DGL • Thema anzeigen - getting started with VBO (vertex buffer objects)

getting started with VBO (vertex buffer objects)

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noeska

Betreff des Beitrags: getting started with VBO (vertex buffer objects)

Verfasst: Sa Jan 12, 2008 19:12

DGL Member

Registriert: Di Jul 01, 2003 18:59
Beiträge: 887
Wohnort: (The Netherlands)
Programmiersprache: fpc/delphi/java/c#

It is time to go on and use the 'new' ways for rendering namely vertex buffer objects.

So i thougt i start simple an try to render a triangle first, but i must be doing something wrong ...

First i declare 4 vars:

Code:

 
var
    FVBO: TGluInt;
    FVBOPointer: PGlVoid;  
    i: integer;
    VertexPointer: PVertex;
 

Then i initialize the vertex buffer object and fill it with the vertexes making up the triangle.

Code:

 
glGenBuffersARB(1, @FVBO); //create a vertex buffer
glBindBufferARB(GL_ARRAY_BUFFER_ARB, FVBO); //bind the buffer
glEnableClientState(GL_VERTEX_ARRAY); //enable the buffer
 
glBufferDataARB(GL_ARRAY_BUFFER_ARB, 3*SizeOf(TVertex), nil, GL_STATIC_DRAW_ARB); //reserve memory
 
FVBOPointer := glMapBufferARB(GL_ARRAY_BUFFER_ARB, GL_WRITE_ONLY_ARB); //get a pointer to the vbo
 
VertexPointer    := FVBOPointer;
VertexPointer^.X := 1.0;
VertexPointer^.Y := -1.0;
VertexPointer^.Z := 0.0;
inc(Integer(FVBOPointer), SizeOf(TVertex));
 
VertexPointer    := FVBOPointer;
VertexPointer^.X := -1.0;
VertexPointer^.Y := -1.0;
VertexPointer^.Z := 0.0;
inc(Integer(FVBOPointer), SizeOf(TVertex));
 
VertexPointer    := FVBOPointer;
VertexPointer^.X := 0.0;
VertexPointer^.Y := 1.0;
VertexPointer^.Z := 0.0;
inc(Integer(FVBOPointer), SizeOf(TVertex));
 

Now when i want to render it i do:

Code:

 
glInterleavedArrays(GL_V3F, SizeOf(TVertex), nil);
glDrawArrays(GL_TRIANGLES, 0, 3 );

Enlighten me and tell me what i do wrong.
Thanks in advance.

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Lord Horazont

Betreff des Beitrags:

Verfasst: Sa Jan 12, 2008 19:52

DGL Member

Registriert: Do Sep 02, 2004 19:42
Beiträge: 4158
Programmiersprache: FreePascal, C++

You didn't unmap the buffer before rendering with:

Code:

 
// if not bound, bind it first
glUnmapBufferARB(GL_ARRAY_BUFFER_ARB);

And the second parameter of glInterleavedArrays definies the space between one element in the vbo and another. Since your records are lying directly behind each other in the memory as far as i can see, this should be zero.

Also make sure that the client state GL_VERTEX_ARRAY is still activated at the point of rendering.

Greetz Lord Horazont

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noeska

Betreff des Beitrags:

Verfasst: Sa Jan 12, 2008 20:13

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Registriert: Di Jul 01, 2003 18:59
Beiträge: 887
Wohnort: (The Netherlands)
Programmiersprache: fpc/delphi/java/c#

Ok with glUnmapBufferARB subsequent renders work again by the vbo itself is still not drawn.

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Lord Horazont

Betreff des Beitrags:

Verfasst: Sa Jan 12, 2008 20:18

DGL Member

Registriert: Do Sep 02, 2004 19:42
Beiträge: 4158
Programmiersprache: FreePascal, C++

Did you changed the parameter of glInterleavedArrays also?

Does OpenGL returns any error with glGetError?

Greetz Lord Horazont

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noeska

Betreff des Beitrags:

Verfasst: Sa Jan 12, 2008 20:26

DGL Member

Registriert: Di Jul 01, 2003 18:59
Beiträge: 887
Wohnort: (The Netherlands)
Programmiersprache: fpc/delphi/java/c#

Yes i changed the second parameter for glInterleavedArrays also. Same result nothing...
Now let's see what glGetError can tell me...

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noeska

Betreff des Beitrags:

Verfasst: Sa Jan 12, 2008 21:45

DGL Member

Registriert: Di Jul 01, 2003 18:59
Beiträge: 887
Wohnort: (The Netherlands)
Programmiersprache: fpc/delphi/java/c#

To start with: my vertexes were wrong to begin with. It would not even render properly the old way. :oops:

Also the filling part needed to be changed to:

Code:

 
    TVertex(FVBOPointer^).X := -1.0; 
    TVertex(FVBOPointer^).Y := 1.0; 
    TVertex(FVBOPointer^).Z := 0.0; 
    inc(Integer(FVBOPointer), SizeOf(TVertex)); 
 
 
    TVertex(FVBOPointer^).X := -1.0; 
    TVertex(FVBOPointer^).Y := -1.0; 
    TVertex(FVBOPointer^).Z := 0.0; 
    inc(Integer(FVBOPointer), SizeOf(TVertex)); 
 
 
    TVertex(FVBOPointer^).X := 1.0; 
    TVertex(FVBOPointer^).Y := -1.0; 
    TVertex(FVBOPointer^).Z := 0.0; 
    inc(Integer(FVBOPointer), SizeOf(TVertex)); 
 

Now i see the triangle .

GlInterleafedArrays could stay like it originaly was.

To be compete here is the type for PVertex i forgot to post earlier:

Code:

type 
  PVertex = ^TVertex; 
  TVertex = packed record 
    X,Y,Z : TGLFloat; 
  end; 

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noeska

Betreff des Beitrags:

Verfasst: So Jan 13, 2008 16:49

DGL Member

Registriert: Di Jul 01, 2003 18:59
Beiträge: 887
Wohnort: (The Netherlands)
Programmiersprache: fpc/delphi/java/c#

Now larger meshes also work.
Now i read that there is also a thing called buffers with indexed arrays. If i understand it correctly i do not need to feed all the vertexes even double but only the unique ones and pass indices to them. So like i do in gld3ds where i have a list of vertexes and a list containing indices to them.

so i need two vbo's one for the vertexes and one for the indices.

but how are these combined? Simple example anyone?

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noeska

Betreff des Beitrags:

Verfasst: So Jan 13, 2008 17:46

DGL Member

Registriert: Di Jul 01, 2003 18:59
Beiträge: 887
Wohnort: (The Netherlands)
Programmiersprache: fpc/delphi/java/c#

Figured it out:

Code:

procedure TRender.CreateIndexedVertexBufferObject;
var
  i: integer;
begin
  glEnableClientState(GL_VERTEX_ARRAY); //enable the buffer
  glGenBuffersARB(1, @FVBO); //create a vertex buffer
  glBindBufferARB(GL_ARRAY_BUFFER_ARB, FVBO); //bind the buffer
  glBufferDataARB(GL_ARRAY_BUFFER_ARB, (FModels[0].Mesh[0].NumVertex)*SizeOf(TVertex), nil, GL_STATIC_DRAW_ARB); //reserve memory
  FVBOPointer := glMapBufferARB(GL_ARRAY_BUFFER_ARB, GL_WRITE_ONLY_ARB); //get a pointer to the vbo
 
  //copy the vertex data into the vbo buffer
  for i := 0 to (FModels[0].Mesh[0].NumVertex )-1 do
  begin
    FVBOPointer^.X := FModels[0].Mesh[0].Vertex[i].x;
    FVBOPointer^.Y := FModels[0].Mesh[0].Vertex[i].y;
    FVBOPointer^.Z := FModels[0].Mesh[0].Vertex[i].z;
    if i < (FModels[0].Mesh[0].NumFaces)-1 then
      inc(Cardinal(FVBOPointer), SizeOf(TVertex));
  end;
 
  glUnMapBufferARB(GL_ARRAY_BUFFER_ARB); //after filling unmap the filled buffer
 
  //now create indices buffer
  glGenBuffersARB(1, @FIVBO);
    glBindBufferARB(GL_ELEMENT_ARRAY_BUFFER_ARB, FIVBO);
    glBufferDataARB(GL_ELEMENT_ARRAY_BUFFER_ARB, (FModels[0].Mesh[0].NumFaces)*sizeof(GLushort), nil, GL_STREAM_DRAW_ARB); //allocate memory on board
 
    FIVBOPointer := glMapBufferARB(GL_ELEMENT_ARRAY_BUFFER_ARB, GL_WRITE_ONLY_ARB); //get pointer to memory on board
    for i:= 0 to (FModels[0].Mesh[0].NumFaces)-1 do
  begin
    FIVBOPointer^ := FModels[0].Mesh[0].Face[i]; //upload indices
    inc(Cardinal(FIVBOPointer), SizeOf(TGluShort));
  end;
 
    glUnmapBufferARB(GL_ELEMENT_ARRAY_BUFFER_ARB);
end;

and i render this with:

Code:

glVertexPointer(3, GL_FLOAT, 0, nil);
  glDrawElements(GL_TRIANGLES, FModels[0].Mesh[0].NumFaces, GL_UNSIGNED_SHORT, 0);

Now is there a best way?
Should i go for indexed or non-indexed?
Does that matter for vertex and or fragment programs?
Should i have used the second render method even for my first example?

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TAK2004

Betreff des Beitrags:

Verfasst: Mo Jan 14, 2008 09:55

DGL Member

Registriert: Di Mai 18, 2004 16:45
Beiträge: 2622
Wohnort: Berlin
Programmiersprache: Go, C/C++

The best way is the second one with index array because smaller memory use and you can sort the tris very easy for vertex cache.
You can use Batches this mean you set a maximum cap for the vertex count.
Good values are 200-20k(r9800-x1950) with newer cards the top cap go up.
The most power come from vertex cache, high usage of cache mean higher FPS.
Test for the optimal balance between batchsize and number of batches.

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