Texture coordinates for rendering a 3d sphere

问题:

I am drawing a 3d sphere using GL_QUAD_STRIP primitive and it works except I don't know how to set up the texture coordinates.

I have a certain number of division that divide my sphere into equal number of latitudes and longitudes. Hence, the sphere's vertices are approximated using the divisions as follows

   float x, y, z, dTheta=180/divisions, dLon=360/divisions, degToRad=3.14/180 ;

    for(float lat =0 ; lat <=180 ; lat+=dTheta)
    {
        glBegin( GL_QUAD_STRIP ) ;
        for(float lon = 0 ; lon <=360; lon+=dLon)
        {  

            x = r*cosf(lat * degToRad) * sinf(lon * degToRad) ;
            y = r*sinf(lat * degToRad) * sinf(lon * degToRad) ;
            z = r*cosf(lon * degToRad) ;

            glNormal3f( x, y, z) ;
            glVertex3f( x, y, z ) ;

            x = r*cosf((lat + dTheta) * degToRad) * sinf(lon * degToRad) ;
            y = r*sinf((lat + dTheta) * degToRad) * sinf(lon * degToRad) ;
            z = r*cosf( lon * degToRad ) ;

            glNormal3f( x, y, z ) ;
            glVertex3f( x, y, z ) ;
        }
        glEnd() ;

    }

回答1:

If you used standardly distortioned sphere texture (like usual earth map) you should use texture coordinates like (lat/180, lon/360), i.e. normalized values in range [0..1].

回答2:

Add new variables :

float texX = 0.0;
float texy = 0.0;
float dTex = 1/ divisions;

then add then to the loop : float x, y, z, dTheta=180/divisions, dLon=360/divisions, degToRad=3.14/180 ;

for(float lat =0 ; lat <=180 ; lat+=dTheta, texX += dTex)
{
    texY = 0.0;
    glBegin( GL_QUAD_STRIP ) ;
    for(float lon = 0 ; lon <=360; lon+=dLon, texY += dTex )
    {  

        x = r*cosf(lat * degToRad) * sinf(lon * degToRad) ;
        y = r*sinf(lat * degToRad) * sinf(lon * degToRad) ;
        z = r*cosf(lon * degToRad) ;

        glNormal3f( x, y, z) ;
        glTexCoord2f( texX, texY );
        glVertex3f( x, y, z ) ;
    }
    glEnd() ;

}

回答3:

Okay the problem was that I was only drawing 2 vertices and GL_QUAD_STRIP requires 4. The following code now has both vertices set up properly and texture working

   double x, y, z, dTheta=180/divisions, dLon=360/divisions, degToRad=3.141592665885/180 ;

    for(double lat =0; lat <=180; lat+=dTheta)
    {

        glBegin( GL_QUAD_STRIP ) ;
        for(double lon =0 ; lon <=360 ; lon+=dLon)
        {  


            //Vertex 1
            x = r*cos(lon * degToRad) * sin(lat * degToRad) ;
            y = r*sin(lon * degToRad) * sin(lat * degToRad) ;
            z = r*cos(lat * degToRad) ;
            glNormal3d( x, y, z) ;
            glTexCoord2d(lon/360-0.25, lat/180);
            glVertex3d( x, y, z ) ;


            //Vetex 2
            x = r*cos(lon * degToRad) * sin( (lat + dTheta)* degToRad) ;
            y = r*sin(lon * degToRad) * sin((lat + dTheta) * degToRad) ;
            z = r*cos( (lat + dTheta) * degToRad ) ;
            glNormal3d( x, y, z ) ;
            glTexCoord2d(lon/360-0.25, (lat + dTheta-1)/(180)); 
            glVertex3d( x, y, z ) ;


            //Vertex 3
            x = r*cos((lon + dLon) * degToRad) * sin((lat) * degToRad) ;
            y = r*sin((lon + dLon) * degToRad) * sin((lat) * degToRad) ;
            z = r*cos((lat) * degToRad ) ;
            glNormal3d( x, y, z ) ;
           glTexCoord2d((lon + dLon)/(360)-0.25 ,(lat)/180);
             glVertex3d( x, y, z ) ;


            //Vertex 4
            x = r*cos((lon + dLon) * degToRad) * sin((lat + dTheta)* degToRad) ;
            y = r*sin((lon + dLon)* degToRad) * sin((lat + dTheta)* degToRad) ;
            z = r*cos((lat + dTheta)* degToRad ) ;
            glNormal3d( x, y, z ) ;
            glTexCoord2d((lon + dLon)/360-0.25, (lat + dTheta)/(180));
             glVertex3d( x, y, z ) ;


        }
        glEnd() ;

    }

Hope people who get stuck may find this useful.