利用osg::ClipPlane 对节点进行多边形裁切

代码参考

ClipPlane是通过AX+BY+CZ+D的符号来判断裁切范围的

利用多个ClipPlane可以对节点进行多边形切割

代码如下：

#include 
#include 
#include 
#include 
#include 
#include 
#include 

#include 
#include   

using namespace Eigen;

///过点A,B，计算一个与normal平行的平面
osg::Vec4d ComputePlane(osg::Vec3d A, osg::Vec3d B, osg::Vec3d normal) {
	osg::Vec3d n = (B-A)^normal;//如果逆时针仍然无法显示，请调整A，B顺序
	return osg::Vec4d(n, -n * A);
}
///用一堆三维点拟合一个平面,返回平面法向，算法：SVD
osg::Vec3d fitPlane(std::vector points) {
	//中心化
	int n = points.size();
	osg::Vec3d center(0, 0, 0);
	for (int i = 0; i < n; i++)
		center += points[i];
	center /= n;
	MatrixXd A(n,3);
	for (int i = 0; i < points.size(); i++) {
		A(i, 0) = points[i].x() - center.x();
		A(i, 1) = points[i].y() - center.y();
		A(i, 2) = points[i].z() - center.z();
	}
	//AX=0的解 是SVD分解中的V的最后一列
	JacobiSVD svd(A, ComputeThinU | ComputeThinV);
	MatrixXd V = svd.matrixV();
	osg::Vec3d normal(V(2,0), V(2, 1), V(2, 2));
	return normal;
}
osg::ref_ptr createClipNode(std::vector trimPolygon, osg::ref_ptr subgraph) {
	osg::ref_ptr root = new osg::Group();
	root->addChild(subgraph.get());
	osg::Vec3d n = fitPlane(trimPolygon);
	trimPolygon.push_back(trimPolygon[0]);//a trick
	for (int i = 1; i < trimPolygon.size(); i++) {
		osg::Vec4d Plane = ComputePlane(trimPolygon[i - 1], trimPolygon[i], n);
		osg::ref_ptr cl= new osg::ClipPlane(i - 1, Plane);
		root->getOrCreateStateSet()->setAttributeAndModes(cl.get(), osg::StateAttribute::ON);
	}
	root->setCullingActive(false);
	return root.release();
}
int main()
{
	//绘制长方体
	osg::ref_ptr node = new osg::Geode;
	osg::ref_ptr box = new osg::Box(osg::Vec3(0.0, 0.0, 0.0),192*2,1,108*2);
	node->addDrawable(new osg::ShapeDrawable(box));
	//指定裁剪多边形顶点，只能裁剪凸多边形  
	//注意要逆时针指定，方向反了就都被裁剪掉了
	std::vector pts;
	pts.push_back(osg::Vec3d(128, 0, 0));
	pts.push_back(osg::Vec3d(64, 0, 72));
	pts.push_back(osg::Vec3d(-64, 0, 72));
	pts.push_back(osg::Vec3d(-128, 0, 0));
	pts.push_back(osg::Vec3d(-64, 0, -72));
	pts.push_back(osg::Vec3d(64, 0, -72));
	//裁切
	osg::ref_ptr Trim = createClipNode(pts, node);
	//渲染
	osgUtil::Optimizer optimizer;
	optimizer.optimize(Trim.get());
	osg::ref_ptr viewer = new osgViewer::Viewer();
	viewer->setSceneData(Trim.get());
	viewer->setUpViewInWindow(600, 100, 800, 600);
	viewer->run();

	return 0;
}