osg.PrimitiveFunctor

osg.PrimitiveFunctor是一个纯虚类，用于定义遍历OpenSceneGraph中几何图形的原语的方法。

概述

在OpenSceneGraph中，几何图形是使用一个个可渲染的原语（primitive）来组成的，每种原语都由一堆顶点、颜色、法线等属性描述。osg.PrimitiveFunctor的作用就是定义处理这些原语的方法，例如绘制、计算表面积等。

osg.PrimitiveFunctor有两个纯虚函数：

• void apply( unsigned int mode, unsigned int count, const void* indices )：处理一个原语的方法。
• void setVertexArray( const osg::Vec3* vertices, unsigned int vtx_num )：设置顶点数组的方法。

其中，apply是必须实现的方法，用于处理具体的原语；而setVertexArray是可选的，用于在处理原语前设置顶点的数组和数量。

在使用osg.PrimitiveFunctor时一般需要补充定义一个继承自它的类，并实现自己的apply方法。

示例

下面是一个简单示例，绘制一个立方体的各个面：

#include <osg/Geode>
#include <osg/Geometry>
#include <osg/PrimitiveFunctor>

struct MyPrimitiveFunctor : public osg::PrimitiveFunctor
{
    void setVertexArray( const osg::Vec3* vertices, unsigned int vtx_num ) override
    {
        vertex_data = vertices;
        vertex_num = vtx_num;
    }

    void apply( unsigned int mode, unsigned int count, const void* indices ) override
    {
        auto* vertices = new osg::Vec3[count];
        for( unsigned int i = 0; i < count; ++i )
        {
            unsigned int idx = *(reinterpret_cast<const unsigned int*>(indices) + i);
            vertices[i] = vertex_data[idx];
        }
        auto* geometry = new osg::Geometry;
        auto* vertices_array = new osg::Vec3Array( count, vertices );
        geometry->setVertexArray( vertices_array );
        auto* geode = new osg::Geode;
        geode->addDrawable( geometry );
        switch( mode )
        {
            case osg::PrimitiveSet::QUADS:
                geometry->addPrimitiveSet( new osg::DrawArrays( osg::PrimitiveSet::QUADS, 0, count ) );
                break;
            case osg::PrimitiveSet::TRIANGLES:
                geometry->addPrimitiveSet( new osg::DrawArrays( osg::PrimitiveSet::TRIANGLES, 0, count ) );
                break;
            default:
                break;
        }

        osg::ref_ptr<osgDB::Options> options = new osgDB::Options;
        options->setOptionString( "OutputTextureFiles" );
        osgDB::writeNodeFile( *geode, "test.osg", options );
        delete[] vertices;
    }

    const osg::Vec3* vertex_data = nullptr;
    unsigned int vertex_num = 0;
};

int main( int argc, char** argv )
{
    osg::ref_ptr<osg::Vec3Array> vertices = new osg::Vec3Array;
    vertices->push_back( {-0.5,-0.5,0.5} );
    vertices->push_back( {0.5,-0.5,0.5} );
    vertices->push_back( {0.5,0.5,0.5} );
    vertices->push_back( {-0.5,0.5,0.5} );
    vertices->push_back( {-0.5,-0.5,-0.5} );
    vertices->push_back( {0.5,-0.5,-0.5} );
    vertices->push_back( {0.5,0.5,-0.5} );
    vertices->push_back( {-0.5,0.5,-0.5} );

    unsigned int indices_sum[][4] = {
        {0,1,2,3},
        {1,5,6,2},
        {5,4,7,6},
        {4,0,3,7},
        {3,2,6,7},
        {0,4,5,1}
    };
    osg::ref_ptr<osg::DrawElementsUInt> indices[6];
    for( int i = 0; i < 6; ++i )
    {
        unsigned int* data = new unsigned int[4];
        for( int j = 0; j < 4; ++j )
            data[j] = indices_sum[i][j];
        indices[i] = new osg::DrawElementsUInt( osg::PrimitiveSet::QUADS, 4, data );
    }

    osg::ref_ptr<osg::Geometry> geometry = new osg::Geometry;
    geometry->setVertexArray( vertices.get() );
    for( int i = 0; i < 6; ++i )
        geometry->addPrimitiveSet( indices[i].get() );

    MyPrimitiveFunctor functor;
    geometry->accept( functor );

    return 0;
}

输出截图：