the execution process, the values ​​of the height parameter h and the angle parameter α should be appropriately increased and the above process should be re-executed from step 101 to ensure that the outer surface of the tree trunk is reconstructed.

[0017] Preferably, the step 3 of subdividing the trunk triangular surface model includes:

[0018] Step 301, a subdivision of the trunk triangular surface model, including: traversing the trunk outer surface triangulation network T O Each edge in each triangle in , and the edge is the edge generated when the outer surface is reconstructed, that is, the edge is not the reverse edge of the boundary edge when the upper and lower surfaces of the trunk are reconstructed, that is, the reverse edge of the edge does not belong to B L With B U, for an edge AB generated during the reconstruction of the outer surface, its reverse edge can be expressed as BA, find the triangle where the edge AB is located, here using ΔABC to represent it, find the triangle where the edge BA is located, here using ΔBAD to represent it, calculate the midpoint F of the edge AB, search for a point in the trunk point cloud P that is closest to point F and whose distance is less than h / 2, and record this point as E, if point E does not intersect with the four triangles ΔAEC, ΔEBC, ΔDBE, ΔDEA newly generated by the other two edges of ΔABC and ΔBAD and other adjacent triangles in their triangulation network, then ΔABC and ΔBAD should be subdivided into ΔAEC, ΔEBC, ΔDBE and ΔDEA, that is, ΔABC and ΔBAD should be removed from the outer surface triangulation network T O Delete △AEC, △EBC, △DBE and △DEA and add them to the outer surface triangulation network T O The four newly generated edges AE, EB, DE, and EC are also called the edges generated during the external surface reconstruction, but they will no longer participate in this subdivision. The above operations are performed one by one on each edge generated during the external surface reconstruction to complete a subdivision of the tree trunk triangular surface model.

[0019] Step 302, iterative subdivision of the trunk triangular surface model, including: subdividing the outer surface model T of the previous trunk triangular surface model O_i Execute step 301 to obtain a subdivided outer surface model T O_i+1 The number of iterations can be set in advance or iterated until a certain iteration ends when triangles intersect and no new triangles are generated, thus obtaining the final subdivided tree trunk outer surface model T O_e ;

[0020] Step 303: transform the trunk upper surface triangulation model T U , trunk lower surface model T L Subdivision model of the outer surface of the trunk O_e The final reconstructed trunk surface triangulated network model T is obtained by merging, that is, T = T U ∪T L ∪T O_e .

[0021] Preferably, the step 4, constructing a trunk tetrahedron model and calculating the trunk volume, includes:

[0022] Step 401: constructing a tetrahedron model of the trunk. The trunk surface triangulated mesh model T is used as input to construct a tetrahedron model of the trunk. The construction method adopts the constrained tetrahedron and boundary recovery method in the tetrahedron mesh generator TetGen, that is, the tetrahedron model is constructed using the input triangulated surface model as the boundary constraint.

[0023] Step 402 , calculating the trunk volume, traverses each tetrahedron in the trunk tetrahedron model one by one and calculates its volume. The sum of the volumes of all tetrahedrons is the trunk volume.

[0024] The beneficial effects of the present invention are:

[0025] The present invention uses the tree trunk point cloud obtained by scanning with a ground-based three-dimensional laser scanner as data to construct a tree trunk geometric model that can accurately reflect the geometric features of the trunk surface and the solid geometric features of the trunk. The initial triangular surface model of the trunk is subdivided to obtain geometric features that can truly reflect the unevenness of the trunk surface. The triangular surface model has closed and non-intersecting geometric properties. The triangular surface model is then used to construct a tetrahedron model of the trunk, and the trunk volume is obtained by accumulating the volumes of each tetrahedron. The triangular surface model constructed using the method of the present invention can effectively reflect the geometric features of the trunk surface, and the constructed tetrahedron model is a solid geometric model. Therefore, the calculated trunk volume has a high degree of accuracy, providing accurate basic data for extracting trunk volume, constructing tree growth equations, calculating stand accumulation, calculating forest carbon reserves, tree biomass, and optimizing trunk volume, thereby providing technical support for precise forest management.

[0026] Other features and advantages of the present invention will be described in the following description, and in part will become apparent from the description, or will be understood by practicing the present invention. The purposes and other advantages of the present invention can be realized and obtained by the structures particularly pointed out in the written description, claims, and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic flow chart of the method of the present invention.

[0028] Figure 2 It is a triangulated network reconstructed from the upper surface of a tree trunk point cloud according to the present invention.

[0029] Figure 3 It is a triangulated network reconstructed from the lower surface of a tree trunk point cloud according to the present invention.

[0030] Figure 4 It is a schematic diagram of triangle intersection judgment during the base edge expansion process of the present invention.

[0031] Figure 5 It is a triangulated network reconstructed from the outer surface of a tree trunk point cloud of the present invention.

[0032] Figure 6 It is a schematic diagram of the triangulated network subdivision of the outer surface of a tree trunk according to the present invention.

[0033] Figure 7 It is a triangulated network of the outer surface of a tree trunk point cloud surface model after iterative subdivision.

[0034] Figure 8 It is a triangulated network obtained by iteratively subdividing the point cloud surface model of a section of a tree trunk according to the present invention.

[0035] Figure 9 This is a surface model diagram of a tree trunk point cloud surface model after iterative subdivision.

[0036] Figure 10 It is a cross-sectional view of the point cloud tetrahedron solid model of a tree trunk section of the present invention on the X-axis.

[0037] Figure 11 It is a cross-sectional view of the point cloud tetrahedron solid model of a tree trunk section of the present invention on the Y axis. DETAILED DESCRIPTION

[0038] The present invention will be further described below with reference to the accompanying drawings and examples.

[0039] like Figure 1 As shown, the method for extracting tree trunk volume of the present invention comprises the following steps:

[0040] Step 1: Obtain the initial point for constructing the triangulated surface model of the tree trunk;

[0041] Step 2, constructing the initial triangular surface model of the tree trunk;

[0042] Step 3, subdivide the triangular surface model of the tree trunk;

[0043] Step 4: Construct the trunk tetrahedron model and calculate the trunk volume.

[0044] In one embodiment, the step 1, obtaining initial points for constructing a triangulated surface model of the tree trunk, includes:

[0045] The step 101, vertical segmentation of the trunk point cloud, includes: taking the trunk point cloud P obtained by scanning a tree with a terrestrial 3D laser scanner as input, calculating the maximum value Z of P on the Z axis max With the minimum value Z min In this embodiment, the height parameter h is set to 5cm, from Z min Start to Z max Divide P evenly into vertical segments.

[0046] The step 102, vertical segmentation angle partitioning, includes: in this embodiment, setting the height parameter α = 15° to divide each vertical segment into The average coordinate value of all points in each angular partition is the centroid of the angular partition.

[0047] The step 103, angle partition sampling point, includes: setting the Z axis coordinate value of each angle partition center point in the first vertical segment to Z axis.min And as the sampling point of each angle partition; set the Z axis coordinate value of the center of gravity of each angle partition in the last vertical segment to Z max And it is used as the sampling point of each angular partition; for each angular partition of other vertical segments, the point with the smallest distance from the center of gravity of the angular partition is searched in the angular partition, and this point is used as the sampling point of the angular partition. The set of sampling points of all angular partitions of all vertical segments constitutes the initial point set P0.

[0048] In one embodiment, the step 2, constructing the initial triangular surface model of the trunk, includes: step 201, reconstructing the upper and lower surfaces of the trunk, including: forming a point set P with all sampling points of the first vertical segment L As input, arrange the point set in counterclockwise order, then every two adjacent sampling points form a directed edge arranged in counterclockwise order, and these directed edges constitute the boundary edge set B of the lower surface L , then P L and B L As input, the Delaunay triangulation algorithm of the two-dimensional plane point set is used to construct the two-dimensional Delaunay triangulation of the first vertical segment sampling point. The triangulation is the triangulation of the lower surface of the trunk T L ,like Figure 2 In the construction process, the triangle is represented by the counterclockwise order of the points. That is, if there are three points A, B and C, and they are sorted counterclockwise as A, C, B, then the triangle formed by these three points should be marked as ΔACB. Similarly, the set P of all sampling points in the last vertical segment is U and the boundary edge set B U The constructed Delaunay triangulation is the upper surface T of the trunk U ,like Figure 3 shown.

[0049] The step 202, reconstructing the outer surface of the trunk, includes: taking the initial point set P0 as input, and using the reverse edges of the boundary edges of the lower surface of the trunk and the reverse edges of the boundary edges of the upper surface of the trunk as the boundary edge set B for reconstructing the surrounding surface of the trunk. O Reconstruct the triangulated mesh of the trunk's outer surface O . Take any edge from the boundary edge set as the initial base edge AB, and perform the operation of finding the third point to maximize the angle between the base edge AB: find a point C in the point set P0 that does not belong to P L With P U , determine whether the triangle formed by the base side and point C satisfies the triangle non-intersection rule, that is, ΔABC and T O Do the triangles with three points A, B and C as nodes already exist in the intersection relationship (such as Figure 4In the middle, the base side AB finds C point, needs to judge ΔABC respectively with ΔGHA, ΔHBA with ΔHDB intersection relation), including in three-dimensional space direct intersection (through direct in three-dimensional space judges two triangles whether to intersect) and in projection space intersection (through two triangles vertex projection to its minimum principal component corresponding feature vector plane, calculates the intersection relation between the projection triangle), if there is no intersection relation as a candidate node one, from candidate node set selects a node D makes the angle value of the included angle ∠ADB maximum, then can constitute a new triangle ΔABD. At this time because the base side AB constitutes a new triangle ΔABD, then AB is no longer the boundary edge, therefore will be from the boundary edge set B O In the middle delete. For ΔABD's other two directed edges BD and DA, if the edge BD exists in B O In the middle, then will delete the edge BD from B O In the middle, otherwise will add the edge BD to B O ; similarly, to the directed edge DA, execute similar operation. Continue from the boundary edge set arbitrarily takes an edge as the base side and executes the above base side's included angle maximization operation until the boundary edge set is empty, if the directed edge BD and DA exist in B O In the middle, then prefer to take these two edges as the base side. If in the execution process, a base side has no candidate node, then should appropriately increase the height parameter h and the angle parameter α value, from step 101 begins to reexecute the above process to ensure that the tree trunk outer surface reconstruction is completed, a reconstructed outer surface model is shown in Figure 5 .

[0050] In an embodiment, the step 3, the tree trunk triangular surface model subdivision, includes:

[0051] The step 301, the tree trunk triangular surface model one-time subdivision, includes: traverses each edge in each triangle in the tree trunk outer surface triangular net T O In the middle, and the edge is the edge generated in the outer surface reconstruction, that is, the edge is not the reverse edge of the boundary edge in the tree trunk upper and lower surface reconstruction, in other words, the reverse edge of the edge does not belong to B L In the middle and B U For an edge AB generated in the outer surface reconstruction, the reverse edge of the edge AB can be expressed as BA, finds the triangle where the edge AB is, here uses ΔABC to express, finds the triangle where the edge BA is, here uses ΔBAD to express, calculates the midpoint F of the edge AB, searches for a point closest to the point F and less than h / 2 in the tree trunk point cloud P, and records the point as E, if the four triangles ΔAEC, ΔEBC, ΔDBE and ΔDEA newly generated by the point E and the other two edges of ΔABC and ΔBAD respectively and the other adjacent triangles in the triangular net do not have intersection relation, then the ΔABC and ΔBAD are subdivided into ΔAEC, ΔEBC, ΔDBE and ΔDEA (as shown in Figure 6As shown), that is, △ABC and ΔBAD are deleted from the outer surface triangle net T O , ΔAEC, ΔEBC, ΔDBE and ΔDEA are added to the outer surface triangle net T O The newly generated four edges AE, EB, DE and EC are also called the edges generated during the outer surface reconstruction, but no longer participate in this subdivision. The above operation is performed on each generated edge during the outer surface reconstruction to complete one subdivision of the trunk triangle surface model.

[0052] The step 302, iterative subdivision of the trunk triangle surface model, includes: performing the operation of the step 301 on the outer surface model T O_i of the last iteration to obtain a subdivided outer surface model T O_i+1 In this embodiment, the number of iterations is set to 10, and the iteration is stopped at most 10 times or ended in the iteration process because the triangles intersect and no new triangle is generated, thereby obtaining a final subdivided trunk outer surface model T O_e A final subdivided trunk outer surface model is shown in Figure 7 .

[0053] The step 303 combines the trunk upper surface triangle net model T U , the trunk lower surface model T L and the subdivided model of the trunk outer surface T O_e to obtain a final reconstructed trunk surface triangle net model T, that is, T = T U ∪ T L ∪ T O_e A final reconstructed trunk outer surface model is shown in Figure 8 .

[0054] In one embodiment, the step 4, trunk tetrahedron model construction and trunk volume calculation, includes:

[0055] The step 401, trunk tetrahedron model construction, takes the trunk surface triangle net model T as input to construct a trunk tetrahedron model. The construction method uses the constrained tetrahedron and boundary recovery method in the tetrahedron grid generator TetGen, that is, the input triangle surface model is used as a boundary constraint to construct a tetrahedron model. The cross-sectional views of a constructed trunk tetrahedron model on the X-axis and Y-axis are shown in Figure 10 and Figure 11 . As can be seen from Figure 10 and Figure 11 , the constructed tetrahedron model fills the enclosed space surrounded by the trunk surface model, which means that the accumulation of the volumes of all tetrahedron models is the accurate volume of the enclosed space surrounded by the trunk surface model, that is, the accurate volume of the trunk.

[0056] Step 402, stem volume calculation, each tetrahedron in the stem tetrahedron model is traversed one by one and its volume is calculated, and the accumulation of all tetrahedron volumes is the stem volume.

[0057] In the present application, the vertices of the reconstructed stem surface triangular net model T are all original points of the stem surface, and the reconstructed surface triangular net model is close to the stem surface and can reflect the convex and concave features of the stem surface, as shown in Figure 9 The height parameter h and the angle parameter a have different values, and different outer surface models are generated, but after iterative subdivision, although the triangular net surface models are not completely the same, since the vertices of the triangular net surface model are all original points of the stem surface, although there is a slight difference, the influence on the calculation accuracy is small.

[0058] The present application can accurately reflect the geometric features of the stem surface by subdividing the initial triangular surface model of the stem to obtain a triangular surface model that can accurately reflect the convex and concave features of the stem surface, and can accurately calculate the stem volume by constructing a tetrahedron model of the stem from the triangular surface model and accumulating the volume of each tetrahedron, and can provide basic data for estimating tree volume, tree biomass, carbon storage and stem volume optimization.

[0059] The present application uses the tree point cloud obtained by scanning with a ground three-dimensional laser scanner as data, and can construct a stem surface geometric model that can accurately reflect the stem surface structure and a stem entity geometric model that can accurately reflect the stem entity structure by using the method of the present application, and can extract the stem volume from the stem entity geometric model, thereby providing technical support for forest precision management.

[0060] It should be noted that the terms "comprising", "including", or any other variant thereof are intended to cover non-exclusive inclusion, so that processes, methods, articles or devices including a series of elements not only include those elements, but also include other elements not explicitly listed, or inherent to such processes, methods, articles or devices. Without more limitations, the element defined by the statement "including a" does not exclude the presence of another identical element in the process, method, article or device including the element.

[0061] Although embodiments of the present application have been shown and described, those skilled in the art can understand that various changes, modifications, replacements and variations can be made to these embodiments without departing from the principles and spirits of the present application, and the scope of the present application is defined by the appended claims and their equivalents.

Claims

1. A method for extracting tree trunk volume based on a triangular surface model and a tetrahedron model, characterized by: The steps include: Step 1: Obtain the initial point set for constructing the tree trunk triangulated surface model; Step 2: construct an initial triangular surface model of the trunk, which includes a triangular mesh model of the trunk outer surface T O , triangulated mesh model of the trunk upper surface T U , triangulated mesh model of the lower surface of the trunk T L ; Step 3: Subdivide the triangulated mesh model of the trunk outer surface O , the triangulated mesh model of the trunk surface T U , triangulated mesh model of the lower surface of the trunk T L Subdivision model of the outer surface of the trunk O_e The final reconstructed trunk surface triangulated mesh model T is obtained by merging; Step 4: construct a trunk tetrahedron model using the trunk surface triangulated mesh model T as input, and calculate the trunk volume based on the constructed trunk tetrahedron model; The step 1 comprises: Step 101, vertical segmentation of the trunk point cloud, including: taking the trunk point cloud P obtained by scanning a tree with a terrestrial 3D laser scanner as input, calculating the maximum value Z of P on the Z axis max With the minimum value Z min , with the selected vertical segment height parameter h as the interval, from Z min Start to Z max Divide P evenly into vertical segments; Step 102, vertical segment angle partitioning, includes: dividing each vertical segment into Angle partitions, the average coordinate value of all points in each angle partition is the centroid of the angle partition; Step 103, angle partition sampling point, including: setting the Z axis coordinate value of each angle partition center point in the first vertical segment to Z axis. min And as the sampling point of each angle partition; set the Z axis coordinate value of the center of gravity of each angle partition in the last vertical segment to Z max And as the sampling point of each angle partition; for each angle partition of other vertical segments, search for the point with the smallest distance to the center of gravity of the angle partition in the angle partition, and use this point as the sampling point of the angle partition. The set of sampling points of all angle partitions of all vertical segments constitutes the initial point set P0; The step 2 includes: Step 201: Reconstructing the upper and lower surfaces of the trunk, including: forming a point set P with all sampling points of the first vertical segment L As input, arrange the point set in counterclockwise order, then every two adjacent sampling points form a directed edge arranged in counterclockwise order, and these directed edges constitute the boundary edge set B of the lower surface L , then P L and B L As input, the Delaunay triangulation algorithm of the two-dimensional plane point set is used to construct the two-dimensional Delaunay triangulation of the first vertical segment sampling point. The triangulation is the triangulation model of the lower surface of the trunk T L ; Similarly, the set P of all sampling points of the last vertical segment U and the boundary edge set B U The constructed Delaunay triangulation is the triangulation model of the trunk upper surface T U During the construction process, triangles are represented by the counterclockwise order of the points. That is, if there are three points A, B, and C, and they are arranged counterclockwise as A, C, and B, then the triangle formed by these three points should be labeled △ACB. Step 202, the outer surface of the trunk is reconstructed, including: taking the initial point set P0 as input, and taking the reverse edge of the boundary edge of the lower surface of the trunk and the reverse edge of the boundary edge of the upper surface of the trunk as the boundary edge set B for reconstructing the surrounding surface of the trunk O Reconstruct the triangulated mesh model of the trunk outer surface O ; Take any edge from the boundary edge set as the initial base edge AB, and perform the operation of finding the third point to maximize the angle between the base edge AB: Find a point C in the point set P0 that does not belong to P L With P U , determine whether the triangle formed by the base side and point C satisfies the triangle non-intersection rule, that is, △ABC and T O Do the existing triangles with three points A, B and C as nodes intersect with each other, including direct intersection in three-dimensional space and intersection in projection space? If there is no intersection, select a node D from the candidate node set as one of the candidate nodes to maximize the angle ∠ADB, thus forming a new triangle △ABD. At this time, since the base side AB forms a new triangle △ABD, AB is no longer a boundary edge, so it is removed from the boundary edge set B. O Delete; for the other two directed edges BD and DA of △ABD, if edge BD is in the boundary edge set B O If exists in the set, remove edge BD from the boundary edge set B. O Otherwise, add edge BD to the boundary edge set B O ; If edge DA is in the boundary edge set B O If exists in the set B, remove edge DA from the boundary edge set B. O Otherwise, add edge DA to the boundary edge set B. O ; Continue to select any edge from the boundary edge set as the base edge and perform the above-mentioned operation of maximizing the angle between the base edges until the boundary edge set is empty. If the directed edges BD and DA are in the boundary edge set B O If there are two edges in the tree trunk, these two edges are prioritized as base edges. If there is no candidate node for a base edge during the execution process, the values ​​of the height parameter h and the angle parameter α should be appropriately increased and the above process should be re-executed from step 101 to ensure that the outer surface of the tree trunk is reconstructed.

2. The method for extracting tree trunk volume based on a triangular surface model and a tetrahedron model according to claim 1, characterized in that: The step 3 includes: Step 301, a subdivision of the trunk triangular surface model, including: traversing the trunk outer surface triangular mesh model T O Each edge in each triangle in , and the edge is the edge generated when the outer surface is reconstructed, that is, the edge is not the reverse edge of the boundary edge when the upper and lower surfaces of the trunk are reconstructed, that is, the reverse edge of the edge does not belong to B L With B U , for an edge AB generated during the reconstruction of the outer surface, its reverse edge is represented as BA, find the triangle where the edge AB is located, here using △ABC to represent it, find the triangle where the edge BA is located, here using △BAD to represent it, calculate the midpoint F of the edge AB, search for a point in the trunk point cloud P that is closest to point F and whose distance is less than h / 2, and record this point as E, if point E does not intersect with the four triangles △AEC, △EBC, △DBE, △DEA newly generated by the other two edges of △ABC and △BAD and other adjacent triangles in the triangulation network, then subdivide △ABC and △BAD into △AEC, △EBC, △DBE and △DEA, that is, remove △ABC and △BAD from the trunk outer surface triangulation model T O Delete △AEC, △EBC, △DBE and △DEA and add them to the trunk outer surface triangulation model T O The four newly generated edges AE, EB, DE, and EC are also called the edges generated during the external surface reconstruction, but they will no longer participate in this subdivision. The above operations are performed one by one on each edge generated during the external surface reconstruction to complete a subdivision of the tree trunk triangular surface model. Step 302, iterative subdivision of the trunk triangular surface model, including: subdividing the outer surface model T of the previous trunk triangular surface model O_i Execute step 301 to obtain a subdivided outer surface model T O_i+1 The number of iterations is set in advance or iterated until a certain iteration ends when no new triangles are generated due to triangle intersection, thus obtaining the subdivision model T of the outer surface of the trunk. O_e ; Step 303: transform the trunk upper surface triangulation model T U , triangulated mesh model of the lower surface of the trunk T L Subdivision model of the outer surface of the trunk O_e The final reconstructed trunk surface triangulated network model T is obtained by merging, that is, T = T U ∪T L ∪T O_e .

3. The method for extracting tree trunk volume based on a triangular surface model and a tetrahedron model according to claim 2, characterized in that: The step 4 comprises: Step 401: constructing a tetrahedron model of the tree trunk. The tree trunk surface triangulated mesh model T is used as input to construct a tetrahedron model of the tree trunk. The construction method adopts the constrained tetrahedron and boundary recovery method in the tetrahedron mesh generator TetGen. That is, the tetrahedron model is constructed using the input triangulated mesh model T of the tree trunk surface as the boundary constraint. Step 402 , calculating the trunk volume, traverses each tetrahedron in the trunk tetrahedron model one by one and calculates its volume. The sum of the volumes of all tetrahedrons is the trunk volume.

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