# Efficient 3D Volume Reconstruction from a Point Cloud Using a Phase-Field Method

Authors: Darae Jeong; Yibao Li; Heon Ju Lee; Sang Min Lee; Junxiang Yang; Seungwoo Park; Hyundong Kim; Yongho Choi; Junseok Kim
Journal: Mathematical Problems in Engineering
 (2018)

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## Abstract

We propose an explicit hybrid numerical method for the efficient 3D volume reconstruction from unorganized point clouds using a phase-field method. The proposed three-dimensional volume reconstruction algorithm is based on the 3D binary image segmentation method. First, we define a narrow band domain embedding the unorganized point cloud and an edge indicating function. Second, we define a good initial phase-field function which speeds up the computation significantly. Third, we use a recently developed explicit hybrid numerical method for solving the three-dimensional image segmentation model to obtain efficient volume reconstruction from point cloud data. In order to demonstrate the practical applicability of the proposed method, we perform various numerical experiments.


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## Body

## 1. Introduction

In this paper, we propose an efficient and robust algorithm for volume reconstruction from a point cloud. Reconstructing the three-dimensional model from a point cloud is important in medical applications. Surface reconstruction from a point cloud is a process of finding a surface model that approximates an unknown surface for a given set of sample points lying on or near the unknown surface [1].Hoppe et al. developed an algorithm to reconstruct a surface in the three-dimensional space from unorganized points scattered on or near the unknown surface. The algorithm is based on the idea of determining the zero level set of a signed distance function [2]. Kazhdan proposed a surface reconstruction method which takes an oriented point set and returns a solid model. The method uses Stokes’ theorem to calculate the characteristic function (one inside the model and zero outside of it) of the solid model [3]. To reconstruct implicit surfaces from scattered unorganized data set, Li et al. presented a novel numerical method for surface embedding narrow volume reconstruction from unorganized points [4, 5]. Yang et al. proposed a 3D reconstruction technique from nonuniform point clouds via local hierarchical clustering [6]. Zhao et al. developed a fast sweeping level set and tagging methods [7]. Yezzi Jr. et al. proposed a new medical image segmentation based on feature-based metrics on a given image [8].Beneš et al. used the Allen-Cahn equation with a forcing term to achieve image segmentation [9]. Caselles et al. proposed a model for active contours which could extract smooth shapes and could be adapted to find several contours simultaneously [10]. Methods using geometric active contour were introduced in [11–14]. Zhang et al. developed a weighted sparse penalty and a weighted grouping effect penalty in modeling the subspace structure [15]. Chen used an ICKFCM method (ICA analysis and KFCM algorithm) in medical image segmentation and made a good result in extracting the complicated images [16]. Zhang et al. proposed a novel fuzzy level set method based on finding the minimum of energy function to locate the true object boundaries effectively [17]. Other numerical studies based on level set method were also introduced in [18, 19].In this article, we propose an explicit hybrid algorithm for volume reconstruction from a point cloud. Therefore, it does not need implicit solvers such as multigrid methods. The computation is fast and efficient because the proposed algorithm uses a narrow band domain and a good initial condition.This paper is organized as follows. In Section2, we describe a mathematical model and a numerical solution algorithm for volume reconstruction from a point cloud. We present the numerical results for several examples in Section 3. In Section 4, we conclude.


## 2. Mathematical Model and Numerical Solution Algorithm

Now, we propose an explicit hybrid numerical method for volume reconstruction from a point cloud using a phase-field method. ForXl=(Xl,Yl) in the two-dimensional space or Xl=(Xl,Yl,Zl) in the three-dimensional space, S={Xl∣1≤l≤N} denote the point cloud in the two- or the three-dimensional space, respectively. The geometric active contour model based on the mean curvature motion is given by the following evolution equation [20]:(1)∂ϕx,t∂t=gx-F′ϕx,tϵ2+Δϕx,t+λFϕx,t,where g(x) is an edge indicator function, F(ϕ)=0.25(ϕ2-1)2, and ϵ is a constant which is related to the phase transition width. Note that here we use a different edge indicator function and efficient explicit numerical algorithm.For simplicity of exposition, we first discretize (1) in the two-dimensional space Ω=(a,b)×(c,d). Let h=(b-a)/(Nx-1)=(d-c)/(Ny-1) be the uniform mesh size, where Nx and Ny are the number of grid points. Let Ωh={x=(xi,yj):xi=a+(i-1)h,yj=c+(j-1)h,1≤i≤Nx,1≤j≤Ny} be the discrete domain. Let ϕijn be approximations of ϕ(xi,yj,nΔt), where Δt is the time step. Let d(x)=dist(x,S)=min1≤l≤N|Xl-x| be the distance to the data S, where Xl=(Xl,Yl). In fact, we will use the distance function as an edge indicator function, g(x). In practice, for l=1,…,N, we define a local domain Ωl which embeds the point Xl and set the minimum value at the grid point between the point and the grid point. For example, Ωl is a 3×3 grid. Then, the computational narrow band domain is defined as(2)Ωnb=⋃l=1NΩl.Outside the narrow band domain, we set a large value to the edge indicator function, see Figure 1 for the procedure.Figure 1
Schematic of defined distance function. (a) Point cloud data, (b) mesh grid covering the point cloud and local mesh grid, (c) local mesh gridΩl embedding a point Xl, (d) narrow band domain, and (e) edge indicator function, g(x).
(a)
(b)
(c)
(d)
(e)In this study, we apply the simplest sequential splitting procedure. We split (1) into two equations by using the operator splitting method:(3)∂ϕx,t∂t=gxΔϕx,t+λFϕx,t,(4)∂ϕx,t∂t=-gxF′ϕx,tϵ2=gxϕx,t-ϕ3x,tϵ2.For a good initial configuration, we set ϕij0=1 inside on the narrow band domain and set ϕij0=-1 outside the narrow band domain.Givenϕn, we solve (3) on the narrow band domain Ωnb by using the explicit Euler method:(5)ϕij∗-ϕijnΔt=gijΔdϕijn+λFϕijn,where ϕij∗ is the intermediate value which is defined at point (xi,yj). The initial values at inside and outside region of the narrow band domain Ωnb act as Dirichlet boundary condition in computing the discrete Laplace operator, Δd.Then, we analytically solve (4) by the method of separation of variables [21, 22]. That is, ϕijn+1=ψ(Δt) by analytically solving(6)dψtdt=gijψt-ψ3tϵ2withtheinitialconditionψ0=ϕij∗.The analytic solution is given as(7)ϕijn+1=ϕij∗e-2gijΔt/ϵ2+ϕij∗21-e-2gijΔt/ϵ2.Therefore, (5) and (7) consist of an efficient and robust algorithm for volume reconstruction from a point cloud. We should note that the proposed numerical solution algorithm is fully explicit. Therefore, we do not need an iterative method such as multigrid method to solve the governing equation. Also, the implementation of the algorithm is straightforward.


## 3. Computational Experiments

### 3.1. Two-Dimensional Experiments

#### 3.1.1. Motion by Mean Curvature

To test the proposed numerical scheme, we perform a numerical experiment. The test is motion by mean curvature. If we setg(x)=1 and λ=0, then the governing equation (1) becomes the original Allen-Cahn equation [23], which is a reaction-diffusion equation describing the process of phase separation in a binary alloy mixture. In the two-dimensional case, as ϵ approaches zero, the zero level set of ϕ evolves with the following velocity:(8)V=-κ=-1R,where V is the normal velocity, κ is the curvature, and R is the radius of curvature at the point of the zero level set [24]. Then, (8) is rewritten by dR(t)/dt=-1/R(t) with R(0)=R0. Therefore, analytic solution is given as R(t)=R02-2t.On the computational domainΩ=(0,1)×(0,1), we investigate the motion by mean curvature of the circle in the annulus narrow band domain: {(x,y)∣0.35≤(x-0.5)2+(y-0.5)2≤0.43} as shown in Figure 2(a). We define an initial condition as(9)ϕx,y,0=tanh⁡R0-x-0.52+y-0.522ϵ.In this numerical simulation, we use the following parameters: R0=0.4, ϵ=0.011, h=1/256, Δt=0.15h2, and T=5000Δt. Figures 2(a) and 2(b) show the temporal evolution of the initial circle and its radius with respect to time, respectively. For verification of our numerical results, we include the results of the analytic solution. As shown in Figure 2, the initial circle shrinks under the motion by mean curvature.Figure 2
Temporal evolutions of the radius withΔt=0.15h2 up to t=5000Δt in the two-dimensional space. (a) Zero level contour and (b) radius R(t) of circle with respect to time.
(a)
(b)


#### 3.1.2. The Basic Working Mechanism of the Algorithm

The edge indicator functiong(x) is close to zero where the point cloud exists. Therefore, the evolution will stop or slow down in the neighborhood of the point cloud. In (1), ∂ϕ/∂t=-F′(ϕ)/ϵ2+Δϕ makes the phase-field shrink until it reaches the point cloud by the mean curvature flow. If the geometry of the point cloud is not convex, then the term λF(ϕ) makes the level set of the phase-field further shrink. For more details, please refer to [20]. To confirm the working mechanism of the algorithm, the temporal evolution of the interface in the two-dimensional space is shown in Figure 3. Here, we use the following parameters: ϵ=0.0096, h=1/200, and Δt=2.8284e-5.Figure 3
Temporal evolution of the interface: (a) initial condition, (b) 100 iterations, and (c) 1000 iterations.
(a)
(b)
(c)


### 3.2. Three-Dimensional Experiments

Next, we discretize (1) in the three-dimensional space, that is, Ω=(a,b)×(c,d)×(e,f). Let h=(b-a)/(Nx-1)=(d-c)/(Ny-1)=(f-e)/(Nz-1) be the uniform mesh size, where Nx, Ny, and Nz are the total number of grid points. Let Ωh={x=(xi,yj,zk):xi=a+(i-1)h,yj=c+(j-1)h,zk=e+(k-1)h,1≤i≤Nx,1≤j≤Ny,1≤k≤Nz} be the discrete domain. We define ϕijkn as approximations of ϕ(xi,yj,zk,nΔt), where Δt is the time step size. Let d(x)=dist(x,S)=min1≤l≤N|Xl-x| be the distance to the data S, where Xl=(Xl,Yl,Zl).Figure4 represents construction of the three-dimensional distance function d(x) and the narrow band domain. In Figures 4(a) and 4(b), we can see the given point cloud data on computational grid and a local mesh Ωl of single point Xl. Here, we calculate the distance between the given point Xl and the grid points x on the local mesh Ωl. The distance function d(x) is defined by the shortest one among the distance. Then, we obtain the narrow band domain which is determined by the distance function.Figure 4
Construction of the three-dimensional distance functiond(x) and the narrow band domain: (a) point cloud data and mesh, (b) cross section of point cloud and a local mesh Ωl of single point Xl, (c) local mesh grid Ωl embedding a point Xl, and (d) narrow band domain which is determined by the distance function.
(a)
(b)
(c)
(d)Now, we can straightforwardly extend the two-dimensional numerical solutions (5) and (7) to the following three-dimensional solutions:(10)ϕijk∗-ϕijknΔt=gijkΔdϕijkn+λFϕijkn,ϕijkn+1=ϕijk∗e-2gijkΔt/ϵ2+ϕijk∗21-e-2gijkΔt/ϵ2.


#### 3.2.1. Reconstruction from Various Point Clouds

First, we reconstruct volume of Happy Buddha from the given scattered points (N=1621848) as shown in Figure 5(a) [25]. For numerical test, we use the following parameters: ϵ=0.0069, h=0.0048, Δt=h/(2003), Nx=250, Ny=500, Nz=250, Ω=(0,1.2)×(0,2.4)×(0,1.2), and λ=100. In the first and second rows in Figure 5, we can see the front and back views of Happy Buddha. By the proposed scheme, we obtain the numerical solution ϕ after 200 iterations (see Figure 5(c)) with the initial condition in Figure 5(b).Figure 5
Front and back views of Happy Buddha: (a) point clouds, (b) initial condition ofϕ, and (c) numerical solution ϕ after 200 iterations.
(a)
(b)
(c)Next, we reconstruct volume of Armadillo model from the given scattered points (N=129732) as shown in Figure 6(a) [25]. For numerical test, we use the following parameters: ϵ=0.0127, h=0.0088, Δt=h/(2003), Nx=220, Ny=250, Nz=200, Ω=(0,2.2)×(0,2.5)×(0,2), and λ=1000. In the first and second rows in Figure 6, we can see the front and back views of Armadillo model. By the proposed scheme, we obtain the numerical solution ϕ after 400 iterations (see Figure 6(c)) with the initial condition in Figure 6(b).Figure 6
Front and back views of Armadillo model: (a) initial point clouds, (b) initial condition ofϕ, and (c) numerical solution ϕ after 400 iterations.
(a)
(b)
(c)As the final example, we reconstruct volume of Stanford Dragon from the given scattered points (N=656469) as shown in Figure 7(a) [25]. For numerical test, we use the following parameters: ϵ=0.00432, h=0.003, Δt=h/(2003), Nx=400, Ny=320, Nz=240, Ω=(0,1.2)×(0,1)×(0,0.8), and λ=100. In the first and second rows in Figure 7, we can see the front and back views of Stanford Dragon. By the proposed scheme, we obtain the numerical solution ϕ after 400 iterations (see Figure 7(c)) with the initial condition in Figure 7(b).Figure 7
Front and back views of Stanford Dragon model: (a) initial point clouds, (b) initial condition ofϕ, and (c) numerical solution ϕ after 400 iterations.
(a)
(b)
(c)


#### 3.2.2. Effect ofλ

In this section, we investigate the effect ofλ parameter on the three-dimensional volume reconstruction. The parameter makes the level set of ϕ shrink to the given points. We use the same parameters in Figure 7 except for the λ value. As shown in Figure 8, if the value of λ is small, then the surface is oversmoothed by the motion by mean curvature. On the other hand, if it is too large, then the surface is rough.Figure 8
Front and back views of Stanford Dragon model with variousλ. Numerical solution ϕ after 1000 iterations.


## 3.1. Two-Dimensional Experiments

### 3.1.1. Motion by Mean Curvature

To test the proposed numerical scheme, we perform a numerical experiment. The test is motion by mean curvature. If we setg(x)=1 and λ=0, then the governing equation (1) becomes the original Allen-Cahn equation [23], which is a reaction-diffusion equation describing the process of phase separation in a binary alloy mixture. In the two-dimensional case, as ϵ approaches zero, the zero level set of ϕ evolves with the following velocity:(8)V=-κ=-1R,where V is the normal velocity, κ is the curvature, and R is the radius of curvature at the point of the zero level set [24]. Then, (8) is rewritten by dR(t)/dt=-1/R(t) with R(0)=R0. Therefore, analytic solution is given as R(t)=R02-2t.On the computational domainΩ=(0,1)×(0,1), we investigate the motion by mean curvature of the circle in the annulus narrow band domain: {(x,y)∣0.35≤(x-0.5)2+(y-0.5)2≤0.43} as shown in Figure 2(a). We define an initial condition as(9)ϕx,y,0=tanh⁡R0-x-0.52+y-0.522ϵ.In this numerical simulation, we use the following parameters: R0=0.4, ϵ=0.011, h=1/256, Δt=0.15h2, and T=5000Δt. Figures 2(a) and 2(b) show the temporal evolution of the initial circle and its radius with respect to time, respectively. For verification of our numerical results, we include the results of the analytic solution. As shown in Figure 2, the initial circle shrinks under the motion by mean curvature.Figure 2
Temporal evolutions of the radius withΔt=0.15h2 up to t=5000Δt in the two-dimensional space. (a) Zero level contour and (b) radius R(t) of circle with respect to time.
(a)
(b)


### 3.1.2. The Basic Working Mechanism of the Algorithm

The edge indicator functiong(x) is close to zero where the point cloud exists. Therefore, the evolution will stop or slow down in the neighborhood of the point cloud. In (1), ∂ϕ/∂t=-F′(ϕ)/ϵ2+Δϕ makes the phase-field shrink until it reaches the point cloud by the mean curvature flow. If the geometry of the point cloud is not convex, then the term λF(ϕ) makes the level set of the phase-field further shrink. For more details, please refer to [20]. To confirm the working mechanism of the algorithm, the temporal evolution of the interface in the two-dimensional space is shown in Figure 3. Here, we use the following parameters: ϵ=0.0096, h=1/200, and Δt=2.8284e-5.Figure 3
Temporal evolution of the interface: (a) initial condition, (b) 100 iterations, and (c) 1000 iterations.
(a)
(b)
(c)


## 3.1.1. Motion by Mean Curvature

To test the proposed numerical scheme, we perform a numerical experiment. The test is motion by mean curvature. If we setg(x)=1 and λ=0, then the governing equation (1) becomes the original Allen-Cahn equation [23], which is a reaction-diffusion equation describing the process of phase separation in a binary alloy mixture. In the two-dimensional case, as ϵ approaches zero, the zero level set of ϕ evolves with the following velocity:(8)V=-κ=-1R,where V is the normal velocity, κ is the curvature, and R is the radius of curvature at the point of the zero level set [24]. Then, (8) is rewritten by dR(t)/dt=-1/R(t) with R(0)=R0. Therefore, analytic solution is given as R(t)=R02-2t.On the computational domainΩ=(0,1)×(0,1), we investigate the motion by mean curvature of the circle in the annulus narrow band domain: {(x,y)∣0.35≤(x-0.5)2+(y-0.5)2≤0.43} as shown in Figure 2(a). We define an initial condition as(9)ϕx,y,0=tanh⁡R0-x-0.52+y-0.522ϵ.In this numerical simulation, we use the following parameters: R0=0.4, ϵ=0.011, h=1/256, Δt=0.15h2, and T=5000Δt. Figures 2(a) and 2(b) show the temporal evolution of the initial circle and its radius with respect to time, respectively. For verification of our numerical results, we include the results of the analytic solution. As shown in Figure 2, the initial circle shrinks under the motion by mean curvature.Figure 2
Temporal evolutions of the radius withΔt=0.15h2 up to t=5000Δt in the two-dimensional space. (a) Zero level contour and (b) radius R(t) of circle with respect to time.
(a)
(b)


## 3.1.2. The Basic Working Mechanism of the Algorithm

The edge indicator functiong(x) is close to zero where the point cloud exists. Therefore, the evolution will stop or slow down in the neighborhood of the point cloud. In (1), ∂ϕ/∂t=-F′(ϕ)/ϵ2+Δϕ makes the phase-field shrink until it reaches the point cloud by the mean curvature flow. If the geometry of the point cloud is not convex, then the term λF(ϕ) makes the level set of the phase-field further shrink. For more details, please refer to [20]. To confirm the working mechanism of the algorithm, the temporal evolution of the interface in the two-dimensional space is shown in Figure 3. Here, we use the following parameters: ϵ=0.0096, h=1/200, and Δt=2.8284e-5.Figure 3
Temporal evolution of the interface: (a) initial condition, (b) 100 iterations, and (c) 1000 iterations.
(a)
(b)
(c)


## 3.2. Three-Dimensional Experiments

Next, we discretize (1) in the three-dimensional space, that is, Ω=(a,b)×(c,d)×(e,f). Let h=(b-a)/(Nx-1)=(d-c)/(Ny-1)=(f-e)/(Nz-1) be the uniform mesh size, where Nx, Ny, and Nz are the total number of grid points. Let Ωh={x=(xi,yj,zk):xi=a+(i-1)h,yj=c+(j-1)h,zk=e+(k-1)h,1≤i≤Nx,1≤j≤Ny,1≤k≤Nz} be the discrete domain. We define ϕijkn as approximations of ϕ(xi,yj,zk,nΔt), where Δt is the time step size. Let d(x)=dist(x,S)=min1≤l≤N|Xl-x| be the distance to the data S, where Xl=(Xl,Yl,Zl).Figure4 represents construction of the three-dimensional distance function d(x) and the narrow band domain. In Figures 4(a) and 4(b), we can see the given point cloud data on computational grid and a local mesh Ωl of single point Xl. Here, we calculate the distance between the given point Xl and the grid points x on the local mesh Ωl. The distance function d(x) is defined by the shortest one among the distance. Then, we obtain the narrow band domain which is determined by the distance function.Figure 4
Construction of the three-dimensional distance functiond(x) and the narrow band domain: (a) point cloud data and mesh, (b) cross section of point cloud and a local mesh Ωl of single point Xl, (c) local mesh grid Ωl embedding a point Xl, and (d) narrow band domain which is determined by the distance function.
(a)
(b)
(c)
(d)Now, we can straightforwardly extend the two-dimensional numerical solutions (5) and (7) to the following three-dimensional solutions:(10)ϕijk∗-ϕijknΔt=gijkΔdϕijkn+λFϕijkn,ϕijkn+1=ϕijk∗e-2gijkΔt/ϵ2+ϕijk∗21-e-2gijkΔt/ϵ2.


### 3.2.1. Reconstruction from Various Point Clouds

First, we reconstruct volume of Happy Buddha from the given scattered points (N=1621848) as shown in Figure 5(a) [25]. For numerical test, we use the following parameters: ϵ=0.0069, h=0.0048, Δt=h/(2003), Nx=250, Ny=500, Nz=250, Ω=(0,1.2)×(0,2.4)×(0,1.2), and λ=100. In the first and second rows in Figure 5, we can see the front and back views of Happy Buddha. By the proposed scheme, we obtain the numerical solution ϕ after 200 iterations (see Figure 5(c)) with the initial condition in Figure 5(b).Figure 5
Front and back views of Happy Buddha: (a) point clouds, (b) initial condition ofϕ, and (c) numerical solution ϕ after 200 iterations.
(a)
(b)
(c)Next, we reconstruct volume of Armadillo model from the given scattered points (N=129732) as shown in Figure 6(a) [25]. For numerical test, we use the following parameters: ϵ=0.0127, h=0.0088, Δt=h/(2003), Nx=220, Ny=250, Nz=200, Ω=(0,2.2)×(0,2.5)×(0,2), and λ=1000. In the first and second rows in Figure 6, we can see the front and back views of Armadillo model. By the proposed scheme, we obtain the numerical solution ϕ after 400 iterations (see Figure 6(c)) with the initial condition in Figure 6(b).Figure 6
Front and back views of Armadillo model: (a) initial point clouds, (b) initial condition ofϕ, and (c) numerical solution ϕ after 400 iterations.
(a)
(b)
(c)As the final example, we reconstruct volume of Stanford Dragon from the given scattered points (N=656469) as shown in Figure 7(a) [25]. For numerical test, we use the following parameters: ϵ=0.00432, h=0.003, Δt=h/(2003), Nx=400, Ny=320, Nz=240, Ω=(0,1.2)×(0,1)×(0,0.8), and λ=100. In the first and second rows in Figure 7, we can see the front and back views of Stanford Dragon. By the proposed scheme, we obtain the numerical solution ϕ after 400 iterations (see Figure 7(c)) with the initial condition in Figure 7(b).Figure 7
Front and back views of Stanford Dragon model: (a) initial point clouds, (b) initial condition ofϕ, and (c) numerical solution ϕ after 400 iterations.
(a)
(b)
(c)


### 3.2.2. Effect ofλ

In this section, we investigate the effect ofλ parameter on the three-dimensional volume reconstruction. The parameter makes the level set of ϕ shrink to the given points. We use the same parameters in Figure 7 except for the λ value. As shown in Figure 8, if the value of λ is small, then the surface is oversmoothed by the motion by mean curvature. On the other hand, if it is too large, then the surface is rough.Figure 8
Front and back views of Stanford Dragon model with variousλ. Numerical solution ϕ after 1000 iterations.


## 3.2.1. Reconstruction from Various Point Clouds

First, we reconstruct volume of Happy Buddha from the given scattered points (N=1621848) as shown in Figure 5(a) [25]. For numerical test, we use the following parameters: ϵ=0.0069, h=0.0048, Δt=h/(2003), Nx=250, Ny=500, Nz=250, Ω=(0,1.2)×(0,2.4)×(0,1.2), and λ=100. In the first and second rows in Figure 5, we can see the front and back views of Happy Buddha. By the proposed scheme, we obtain the numerical solution ϕ after 200 iterations (see Figure 5(c)) with the initial condition in Figure 5(b).Figure 5
Front and back views of Happy Buddha: (a) point clouds, (b) initial condition ofϕ, and (c) numerical solution ϕ after 200 iterations.
(a)
(b)
(c)Next, we reconstruct volume of Armadillo model from the given scattered points (N=129732) as shown in Figure 6(a) [25]. For numerical test, we use the following parameters: ϵ=0.0127, h=0.0088, Δt=h/(2003), Nx=220, Ny=250, Nz=200, Ω=(0,2.2)×(0,2.5)×(0,2), and λ=1000. In the first and second rows in Figure 6, we can see the front and back views of Armadillo model. By the proposed scheme, we obtain the numerical solution ϕ after 400 iterations (see Figure 6(c)) with the initial condition in Figure 6(b).Figure 6
Front and back views of Armadillo model: (a) initial point clouds, (b) initial condition ofϕ, and (c) numerical solution ϕ after 400 iterations.
(a)
(b)
(c)As the final example, we reconstruct volume of Stanford Dragon from the given scattered points (N=656469) as shown in Figure 7(a) [25]. For numerical test, we use the following parameters: ϵ=0.00432, h=0.003, Δt=h/(2003), Nx=400, Ny=320, Nz=240, Ω=(0,1.2)×(0,1)×(0,0.8), and λ=100. In the first and second rows in Figure 7, we can see the front and back views of Stanford Dragon. By the proposed scheme, we obtain the numerical solution ϕ after 400 iterations (see Figure 7(c)) with the initial condition in Figure 7(b).Figure 7
Front and back views of Stanford Dragon model: (a) initial point clouds, (b) initial condition ofϕ, and (c) numerical solution ϕ after 400 iterations.
(a)
(b)
(c)


## 3.2.2. Effect ofλ

In this section, we investigate the effect ofλ parameter on the three-dimensional volume reconstruction. The parameter makes the level set of ϕ shrink to the given points. We use the same parameters in Figure 7 except for the λ value. As shown in Figure 8, if the value of λ is small, then the surface is oversmoothed by the motion by mean curvature. On the other hand, if it is too large, then the surface is rough.Figure 8
Front and back views of Stanford Dragon model with variousλ. Numerical solution ϕ after 1000 iterations.


## 4. Conclusions

In this article, we developed an explicit hybrid numerical algorithm for the efficient 3D volume reconstruction from unorganized point clouds using a modified Allen-Cahn equation. The 3D volume reconstruction algorithm is based on the 3D binary image segmentation method. The proposed algorithm has potential to be used in various practical industry such as 3D model printing from scattered scanned data. The computational results confirmed that the algorithm is very efficient and robust in reconstructing 3D volume from point clouds.



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*Source: 7090186-2018-02-06.xml*