我有std::vector<osg::ref_ptr<osg::Vec3dArray>> 的容器，里面装着4个点的坐标，我想计算由他们构成的面的面积是多大，怎么算?

您可以使用osgUtil::Tessellator工具将四个点构成的面转化为三角形网格，然后计算每个三角形的面积并累加得到总面积。具体实现方法可以参考以下代码：

#include <osg/Array>
#include <osg/TriangleIndexFunctor>
#include <osgUtil/Tessellator>

// 计算三角形面积
double calcTriangleArea(const osg::Vec3d& v1, const osg::Vec3d& v2, const osg::Vec3d& v3) {
    double a = (v1 - v2).length();
    double b = (v2 - v3).length();
    double c = (v3 - v1).length();
    double s = (a + b + c) / 2.0;
    return sqrt(s * (s - a) * (s - b) * (s - c));
}

// 计算由容器中的点构成的面的面积
double calcSurfaceArea(const std::vector<osg::ref_ptr<osg::Vec3dArray>>& container) {
    osg::ref_ptr<osg::Vec3Array> vertices = new osg::Vec3Array;
    osg::DrawElementsUInt* indices = new osg::DrawElementsUInt(GL_TRIANGLES);
    for (const auto& array : container) {
        for (const auto& vertex : *array) {
            vertices->push_back(vertex);
        }
        indices->push_back(vertices->size() - 3);
        indices->push_back(vertices->size() - 2);
        indices->push_back(vertices->size() - 1);
    }
    osg::ref_ptr<osg::TriangleIndexFunctor<>> triangleIndexFunctor = new osg::TriangleIndexFunctor<>(indices->begin());
    osg::ref_ptr<osgUtil::Tessellator> tessellator = new osgUtil::Tessellator;
    tessellator->setTessellationType(osgUtil::Tessellator::TESS_TYPE_GEOMETRY);
    tessellator->setBoundaryOnly(false);
    tessellator->setWindingType(osgUtil::Tessellator::TESS_WINDING_ODD);
    tessellator->setNormal(osg::Vec3(0.0, 0.0, 1.0));
    tessellator->setCallback(triangleIndexFunctor.get());
    osg::ref_ptr<osg::Geometry> geometry = new osg::Geometry;
    geometry->setVertexArray(vertices);
    geometry->addPrimitiveSet(indices);
    tessellator->retessellatePolygons(*geometry);
    double area = 0.0;
    const osg::Vec3Array* triangles = dynamic_cast<const osg::Vec3Array*>(geometry->getVertexArray());
    if (triangles) {
        for (size_t i = 0; i < triangles->size(); i += 3) {
            area += calcTriangleArea(triangles->at(i), triangles->at(i + 1), triangles->at(i + 2));
        }
    }
    return area;
}

您可以将容器中的点构成的数组传递给calcSurfaceArea函数，即可计算出由它们构成的面的面积。

std::vector<osg::ref_ptr<osg::Vec3dArray>> container;
// ...
double area = calcSurfaceArea(container);