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OpenGL Vertex Declarations in OpenGL and DirectX ?

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xelanoimis    172
Hi, What is the OpenGL echivalent for DirectX9 vertex declarations ? I know that OpenGL can use vertex arrays that are echivalent for vertexbuffers. But how to set vertex declarations, and can it use multiple streams (vertex arrays) ? I want to know this for use with CG effects or vertex programs available for OpenGL. Can anyone give a small code example (not necessarly functional, just curios about the functions used). I am also interested in a platform independent approach to meshes and effects. Can vertex shaders and vertex declarations be wraped in some engine classes to use with both OpenGL and DirectX effects (eventualy CG) ? Thanks

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jollyjeffers    1570
Moved to OpenGL

For the first part of your question you're more likely to get a better answer from the OpenGL experts - they're the guys who know the API best [wink]

AS for an API/platform independent method, yes it's probably possible - but you can't really take it any further until you know what form both API's can use (or require). From there you can either decide whether you want to take a subset approach (basic functionality that both API's can handle) or some higher-level approach that involves work-arounds/emulation for parts of the respective API's that don't natively handle what you want. For example, if you really want streams, but OpenGL can't do that you could (presumably) emulate it by creating your own Input Assembler - not necessarily efficient, but it'd probably work.


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Alex Baker    172

i can't follow your explanation; so, plesae tell me, what do you mean exactly by the term "subset approach" ?

(do you mean splitting up the rendering-work into more/simple subsets by switching only the vertex-dec.format ?)

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Promit    13246
What it comes down to in OpenGL is that you get to define vertex declarations yourself, and then you get to write the rendering backend code that translates a vertex declaration into BindBuffer and Pointer calls.

It's fairly simple to do. You define an array of structures that have all of the information about the vertex declaration -- what its usage is, what type it is, offset and stream index, etc. Then you loop through that array, calling BindBuffer and pointer as necessary.

I basically have a header that defines stuff for a declaration:

#ifndef _GEOMETRY_H
#define _GEOMETRY_H

#include "../HoverEngine.h"

#include "RendererTypes.h"
#include "VertexBuffer.h"
#include "IndexBuffer.h"

#include <vector>

//type of element, i.e. what it specifies
enum VertexElementType

//'format' of element, basically the data type
//it's recommended that you avoid the INT64 types, as well as long double
enum VertexElementFormat





//FVF quick-system
#define FVF_XYZ 0x00000001
#define FVF_NORMAL 0x00000002
#define FVF_DIFFUSE 0x00000004
#define FVF_SPECULAR 0x00000008

#define FVF_TEXTURE0 0x00010000
#define FVF_TEXTURE1 0x00020000
#define FVF_TEXTURE2 0x00040000
#define FVF_TEXTURE3 0x00080000
#define FVF_TEXTURE(X) (0x00010000 << (X))

//specifies a single part of a vertex, e.g. one position, or one texcoord, etc.
struct VertexElement
unsigned int Stream;
unsigned int Count;
std::size_t Offset; //offset in the structure
VertexElementFormat Format;
VertexElementType Type;

//normal ctor
VertexElement() : Stream( 0 ), Count( 0 ), Format( VEF_FLOAT ),
Type( VET_XYZ ), Offset( 0 )
{ }

//inline ctor for laziness
VertexElement( unsigned int vStream, unsigned int vCount, std::size_t vOffset,
VertexElementFormat vFormat, VertexElementType vType )
: Stream( vStream ), Count( vCount ), Format( vFormat), Type( vType ),
Offset( vOffset )
{ }

static std::size_t FormatSize( VertexElementFormat vef );

//Compute the size of this element in bytes
std::size_t SizeBytes() { return FormatSize( Format ) * Count; }

//specifies a complete vertex, basically just a list of elements
struct VertexDeclaration
typedef std::vector<VertexElement> ElementList;
typedef ElementList::iterator Iterator;

ElementList Elements;

Elements.reserve( 4 );

static VertexDeclaration CreateFromFVF( unsigned int FVF );

//in D3D, streams will correspond to real streams
//in OGL, the streams will be somewhat virtualised, but effectively the same
//holds all the data for a single stream
struct StreamSource
VertexBuffer* Source;
std::size_t Offset;
std::size_t Stride;

StreamSource() : Source( NULL ), Offset( 0 ), Stride( 0 )
{ }

//holds everything geometric about an object
class Geometry
VertexDeclaration m_Decl;

RenderMode m_Mode;
std::size_t m_PrimitiveCount;
std::size_t m_IndexCount;

StreamSource m_Streams[MAX_VERTEX_STREAMS];
IndexBuffer* m_Indices; //if IB is NULL, use non-indexed primitive
std::size_t m_IndexOffset;

std::size_t m_FirstVertex;
std::size_t m_NumVertices;

virtual ~Geometry();

//Sets the rendering mode and primitive count for this geom
void SetRenderMode( RenderMode Mode, std::size_t IndexCount );

//Sets the vertex buffer for the specified stream
void SetStreamSource( unsigned int Stream, VertexBuffer* Source, std::size_t Offset, std::size_t Stride );
//Sets the indices for this geom (NULL for no indexing -- Offset is used for VB)
void SetIndices( IndexBuffer* Indices, std::size_t Offset );
//Sets the range of vertex used by the indices (ignored if not using indices)
void SetRange( std::size_t First, std::size_t Count );

//used to access the vertex declaration
VertexDeclaration& Decl() { return m_Decl; }
//access the streams
StreamSource* Stream( unsigned int Idx ) { assert( Idx < MAX_VERTEX_STREAMS ); return &m_Streams[Idx]; }
//access the indices
IndexBuffer* Indices() { return m_Indices; }
//index buffer offset
std::size_t IndexOffset() const { return m_IndexOffset; }
//Primitive count
std::size_t PrimitiveCount() const { return m_PrimitiveCount; }
std::size_t IndexCount() const { return m_IndexCount; }
//render mode for this geom
RenderMode Mode() const { return m_Mode; }
//Get the index range
void GetRange( std::size_t& First, std::size_t& Count ) const { First = m_FirstVertex; Count = m_NumVertices; }



And then the function to parse this into OpenGL goes like this:
void OGLRenderer::BeginRender( Geometry* Geom )
if( m_RenderBegun )
if( Geom == NULL )

//keeps track of tex coord sets in use
unsigned int TexCoord = 0;

//first, get the Geom's declaration and set stuff up
VertexDeclaration::Iterator it = Geom->Decl().Elements.begin();
while( it != Geom->Decl().Elements.end() )
if( m_CurrentVB != Geom->Stream( it->Stream )->Source )
m_CurrentVB = down_cast<OGLVertexBuffer*>( Geom->Stream( it->Stream )->Source );
if( m_CurrentVB != NULL )
else if( GLEE_ARB_vertex_buffer_object )

//initialize the pointers for this element
GLsizei Stride = (GLsizei) Geom->Stream( it->Stream )->Stride;
GLsizei StreamOffset = (GLsizei) Geom->Stream( it->Stream)->Offset;
switch( it->Type )
case VET_XYZ:
glEnableClientState( GL_VERTEX_ARRAY );
glVertexPointer( it->Count, TranslateVertexFormat( it->Format ), Stride, m_CurrentVB->GetPointer() + it->Offset + StreamOffset );
glEnableClientState( GL_NORMAL_ARRAY );
glNormalPointer( TranslateVertexFormat( it->Format ), Stride, m_CurrentVB->GetPointer() + it->Offset + StreamOffset );
glEnableClientState( GL_COLOR_ARRAY );
glColorPointer( it->Count, TranslateVertexFormat( it->Format ), Stride, m_CurrentVB->GetPointer() + it->Offset + StreamOffset );
glEnableClientState( GL_SECONDARY_COLOR_ARRAY );
glSecondaryColorPointer( it->Count, TranslateVertexFormat( it->Format ), Stride, m_CurrentVB->GetPointer() + it->Offset + StreamOffset );
glClientActiveTexture( GL_TEXTURE0 + TexCoord );
glEnableClientState( GL_TEXTURE_COORD_ARRAY );
glTexCoordPointer( it->Count, TranslateVertexFormat( it->Format ), Stride, m_CurrentVB->GetPointer() + it->Offset + StreamOffset );


//set up indices if we have any
if( m_Indices != Geom->Indices() )
m_Indices = down_cast<OGLIndexBuffer*>( Geom->Indices() );
if( m_Indices != NULL )
else if( GLEE_ARB_vertex_buffer_object )

m_RenderBegun = true;
m_CurGeom = Geom;

(Note: This appears to be an older version of my source. The newer version has some modifications that allow arbitrary VertexElementTypes, which is useful for shader stuff.)

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