View Full Version : occlusion culling causes flickering

04-04-2007, 02:30 PM
this is my (unorganized, unoptimitzed and ugly i know) occlusion culling code. it works fine - except for some frames in which meshes that cover a large portion of the screen seem to flicker.

<vector> m_vpkVisibleGroups contains meshes that passed view frustum culling.
<vector> m_vpkVisibleNotOccludedGroups contains the meshes from the first array that passed occlusion culling.

// generate query id's
unsigned int uiCount = m_vpkVisibleGroups.size();
unsigned int *uiQueryIds = new unsigned int[uiCount];

glGenQueries( uiCount, &amp;uiQueryIds[0] );

for( unsigned int i = 0; i < m_vpkVisibleGroups.size(); i++ )
// get group
Group *pkGroup = m_vpkVisibleGroups[i];

// begin query
glBeginQuery( GL_SAMPLES_PASSED, uiQueryIds[i] );

// render bounding box
pkGroup->GetBoundingBox()->Render( false );

// end query

unsigned int uiResult = 0;

glGetQueryObjectuiv( uiQueryIds[uiCount-1], GL_QUERY_RESULT_AVAILABLE, &amp;uiResult );
while( !uiResult );

for( unsigned int i = 0; i < uiCount; i++ )
// get result
unsigned int uiResult;
glGetQueryObjectuiv( uiQueryIds[i], GL_QUERY_RESULT, &amp;uiResult );

if( uiResult )
m_vpkVisibleNotOccludedGroups.push_back( m_vpkVisibleGroups[i] );

// delete query id's
glDeleteQueries( uiCount, &amp;uiQueryIds[0] );any idea what's wrong? glFlush() didn't help.

04-05-2007, 06:38 AM

the problem is quite easy to explain - you render all the BBs into the depth buffer - so it might appear that one of the BBs (call it A) is completely occluded by another one (B) - BUT - the actual object (B) which lies inside its BB might not occlude the actual object (A)!
Your code will not work in a 100% conservative way of you do it like this, but there are other ways to do it correctly with occlusion queries.