[英]Creating 3D Alpha Shapes in CGAL and the visualization
I am a new CGAL user with basic C++ experience, trying to find the 3D Alpha shapes of a set of points. 我是具有基本C ++经验的CGAL新用户,试图查找一组点的3D Alpha形状。 I am using the ex_alpha_shapes_3 example code, and then using the instructions in saving CGAL alpha shape surface mesh for visualization of the results.
我正在使用ex_alpha_shapes_3示例代码,然后使用说明保存CGAL alpha形状表面网格以可视化结果。 Everything seems to work properly, but when I try to change the value of alpha by replacing
一切似乎都能正常工作,但是当我尝试通过替换来更改alpha值时
Alpha_shape_3 as(lp.begin(),lp.end());
with 同
Alpha_shape_3 as(lp.begin(),lp.end(),1, Alpha_shape_3::GENERAL);
assuming the third variable is the value of alpha (= 1) and change this value each time, no change in the results is obtained. 假设第三个变量是alpha的值(= 1),并且每次更改此值,结果都不会改变。
More specifically, in my set of particles, some are detached from the majority and I would like to represent them with separate volumes (similar to the Figure 41.1 ) using the concave hull or alpha shapes. 更具体地说,在我的一组粒子中,有些粒子与多数粒子分离,我想使用凹壳或alpha形状用单独的体积(类似于图41.1 )表示它们。 Currently what I get (using the Tecplot for visualization) is:
目前,我得到的(使用Tecplot进行可视化处理)是:
and as you can see, the detached particles are connected to the other particles.
如您所见,分离的粒子与其他粒子相连。 I am also attaching my code, at the end.
最后,我还将附加我的代码。 I would appreciate any help on this matter.
对此,我将不胜感激。
#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
#include <CGAL/Delaunay_triangulation_3.h>
#include <CGAL/Alpha_shape_3.h>
#include <iostream>
#include <fstream>
#include <list>
#include <cassert>
typedef CGAL::Exact_predicates_inexact_constructions_kernel Gt;
typedef CGAL::Alpha_shape_vertex_base_3<Gt> Vb;
typedef CGAL::Alpha_shape_cell_base_3<Gt> Fb;
typedef CGAL::Triangulation_data_structure_3<Vb,Fb> Tds;
typedef CGAL::Delaunay_triangulation_3<Gt,Tds> Triangulation_3;
typedef CGAL::Alpha_shape_3<Triangulation_3> Alpha_shape_3;
typedef Gt::Point_3 Point;
typedef Alpha_shape_3::Alpha_iterator Alpha_iterator;
using namespace std;
int main()
{
std::list<Point> lp;
//read input
std::ifstream is("./data/finalwater4.dat");
int n;
is >> n;
std::cout << "Reading " << n << " points " << std::endl;
Point p;
for( ; n>0 ; n--) {
is >> p;
lp.push_back(p);
}
// compute alpha shape
// Alpha_shape_3 as(lp.begin(),lp.end());
Alpha_shape_3 as(lp.begin(),lp.end(),0.001, Alpha_shape_3::GENERAL);
// find optimal alpha value
Alpha_iterator opt = as.find_optimal_alpha(1);
std::cout << "Optimal alpha value to get one connected component is "
<< *opt << std::endl;
as.set_alpha(*opt);
assert(as.number_of_solid_components() == 1);
/// the rest of the code, prepares the output to be written into a file
/// collect all regular facets (fetch regular facets from as and inserts in facets)
std::vector<Alpha_shape_3::Facet> facets;
as.get_alpha_shape_facets(std::back_inserter(facets), Alpha_shape_3::REGULAR);
std::stringstream pts;
std::stringstream ind;
std::size_t nbf=facets.size();
for (std::size_t i=0;i<nbf;++i)
{
//To have a consistent orientation of the facet, always consider an exterior cell
if ( as.classify( facets[i].first )!=Alpha_shape_3::EXTERIOR )
facets[i]=as.mirror_facet( facets[i] );
CGAL_assertion( as.classify( facets[i].first )==Alpha_shape_3::EXTERIOR );
int indices[3]={
(facets[i].second+1)%4,
(facets[i].second+2)%4,
(facets[i].second+3)%4,
};
/// according to the encoding of vertex indices, this is needed to get
/// a consistent orienation
if ( facets[i].second%2==0 ) std::swap(indices[0], indices[1]);
pts <<
facets[i].first->vertex(indices[0])->point() << "\n" <<
facets[i].first->vertex(indices[1])->point() << "\n" <<
facets[i].first->vertex(indices[2])->point() << "\n";
ind << 3*i+1 << " " << 3*i+2 << " " << 3*i+3 << "\n";
}
ofstream myfile;
myfile.open ("output.dat");
myfile << "variables = x, y, z\n";
myfile << "zone n="<< 3*nbf << " , e=" << nbf << " , f=fepoint, et=triangle\n";
myfile << pts.str();
myfile << ind.str();
myfile.close();
return 0;
}
If you want to filter things out, you need to take a alpha shape with value less than the one returned by as.find_optimal_alpha(1). 如果要过滤掉内容,则需要采用alpha形状,其值小于as.find_optimal_alpha(1)返回的值。 I suggest to run the alpha-shape demo which feature a slider that will display what the alpha-shape looks like depending on the value of alpha.
我建议运行带有滑块的alpha形状演示,该滑块将显示取决于alpha值的alpha形状。 Input file should be with a .pts extension and must contains the number of points, followed by the coordinates of the points.
输入文件的扩展名应为.pts,并且必须包含点数,后跟点的坐标。
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