STEP BY STEP

使用CGAL实现简单的模型分割

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第一步，先把鼠标轨迹点逆投影到三维世界的观察坐标系。
第二步，把第一步获得的三维坐标点集合投射到模型上面，获得笔画，就是模型内部的一些相互连接的边。
第三步，沿着第二步获得那些相互连接的边进行欧拉操作分裂边。可能需要多分割几次才会分割出模型的一部分。
第四步，搜索整个模型，对不同的部分进行标记。

下面是实现的一些效果。绿色线是鼠标轨迹，红色线是投影到模型上面的边的连接线。


下面具体解释下以上几个步骤。
步骤一，比较简单，网上代码一堆，不过要自己注意理解。

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glGetIntegerv(GL_VIEWPORT, m_nViewPort);
glGetDoublev(GL_PROJECTION_MATRIX, m_fProjmatrix);
glGetDoublev(GL_MODELVIEW_MATRIX, m_fMvmatrix);

float fWinX, fWinY, fWinZ;
int nX, nY;
double fX, fY, fZ;

m_pt3ds.clear();
for (int i = 0; i < m_ptStroke.size(); ++i)
{
    nX = m_ptStroke[i].x;
    nY = m_nViewPort[3] - m_ptStroke[i].y - 1;
    glReadBuffer(GL_BACK);
    glReadPixels(nX, nY, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, fWinZ);
    //if (fabs(fWinZ - 1.0) > 1e-8)
    {
        fWinX = nX;
        fWinY = nY;
        gluUnProject(fWinX, fWinY, fWinZ, m_fMvmatrix, m_fProjmatrix, m_nViewPort,
            fX, fY, fZ);
        m_pt3ds.push_back(Point_3(fX, fY, fZ));
    }
}
m_ptStroke.clear();

该代码就是把m_ptStroke的点转化为观察坐标系的点m_pt3ds。
步骤二，大致思路如下：第一步，把所有鼠标经过的点，都投影回观察空间。第二步，找到离第一个鼠标点p[0]，最近的模型内的顶点pClosed。第三步，循环剩余的鼠标点，如果当前鼠标点离pClosed的相邻顶点更近的话，那么将pClosed更新为上一次找到的pClosed的相邻顶点。所有的这些，pClosed点就是笔画的投影点。

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FILE* fpLog = fopen("log.txt", "w");
char szStr[MAX_PATH];
CString str;
str.Format("%d", pt3ds.size());
//AfxMessageBox(str);
sprintf(szStr, "pt3ds.size():%d\n", pt3ds.size());
fprintf(fpLog, "%s", szStr);

if (pt3ds.size() == 0)
{
    return;
}

double fX = CGAL::to_double(pt3ds[0].x());
double fY = CGAL::to_double(pt3ds[0].y());
double fZ = CGAL::to_double(pt3ds[0].z());
double fX1, fY1, fZ1;

//获得离第一个投影点最近的halfedge_handle
Halfedge_handle heClosed = facets_begin()->halfedge();
double fDiff = 1e10;
for(Face_iterator pf = facets_begin();
    pf != facets_end(); pf++)
{
    Halfedge_handle he = pf->halfedge();

    do
    {
        Point_3 p = he->vertex()->point();
        fX1 = CGAL::to_double(p.x());
        fY1 = CGAL::to_double(p.y());
        fZ1 = CGAL::to_double(p.z());
        double fTemp = (fX1 - fX) * (fX1 - fX) + (fY1 - fY) * (fY1 - fY)
            + (fZ1 - fZ) * (fZ1 - fZ);
        if (fTemp < fDiff)
        {
            fDiff = fTemp;
            heClosed = he;
        }
        he = he->next();
    }while(he!= pf->halfedge());
}

vector<Halfedge_handle> vh;
vh.push_back(heClosed);
for (unsigned i = 1; i < pt3ds.size(); ++i)
{
    sprintf(szStr, "处理第:%d个笔画点\n", i);
    fprintf(fpLog, szStr);

    fX = CGAL::to_double(pt3ds[i].x());
    fY = CGAL::to_double(pt3ds[i].y());
    fZ = CGAL::to_double(pt3ds[i].z());
    sprintf(szStr, "笔画点坐标为:%f,%f,%f\n", fX, fY, fZ);
    fprintf(fpLog, szStr);

    Halfedge_handle next = heClosed;
    Point_3 p = heClosed->vertex()->point();

    fX1 = CGAL::to_double(p.x());
    fY1 = CGAL::to_double(p.y());
    fZ1 = CGAL::to_double(p.z());
    sprintf(szStr, "heClosed坐标为:%f,%f,%f\n", fX1, fY1, fZ1);
    fprintf(fpLog, szStr);

    fDiff = (fX1 - fX) * (fX1 - fX) + (fY1 - fY) * (fY1 - fY)
        + (fZ1 - fZ) * (fZ1 - fZ);

    Vertex_handle v = heClosed->vertex();
    Halfedge_around_vertex_circulator he, end;
    he = end = v->vertex_begin();
    CGAL_For_all(he, end)
    {
        Halfedge_handle heTemp = he->opposite();
        p = heTemp->vertex()->point();
        fX1 = CGAL::to_double(p.x());
        fY1 = CGAL::to_double(p.y());
        fZ1 = CGAL::to_double(p.z());

        double fTemp = (fX1 - fX) * (fX1 - fX) + (fY1 - fY) * (fY1 - fY)
            + (fZ1 - fZ) * (fZ1 - fZ);

        if (fTemp < fDiff  heClosed != he)
        {
            sprintf(szStr, "heTemp坐标为:%f,%f,%f\n", fX1, fY1, fZ1);
            fprintf(fpLog, szStr);

            sprintf(szStr, "fDiff:%f,fTemp:%f\n", fDiff, fTemp);
            fprintf(fpLog, szStr);

            if (std::find(vh.begin(), vh.end(), heTemp) != vh.end())
            {
                continue;
            }

            //if (vh.size() != 1  heClosed->facet() == heTemp->facet())
            {
            //  continue;
            }
            fDiff = fTemp;
            next = heTemp;
            //sprintf(szStr, "next坐标为:%f,%f,%f\n", fX1, fY1, fZ1);
            //fprintf(fpLog, szStr);
        }
    }

    //去掉已经出现过的顶点
    if (next != heClosed)
    {
        heClosed = next;
        vh.push_back(heClosed);
    }
}

glDisable(GL_LIGHTING);
glDisable(GL_DEPTH_TEST);
glColor3f(1.0, 0.0, 0.0);
glLineWidth(3.0);
glBegin(GL_LINE_STRIP);
for (int i = 0; i < vh.size(); ++i)
{
    glVertex3d(CGAL::to_double(vh[i]->vertex()->point().x()),
        CGAL::to_double(vh[i]->vertex()->point().y()),
        CGAL::to_double(vh[i]->vertex()->point().z()));
    sprintf(szStr, "i:%d, x:%f,y:%f,z:%f\n", i,
        CGAL::to_double(vh[i]->vertex()->point().x()),
        CGAL::to_double(vh[i]->vertex()->point().y()),
        CGAL::to_double(vh[i]->vertex()->point().z()));
    fprintf(fpLog, szStr);
}
glEnd();
glFlush();

步骤三是欧拉操作。我的思路是将经过的边都变成边界边，方便第四步的搜索。首先，split_edge。然后，把沿着边的相邻的两个面沿着原来的边split_facet，再删除多余的面。那么，该经过的边就会变成边界边。代码如下：

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for (int i = 1; i < vh.size(); ++i)
{
    Halfedge_handle hBegOne = vh[i]->next()->next();
    fprintf(fpLog ,"处理i:%d\n", i);
    split_edge(vh[i]);

    Halfedge_handle hBegTwo = vh[i]->opposite()->next();
    Halfedge_handle hEndOne = vh[i];
    Halfedge_handle hEndTwo = hBegTwo->next()->next();
    split_facet(hBegOne, hEndOne);
    split_facet(hBegTwo, hEndTwo);
    erase_facet(hEndOne);
    erase_facet(hBegTwo);
    fprintf(fpLog, "is border:%d\n", hBegOne->next()->is_border_edge());
}

步骤五，对整个模型的面进行dfs即可了。代码如下：

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void SetComponent()
{
    for (Facet_iterator i = facets_begin(); i != facets_end(); ++i)
    {
        i->component(-1);
    }

    int nComponent = 1;
    for (Facet_iterator i = facets_begin(); i != facets_end(); ++i)
    {
        if (i->component() == -1)
        {
            DfsComponent(i, nComponent);
            nComponent++;
        }

    }

}

void DfsComponent(Facet_iterator i, int nComponent)
{
    CString str;
    str.Format("处理第:%d个面", i->tag());
    //AfxMessageBox(str);
    i->component(nComponent);

    Halfedge_around_facet_circulator  j = i->facet_begin();

    do {
        Halfedge_handle hfTemp = j;
        if (hfTemp->is_border_edge())
        {
            //AfxMessageBox("找到边界边");
            continue;
        }
        Halfedge_handle hf = j->opposite();
        Facet_handle fh = hf->facet();

        if (fh->component() == -1)
        {
            DfsComponent(fh, nComponent);
        }
    } while ( ++j != i->facet_begin());

}

因为每个面里面都有个代表属于哪个组件的标记，这样就实现了模型的分割。利用该方法就能实现分割出狗的腿和头部等等。

代码分享链接：链接: http://pan.baidu.com/s/1eQ50rf0 密码: 7f6v

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