[osg-users] Why is setTexCoordArray(0, texcoords) needed if array is modified in-place

[Florian Castellane]

Greetings fellow OSG users :) 

There is a detail in my code that I found disturbing:


Code:

void applyTextureToCenter( MyNameSpace::SpecificNode* floor ) 
{
  MyNameSpace::MyGeode* center = floor->getCenterGeode() ;
  osg::StateSet* stateSet = center->getOrCreateStateSet() ;
  MyNameSpace::MyGeometry* geometry = center -> getGeometry() ;
  const osg::DrawElementsUShort* indices = geometry -> getIndexArray() ;
  const osg::Vec3Array* vtxArray = static_cast<osg::Vec3Array*>( geometry->getVertexArray() ) ;
  osg::Vec2Array* texArray = floor ->getTexCoordArray() ;
  const unsigned numVertex = geometry->getVertexArray()->getNumElements() ;
  const unsigned numIndices = geometry->getIndexArray() ->getNumIndices() ;

  for ( unsigned id = 0 ; id < numIndices ; ++id )
  {
    const unsigned idx = (*indices)[id] ;
    osg::Vec2 uv ;

    /* *snip* 
     * some texture coordinates computation
     */
  
    (*texArray)[idx].set( uv.x(), uv.y() ) ;
  }

  geometry->setTexCoordArray( 0, texArray ) ; //Why is this needed ? 

    /* *snip*
     * Code setting the stateSet parameters (shader program, uniforms...)
     */
  return ;
}




I don't understand why the line 
Code:
geometry->setTexCoordArray( 0, texArray ) ;

 is needed. AFAIK since texArray points to the value given by getTexCoordArray(), we are not actually changing anything...  :? 
I was thinking that maybe it was because setTexCoordArray() caused dirty() to be called, causing re-evaluation of the texture coordinates, but calling

Code:
geometry->getTexCoordArray(0)->dirty() ;


is not enough for my texture to be displayed.

So I went on and looked at the code for setTexCoordArray:

Code:
openscenegraph/OpenSceneGraph/blob/master/src/osg/Geometry.cpp


I see that it is calling 
Code:
geometry->dirtyDisplayList() ;

 but since we are using VBOs (Vertex Buffer Objects) and not display lists that should not do anything.
The code is also calling 
Code:
geometry->addVertexBufferObjectIfRequired()

, which should itself call

Code:
texArray->setVertexBufferObject(geometry->getOrCreateVertexBufferObject());


, which AFAIK should not be changing anything here either, since it is setting the array to use the existing VBOs (which should already be the case).

What is your take on this? There must be something I'm missing.

Thank you!

Best regards,
Florian.

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http://forum.openscenegraph.org/viewtopic.php?p=68035#68035