OSG学习：计算纹理坐标

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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NodeVisitorclass 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 ;}

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