view frustum culling and determination

[frogger]

How can one determine if something is in the viewing volume (I am using a perspective not ortho view)?

[Gabriel]

I dont know but I would be relly interested to know… What is the process openGL uses to achive vertex culling?

[onyXMaster]

OpenGL culling is not the thing you can control, sometimes you just can disable it ( GL_EXT_clip_volume_hint extension ).
Though, if you need to determin if a set of vertices lies within your frustum, there are two ways. One - mirror transforms with your own matrices, or fetch OpenGL ones, transform and project vertices, etc. Two - use feedback buffer, it may be a little tricky but it worth it. One good thing, that second way utilizes T&L for gross visibility determination, and is especially effective if you use bounding boxes. Details by mail … ( I am rare visitor )

[onyXMaster]

Sorry about pointing you to Gamasutra in email

Look at these two cuts from my sources
should be pretty straitforward

D:\Dev\XIE\Engine\Src\Math\Geometry.h:
// Enums
// Vector list visibility ( return codes of Geometry_IsVisible ( ) )
enum
{
Geometry_Visibility_False, // Completely invisible ( should be zero for easy checks like “if ( ! Geometry_IsVisible ( … ) )” )
Geometry_Visibility_True, // Visible ( partially or fully )
Geometry_Visibility_Full // Completely visible
} Geometry_Visibility_e;

// Typedefs
// Bounding box typedef
typedef struct
{
Vector_t Min, Max; // Min and max coords
} Geometry_Box_t;

D:\Dev\XIE\Engine\Src\Math\Geometry.c:
// Check list of vectors to be visible using modelview and projection
// Generally computationally intensive because it uses OpenGL transformations and feedback buffer
// It is effective only with hardware transformations - so it should not be used for large vertex arrays, but bounding box test will do ok
// Returns one of the Geometry_Visibility_e members
int Geometry_IsVisibleBox ( Geometry_Box_t * Box )
{
int Return;
bool CullFace;
#ifdef Common_DebugMode
uint i;
#endif
// Some assertions
Debug_Assert ( Box );
// Setup buffer
glFeedbackBuffer ( 24, GL_2D, Buffer );
// Put OpenGL in feedback mode
glRenderMode ( GL_FEEDBACK );
// Feed vertices for fast point test
glBegin ( GL_POINTS );
glVertex3fv ( ( float * ) & Box -> Max );
glVertex3fv ( ( float * ) & Box -> Min );
glVertex3f ( Box -> Min.X, Box -> Max.Y, Box -> Min.Z );
glVertex3f ( Box -> Max.X, Box -> Max.Y, Box -> Min.Z );
glVertex3f ( Box -> Max.X, Box -> Min.Y, Box -> Min.Z );
glVertex3f ( Box -> Min.X, Box -> Min.Y, Box -> Max.Z );
glVertex3f ( Box -> Min.X, Box -> Max.Y, Box -> Max.Z );
glVertex3f ( Box -> Max.X, Box -> Min.Y, Box -> Max.Z );
glEnd ( );
Return = glRenderMode ( GL_RENDER );
#ifdef Common_DebugMode
// Paranoid checks ( it is rumored that some cards may insert color tokens for each vertex with point rendering … stupidity )
if ( Return < 0 )
Debug_Error ( “Geometry_IsVisible - invalid feedback buffer value count” );
for ( i = 0 ; i < ( size_t ) Return ; i += 3 )
if ( Buffer [ i ] != GL_POINT_TOKEN )
Debug_Error ( “Geometry_IsVisible - invalid feedback buffer contents” );
#endif
// All points inside - fully visible
if ( Return == 24 )
return Geometry_Visibility_Full;
else
// Some points inside - partially visible
if ( Return )
return Geometry_Visibility_True;
// Do a more complete test now for point test can fail if box bounds clipping faces in some plane
// Setup buffer
glFeedbackBuffer ( 1, GL_2D, Buffer );
// Put OpenGL in feedback mode
glRenderMode ( GL_FEEDBACK );
// Disable face culling, storing old mode
if ( CullFace = glIsEnabled ( GL_CULL_FACE ) )
glDisable ( GL_CULL_FACE );
// Feed vertices
glBegin ( GL_TRIANGLES );
glVertex3fv ( ( float * ) & Box -> Min );
glVertex3f ( Box -> Min.X, Box -> Max.Y, Box -> Min.Z );
glVertex3f ( Box -> Max.X, Box -> Max.Y, Box -> Min.Z );

glVertex3fv ( ( float * ) & Box -> Min );
glVertex3f ( Box -> Max.X, Box -> Max.Y, Box -> Min.Z );
glVertex3f ( Box -> Max.X, Box -> Min.Y, Box -> Min.Z );

glVertex3f ( Box -> Min.X, Box -> Min.Y, Box -> Max.Z );
glVertex3fv ( ( float * ) & Box -> Max );
glVertex3f ( Box -> Min.X, Box -> Max.Y, Box -> Max.Z );

glVertex3f ( Box -> Min.X, Box -> Min.Y, Box -> Max.Z );
glVertex3f ( Box -> Max.X, Box -> Min.Y, Box -> Max.Z );
glVertex3fv ( ( float * ) & Box -> Max );

glVertex3fv ( ( float * ) & Box -> Min );
glVertex3f ( Box -> Max.X, Box -> Min.Y, Box -> Max.Z );
glVertex3f ( Box -> Min.X, Box -> Min.Y, Box -> Max.Z );

glVertex3fv ( ( float * ) & Box -> Min );
glVertex3f ( Box -> Max.X, Box -> Min.Y, Box -> Min.Z );
glVertex3f ( Box -> Max.X, Box -> Min.Y, Box -> Max.Z );

glVertex3f ( Box -> Min.X, Box -> Max.Y, Box -> Min.Z );
glVertex3f ( Box -> Min.X, Box -> Max.Y, Box -> Max.Z );
glVertex3fv ( ( float * ) & Box -> Max );

glVertex3f ( Box -> Min.X, Box -> Max.Y, Box -> Min.Z );
glVertex3fv ( ( float * ) & Box -> Max );
glVertex3f ( Box -> Max.X, Box -> Max.Y, Box -> Min.Z );

glVertex3fv ( ( float * ) & Box -> Min );
glVertex3f ( Box -> Min.X, Box -> Min.Y, Box -> Max.Z );
glVertex3f ( Box -> Min.X, Box -> Max.Y, Box -> Max.Z );

glVertex3fv ( ( float * ) & Box -> Min );
glVertex3f ( Box -> Min.X, Box -> Max.Y, Box -> Max.Z );
glVertex3f ( Box -> Min.X, Box -> Max.Y, Box -> Min.Z );

glVertex3f ( Box -> Max.X, Box -> Min.Y, Box -> Min.Z );
glVertex3fv ( ( float * ) & Box -> Max );
glVertex3f ( Box -> Max.X, Box -> Min.Y, Box -> Max.Z );

glVertex3f ( Box -> Max.X, Box -> Min.Y, Box -> Min.Z );
glVertex3f ( Box -> Max.X, Box -> Max.Y, Box -> Min.Z );
glVertex3fv ( ( float * ) & Box -> Max );
glEnd ( );
if ( CullFace )
	glEnable ( GL_CULL_FACE );

// Get back, counting values stored
// If buffer is empty - box is invisible, elseway it is partially visible
return glRenderMode ( GL_RENDER ) ? Geometry_Visibility_True : Geometry_Visibility_False;
}

[This message has been edited by onyXMaster (edited 06-07-2000).]