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BSchoen

OpenGL Vertex Buffer Object (VBO)

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Hi! I am using OpenGL > C# > Tao, for drawing large datasets. I managed to draw all data using display lists, but I want more flexibility, because I have to be able to show/hide certain points. I fill a 2D array with MyVertex, which is a struct containing x, y and z coordinates. When drawing this it only shows one vertex (Gl_POINTS) at the centre of the screen. Help appreciated. * I have tried this, using one big array containing vertices data. Where every coordinate is at its own index. So MyVertex vertices = new MyVertex[nrChannel * DRAWDATASIZE * 3]. This gave me the same results.
int vboId;
MyVertex[,] vertices;

struct MyVertex
{
    public float x, y, z;
};

...


I am filling the buffer with vertices here:
vertices = new MyVertex[nrChannels, DRAWDATASIZE]; //nrChannels = 10, DRAWDATASIZE = 100

for (Byte channel = 0; channel < nrChannels; channel++)
{
    for (int position = 0; position < DRAWDATASIZE; position++)
    {
        vertices[channel, position].x = position;
        vertices[channel, position].y = depth; // Depth has been initialized, but not been included in the code snippet
        vertices[channel, position].z = 0;
    }
}

// generate a new VBO and get the associated ID
Gl.glGenBuffers(1, out vboId);

// bind VBO in order to use
Gl.glBindBuffer(Gl.GL_ARRAY_BUFFER, vboId);

// upload data to VBO
Gl.glBufferData(Gl.GL_ARRAY_BUFFER, (IntPtr)(sizeof(float) * 3 * vertices.Length), vertices, Gl.GL_STATIC_DRAW);



Then Drawing the scene:
...

Gl.glEnableClientState(Gl.GL_VERTEX_ARRAY);
                
Gl.glBindBuffer(Gl.GL_ARRAY_BUFFER, vboId);
Gl.glVertexPointer(3, Gl.GL_FLOAT, 0, vertices);
Gl.glDrawArrays(Gl.GL_POINTS, 0, vertices.Length);

...


What am I doing wrong? Regards, Bas

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Although you are completely right, it doesn't fix the problem. I think I found a solution on OpenTK. By using Stride, defined in the MyVertex struct, instead of using (IntPtr)(sizeof(float) * 3 * vertices.Length).

I am able to draw the complete VBO. But it still doesn't give my the flexibility to hide/show certain point. I basically have an 2D array with [channel, vertexdata]. So every channel contains some vertices, which I want to draw. Is it possible to just draw one channel, or some specific channels? I can be using the following for loop, this works when every channel contains the same amount of data. But my channels don't.


for (int i = 0; i < nrChannels; i++)
{
if(somecondition)
Gl.glDrawArrays(Gl.GL_POINTS, i * DRAWDATASIZE, vertices.GetLength(1));
}


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A channel is basically a property of my data points. So every data point has a position (x,y,z) and has got some other properties, e.g. channel.

I want to be able to show or hide points belonging to the same channel. But is not really important what a channel is. My question should perhaps be asked more generally: Can I use multiple Vertex Buffer Objects (i.e. One per channel). And how am I able to draw vertices belonging to one VBO?

Do I just have to bind the buffer for each VBO, with a different index?

Like: Gl.glBindBuffer(Gl.GL_ARRAY_BUFFER, vboId[ChannelNumber]);

Regards,

Bas

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Sure, you can use multiple vertex buffer objects. You can use a single vertex buffer and multiple index buffers as well. The indices in the index buffers will reference the data in the vertex buffer. The draw call itself will be then glDrawElements instead of glDrawArrays.

You have to enable each vertex attribute separately. Have you called glEnableVertexAttribArray(0)?

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Thanks, V-Man and Decibit. I think I've got it right.

Just to make sure:

I only have to call glEnableVertexAttribArray(0) or glEnableClient when I am using index buffers, right? I am not using index buffers, so I should be fine by only calling Gl.glEnableClientState(Gl.GL_VERTEX_ARRAY);?

Regards,

Bas

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Quote:
Original post by BSchoen
Thanks, V-Man and Decibit. I think I've got it right.

Just to make sure:

I only have to call glEnableVertexAttribArray(0) or glEnableClient when I am using index buffers, right? I am not using index buffers, so I should be fine by only calling Gl.glEnableClientState(Gl.GL_VERTEX_ARRAY);?

Regards,

Bas

Not quite. The both functions glEnableVertexAttribArray and glEnableClientState do the same. glEnableVertexAttribArray is however more general and treats all the vertex attributes (positions, normals, etc.) same. Moreover glEnableClientState may be deprecated in some of the newest OpenGL profiles.

Still you have to call at least one of them if you are using vertex buffers.

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Since he is using glVertexPointer, he must use glEnableClientState.
If you use glVertexAttribPointer, then you call glEnableVertexAttribArray.
These things go in pairs.

If glEnableClientState is deprecated, then so is glVertexPointer. You are probably thinking of GL 3.0

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Quote:
Original post by V-man
Since he is using glVertexPointer, he must use glEnableClientState.
If you use glVertexAttribPointer, then you call glEnableVertexAttribArray.
These things go in pairs.

If glEnableClientState is deprecated, then so is glVertexPointer. You are probably thinking of GL 3.0

That's correct.

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Thanks!

I get it. So it seems like I am using deprecated functions. Are there any disadvantages by doing this. Everything seems to be working perfect.

If I use glVertexAttribPointer and glEnableVertexAttribArray does that mean it will not run on Hardware using lower than GL 3.0? Or is it backwards-compatible?

Regards

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Quote:
Original post by BSchoen
If I use glVertexAttribPointer and glEnableVertexAttribArray does that mean it will not run on Hardware using lower than GL 3.0? Or is it backwards-compatible?

You don't have to worry about it. Everything will work fine as the main hardware developers promise that they will never remove any of the old features: link.

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Decibit, I think you might have gotten the question backwards, glVertexAttrib is the new feature, not the depreciated one.

Quote:

If I use glVertexAttribPointer and glEnableVertexAttribArray does that mean it will not run on Hardware using lower than GL 3.0? Or is it backwards-compatible?


These functions are included in OpenGL 2.0 and above, and were probably extensions on cards before that. I think these should be OK on any cards from the last 5-7 years, maybe more.

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Thanks karwosts, appreciate your reply. Then I'll think about rewriting that bit of code. Just to keep up-to-date :)

And Decibit, thanks anyway. You indeed misunderstood the question, or just messed up the mapping of the OpenGL version compared to the function names. But you've been a great help in this topic. Thanks!

Regards,

Bas

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Quote:
Original post by karwosts
Decibit, I think you might have gotten the question backwards, glVertexAttrib is the new feature, not the depreciated one.

I didn't mean any particular function just the fact that the complete set of functions will be there for a long long time. You are certainly right about glVertexAttrib.

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