OSG学习：计算纹理坐标

深碍√TFBOYSˉ_ 2022-06-12 13:50 455阅读 0赞

在很多时候，直接指定纹理坐标是非常不方便的，如曲面纹理坐标，只有少数的曲面（如圆锥、圆柱等）可以在不产生扭曲的情况下映射到平面上，其他的曲面在映射到表面时都会产生一定程度的扭曲。一般而言，曲面表面的曲率越大，纹理所需要的扭曲度就越大。这时，直接指定纹理坐标可能是一件非常困难的事情了。  
下面的示例，通过一个纹理坐标生成器（继承自osg::NodeVisitor访问器）遍历模型的所有顶点及法线，然后根据顶点、法线及一定的比例来确定纹理坐标。

#include <osgViewer/Viewer>
    
    #include <osg/Node>
    #include <osg/Geode>
    #include <osg/Geometry>
    #include <osg/Group>
    #include <osg/Camera>
    #include <osg/MatrixTransform>
    #include <osg/PositionAttitudeTransform>
    #include <osg/TexGen>
    #include <osg/TexEnv>
    #include <osg/NodeVisitor>
    
    #include <osgDB/ReadFile>
    #include <osgDB/WriteFile>
    
    #include <osgUtil/Optimizer>
    
    #include <iostream>
    
    //纹理坐标生成器，继承自NodeVisitor
    class TexCoordGenerator: public osg::NodeVisitor 
    {
    public:
        //遍历所有的子节点
        TexCoordGenerator(): NodeVisitor(NodeVisitor::TRAVERSE_ALL_CHILDREN)
        {
            //
        }
    
        void apply(osg::Geode& geode)
        {
            //通过包围盒来确定合适的比例
            const osg::BoundingSphere &bsphere = geode.getBound();
    
            float scale = 10;
    
            if (bsphere.radius() != 0) 
            {
                scale = 5 / bsphere.radius();
            }
    
            //遍历所有几何体，并设置纹理坐标
            for (unsigned i=0; i<geode.getNumDrawables(); ++i) 
            {
                osg::Geometry* geo = dynamic_cast<osg::Geometry* >(geode.getDrawable(i));
    
                if (geo) 
                {
                    osg::Vec2Array* tc = generate_coords(geo->getVertexArray(), geo->getNormalArray(), scale);
    
                    geo->setTexCoordArray(0, tc);
                }
            }
    
            NodeVisitor::apply(geode);
        }
    
    protected:
    
        //计算纹理坐标
        osg::Vec2Array* generate_coords(osg::Array* vx, osg::Array* nx, float scale)
        {
            osg::Vec2Array* v2a = dynamic_cast<osg::Vec2Array*>(vx);
            osg::Vec3Array* v3a = dynamic_cast<osg::Vec3Array*>(vx);
            osg::Vec4Array* v4a = dynamic_cast<osg::Vec4Array*>(vx);
            osg::Vec2Array* n2a = dynamic_cast<osg::Vec2Array*>(nx);
            osg::Vec3Array* n3a = dynamic_cast<osg::Vec3Array*>(nx);
            osg::Vec4Array* n4a = dynamic_cast<osg::Vec4Array*>(nx);
    
            osg::ref_ptr<osg::Vec2Array> tc = new osg::Vec2Array;
            for (unsigned i=0; i<vx->getNumElements(); ++i) {
    
                osg::Vec3 P;
                if (v2a) P.set((*v2a)[i].x(), (*v2a)[i].y(), 0);
                if (v3a) P.set((*v3a)[i].x(), (*v3a)[i].y(), (*v3a)[i].z());
                if (v4a) P.set((*v4a)[i].x(), (*v4a)[i].y(), (*v4a)[i].z());
    
                osg::Vec3 N(0, 0, 1);
                if (n2a) N.set((*n2a)[i].x(), (*n2a)[i].y(), 0);
                if (n3a) N.set((*n3a)[i].x(), (*n3a)[i].y(), (*n3a)[i].z());
                if (n4a) N.set((*n4a)[i].x(), (*n4a)[i].y(), (*n4a)[i].z());
    
                int axis = 0;
                if (N.y() > N.x() && N.y() > N.z()) axis = 1;
                if (-N.y() > N.x() && -N.y() > N.z()) axis = 1;
                if (N.z() > N.x() && N.z() > N.y()) axis = 2;
                if (-N.z() > N.x() && -N.z() > N.y()) axis = 2;
    
                osg::Vec2 uv;
    
                switch (axis) {
                    case 0: uv.set(P.y(), P.z()); break;
                    case 1: uv.set(P.x(), P.z()); break;
                    case 2: uv.set(P.x(), P.y()); break;
                    default: ;
                }
    
                tc->push_back(uv * scale);
            }
            return tc.release();
        }
    
    };
    
    //创建二维纹理状态对象
    osg::ref_ptr<osg::StateSet> createTexture2DState(osg::ref_ptr<osg::Image> image)
    {
        //创建状态集对象
        osg::ref_ptr<osg::StateSet> stateset = new osg::StateSet();
    
        //创建二维纹理对象
        osg::ref_ptr<osg::Texture2D> texture = new osg::Texture2D();
        texture->setDataVariance(osg::Object::DYNAMIC);
        //设置贴图
        texture->setImage(image.get());
    
        texture->setFilter(osg::Texture::MIN_FILTER, osg::Texture::LINEAR_MIPMAP_LINEAR);
        texture->setFilter(osg::Texture::MAG_FILTER, osg::Texture::LINEAR);
        texture->setWrap(osg::Texture::WRAP_S, osg::Texture::REPEAT);
        texture->setWrap(osg::Texture::WRAP_T, osg::Texture::REPEAT);
    
        stateset->setTextureAttributeAndModes(0,texture.get(),osg::StateAttribute::ON);
    
        return stateset.get() ;
    }
    
    int main()
    {
        osg::ref_ptr<osgViewer::Viewer> viewer = new osgViewer::Viewer();
    
        osg::ref_ptr<osg::Group> root = new osg::Group();
    
        //读取贴图文件
        osg::ref_ptr<osg::Image> image = osgDB::readImageFile("Images/primitives.gif");
    
        osg::ref_ptr<osg::Node> node = osgDB::readNodeFile("dumptruck.osg");
    
        //计算纹理坐标
        TexCoordGenerator tcg ;
    
        node->accept(tcg);
    
        //创建状态集对象
        osg::ref_ptr<osg::StateSet> stateset = new osg::StateSet();
        stateset = createTexture2DState(image.get());
    
        //使用二维纹理
        node->setStateSet(stateset.get());
    
        root->addChild(node.get());
    
        //优化场景数据
        osgUtil::Optimizer optimizer ;
        optimizer.optimize(root.get()) ;
    
        viewer->setSceneData(root.get());
    
        viewer->realize();
    
        viewer->run();
    
        return 0 ;
    }