在OpenSceneGraph中创建一个Sphere（使用osg :: Geometry）

Question

I spent quite some time to get this working, but my Sphere just won't display.
Used the following code to make my function:
Creating a 3D sphere in Opengl using Visual C++

And the rest is simple OSG with osg::Geometry .
(Note: Not ShapeDrawable, as you can't implement custom shapes using that.)
Added the vertices, normals, texcoords into VecArrays.

For one, I suspect something misbehaving, as my saved object is half empty.
Is there a way to convert the existing description into OSG?
Reason? I want to understand how to create objects later on.
Indeed, it is linked with a later assignment, but currently I'm just prepairing beforehand.

Sidenote : Since I have to make it without indices, I left them out.
But my cylinder displays just fine without them.

Answer 1

Caveat: I'm not an OSG expert. But, I did do some research.

OSG requires all of the faces to be defined in counter-clockwise order, so that backface culling can reject faces that are "facing away". The code you're using to generate the sphere does not generate all the faces in counter-clockwise order.

You can approach this a couple ways:

1. Adjust how the code generates the faces, by inserting the faces CCW order.
2. Double up your model and insert each face twice, once with the vertices on each face in their current order and once with the vertices in reverse order.

Option 1 above will limit your total polygon count to what's needed. Option 2 will give you a sphere that's visible from outside the sphere as well as within.

To implement Option 2, you merely need to modify this loop from the code you linked to:

    indices.resize(rings * sectors * 4);
    std::vector<GLushort>::iterator i = indices.begin();
    for(r = 0; r < rings-1; r++) 
        for(s = 0; s < sectors-1; s++) {
            *i++ = r * sectors + s;
            *i++ = r * sectors + (s+1);
            *i++ = (r+1) * sectors + (s+1);
            *i++ = (r+1) * sectors + s;
        }

Double up the set of quads like so:

    indices.resize(rings * sectors * 8);
    std::vector<GLushort>::iterator i = indices.begin();
    for(r = 0; r < rings-1; r++) 
        for(s = 0; s < sectors-1; s++) {
            *i++ = r * sectors + s;
            *i++ = r * sectors + (s+1);
            *i++ = (r+1) * sectors + (s+1);
            *i++ = (r+1) * sectors + s;

            *i++ = (r+1) * sectors + s;
            *i++ = (r+1) * sectors + (s+1);
            *i++ = r * sectors + (s+1);
            *i++ = r * sectors + s;
        }

That really is the "bigger hammer" solution, though.

Personally, I'm having a hard time figuring out why the original loop isn't sufficient; intuiting my way through the geometry, it feels like it's already generating CCW faces, because each successive ring is above the previous, and each successive sector is CCW around the surface of the sphere from the previous. So, the original order itself should be CCW with respect to the face nearest the viewer.

---

EDIT Using the OpenGL code you linked before and the OSG tutorial you linked today, I put together what I think is a correct program to generate the osg::Geometry / osg::Geode for the sphere. I have no way to test the following code, but desk-checking it, it looks correct or at least largely correct.

#include <vector>

class SolidSphere
{
protected:

    osg::Geode      sphereGeode;
    osg::Geometry   sphereGeometry;
    osg::Vec3Array  sphereVertices;
    osg::Vec3Array  sphereNormals;
    osg::Vec2Array  sphereTexCoords;

    std::vector<osg::DrawElementsUInt> spherePrimitiveSets;

public:
    SolidSphere(float radius, unsigned int rings, unsigned int sectors)
    {
        float const R = 1./(float)(rings-1);
        float const S = 1./(float)(sectors-1);
        int r, s;

        sphereGeode.addDrawable( &sphereGeometry );

        // Establish texture coordinates, vertex list, and normals
        for(r = 0; r < rings; r++)
            for(s = 0; s < sectors; s++)
            {
                float const y = sin( -M_PI_2 + M_PI * r * R );
                float const x = cos(2*M_PI * s * S) * sin( M_PI * r * R );
                float const z = sin(2*M_PI * s * S) * sin( M_PI * r * R );

                sphereTexCoords.push_back( osg::Vec2(s*R, r*R) );

                sphereVertices.push_back ( osg::Vec3(x * radius,
                                                     y * radius,
                                                     z * radius) );

                sphereNormals.push_back  ( osg::Vec3(x, y, z) );

            }

        sphereGeometry.setVertexArray  ( &spehreVertices  );
        sphereGeometry.setTexCoordArray( &sphereTexCoords );

        // Generate quads for each face.  
        for(r = 0; r < rings-1; r++)
            for(s = 0; s < sectors-1; s++)
            {
                spherePrimitiveSets.push_back(
                    DrawElementUint( osg::PrimitiveSet::QUADS, 0 )
                );

                osg::DrawElementsUInt& face = spherePrimitiveSets.back();

                // Corners of quads should be in CCW order.
                face.push_back( (r + 0) * sectors + (s + 0) );
                face.push_back( (r + 0) * sectors + (s + 1) );
                face.push_back( (r + 1) * sectors + (s + 1) );
                face.push_back( (r + 1) * sectors + (s + 0) );

                sphereGeometry.addPrimitveSet( &face );
            }
    }

    osg::Geode     *getGeode()     const { return &sphereGeode;     }
    osg::Geometry  *getGeometry()  const { return &sphereGeometry;  }
    osg::Vec3Array *getVertices()  const { return &sphereVertices;  }
    osg::Vec3Array *getNormals()   const { return &sphereNormals;   }
    osg::Vec2Array *getTexCoords() const { return &sphereTexCoords; }

};

You can use the getXXX methods to get the various pieces. I didn't see how to hook the surface normals to anything, but I do store them in a Vec2Array. If you have a use for them, they're computed and stored and waiting to be hooked to something.

Answer 2

That code calls glutSolidSphere() to draw a sphere, but it doesn't make sense to call it if your application is not using GLUT to display a window with 3D context.

There is another way to draw a sphere easily , which is by invoking gluSphere() (you probably have GLU installed):

void gluSphere (GLUquadric* quad , GLdouble radius , GLint slices , GLint stacks );

Parameters

quad - Specifies the quadrics object (created with gluNewQuadric).

radius - Specifies the radius of the sphere.

slices - Specifies the number of subdivisions around the z axis (similar to lines of longitude).

stacks - Specifies the number of subdivisions along the z axis (similar to lines of latitude).

Usage:

// If you also need to include glew.h, do it before glu.h
#include <glu.h>

GLUquadric* _quadratic = gluNewQuadric();
if (_quadratic == NULL)
{
    std::cerr << "!!! Failed gluNewQuadric" << std::endl;
    return;
}

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

glTranslatef(0.0, 0.0, -5.0);
glColor3ub(255, 97, 3);
gluSphere(_quadratic, 1.4f, 64, 64);

glFlush();

gluDeleteQuadric(_quadratic);

It's probably wiser to move the gluNewQuadric() call to the constructor of your class since it needs to be allocated only once, and move the call to gluDeleteQuadric() to the destructor of the class.

Answer 3

@JoeZ's answer is excellent, but the OSG code has some errors/bad practices. Here's the updated code. It's been tested and it shows a very nice sphere.

    osg::ref_ptr<osg::Geode> buildSphere( const double radius,
                                          const unsigned int rings,
                                          const unsigned int sectors )
    {
        osg::ref_ptr<osg::Geode>      sphereGeode = new osg::Geode;
        osg::ref_ptr<osg::Geometry>   sphereGeometry = new osg::Geometry;
        osg::ref_ptr<osg::Vec3Array>  sphereVertices = new osg::Vec3Array;
        osg::ref_ptr<osg::Vec3Array>  sphereNormals = new osg::Vec3Array;
        osg::ref_ptr<osg::Vec2Array>  sphereTexCoords = new osg::Vec2Array;

        float const R = 1. / static_cast<float>( rings - 1 );
        float const S = 1. / static_cast<float>( sectors - 1 );

        sphereGeode->addDrawable( sphereGeometry );

        // Establish texture coordinates, vertex list, and normals
        for( unsigned int r( 0 ); r < rings; ++r ) {
            for( unsigned int s( 0) ; s < sectors; ++s ) {
                float const y = sin( -M_PI_2 + M_PI * r * R );
                float const x = cos( 2 * M_PI * s * S) * sin( M_PI * r * R );
                float const z = sin( 2 * M_PI * s * S) * sin( M_PI * r * R );

                sphereTexCoords->push_back( osg::Vec2( s * R, r * R ) );

                sphereVertices->push_back ( osg::Vec3( x * radius,
                                                       y * radius,
                                                       z * radius) )
                ;
                sphereNormals->push_back  ( osg::Vec3( x, y, z ) );

            }
        }

        sphereGeometry->setVertexArray  ( sphereVertices  );
        sphereGeometry->setTexCoordArray( 0, sphereTexCoords );

        // Generate quads for each face.
        for( unsigned int r( 0 ); r < rings - 1; ++r ) {
            for( unsigned int s( 0 ); s < sectors - 1; ++s ) {

                osg::ref_ptr<osg::DrawElementsUInt> face =
                        new osg::DrawElementsUInt( osg::PrimitiveSet::QUADS,
                                                   4 )
                ;
                // Corners of quads should be in CCW order.
                face->push_back( ( r + 0 ) * sectors + ( s + 0 ) );
                face->push_back( ( r + 0 ) * sectors + ( s + 1 ) );
                face->push_back( ( r + 1 ) * sectors + ( s + 1 ) );
                face->push_back( ( r + 1 ) * sectors + ( s + 0 ) );

                sphereGeometry->addPrimitiveSet( face );
            }
        }

        return sphereGeode;
    }

Changes:

• The OSG elements used in the code now are smart pointers ¹ . Moreover, classes like Geode and Geometry have their destructors protected, so the only way to instantiate them are via dynamic allocation.

• Removed spherePrimitiveSets as it isn't needed in the current version of the code.

• I put the code in a free function, as I don't need a Sphere class in my code. I omitted the getters and the protected attributes. They aren't needed: if you need to access, say, the geometry, you can get it via: sphereGeode->getDrawable(...) . The same goes for the rest of the attributes.

[1] See Rule of thumb #1 here . It's a bit old but the advice maintains.