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Index Buffer help


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#1 greenzone   Members   -  Reputation: 672

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Posted 12 July 2013 - 06:16 AM

So I am still having problems understanding the index buffer completely. There are a few thing related I need cleared up. Such as, how is an index buffer informed. I mean does like 3dsMax or blender generate the index buffer when you export your geometry? Is this index related to the face count and face setup of your geometry. like is the face index and the index buffer the same thing or equal I mean? the reason I ask is because I am trying to cook a mesh with PhysX and Reading on this subject it says this:

  • "Do not duplicate identical vertices. If two triangles are sharing a vertex, this vertex should only occur once in the vertex list, and both triangles should index it in the index list. If you create two copies of the vertex, the collision detection code won't know that it is actually the same vertex, which leads to decreased performance and unreliable results."

After reading this I realize that I need to nail down my understanding of indexing triangles. If any one would help explain any of my questions I would be internally grateful. thanks


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#2 Chananya Freiman   Members   -  Reputation: 140

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Posted 12 July 2013 - 06:41 AM

Suppose you have a 2D rectangle of size [2, 2].

These are the vertices it uses:

[-1, -1]

[-1, 1]

[1, 1]

[1, -1]

 

While we only need four vertices to represent this rectangle, the graphics cards wants triangles, each one having three vertices.

So if we were to split the above rectangle into triangles, we would need to send six coordinates instead of four.

 

Another way to do this, is send only those four vertices, but together with them also tell the graphics card how to form triangles from them.

This is where the index (element in OpenGL) buffer comes in.

The element buffer has numbers that index your vertices.

E.g. 0 would be the first vertex, 1 the second, and so on.

So with an element buffer, to form the triangles needed for the rectangle, we need to send these indices: [0, 1, 2, 0, 2, 3]. If you replace the numbers with the actual vertices they index, you will see you get the original six vertices to form the triangles.

 

Indexing reduces memory and bandwidth (except for very uncommon worst case scenarios), which in turn help rendering speed.

 

Most file formats (to which you export from Blender, 3ds Max, etc.) support indexing, but there are two variants of indexing for file formats.

In modeling tools (and in fact, in your OpenGL code too!), a "vertex" isn't a position, it's a combination of a position, a normal vector, a color, a texture coordinate, and so on.

Every one of these things is called a vertex attribute, and is only one part of the whole vertex.

OpenGL (and Direct3D) only allow 1D indexing, or in other words - you have one index that points to all the vertex attributes.

 

For example, if you have an array of vertex positions and another array of vertex normals, then index 0 would be the first position and the first normal.

 

This might seem obvious, but some file formats don't actually store their data this way.

In most cases, a model doesn't actually need the same amount of positions, normals, and so on.

If there are many vertices that have the same normal, the file might store only one normal, and let them all share it.

You then have different indices for each vertex attribute, which you can't directly use for rendering.

In this case, you will have to "flatten" the arrays and fill them up with all the shared data.

 

This can be seen in the completely obsolete, bad format *.OBJ (it's the most terrible format in existence, but for some reason it's used everywhere).


Edited by Chananya Freiman, 12 July 2013 - 06:43 AM.


#3 marcClintDion   Members   -  Reputation: 431

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Posted 12 July 2013 - 08:10 AM

The object viewer on the following page can handle indices properly. You can easily access them and print them to an array.

 

http://www.dhpoware.com/demos/glObjViewer.html


Consider it pure joy, my brothers and sisters, whenever you face trials of many kinds, because you know that the testing of your faith produces perseverance. Let perseverance finish its work so that you may be mature and complete, not lacking anything.


#4 greenzone   Members   -  Reputation: 672

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Posted 12 July 2013 - 02:51 PM

so with what your saying. say i have a number of vertices and then an index that has been generated properly for efficiency. even though i might not use all the vertices i still include all of them in the VBO?  because with the Index buffer many of those verts will be passed up right because there will be another being used in its place right?


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#5 Chananya Freiman   Members   -  Reputation: 140

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Posted 12 July 2013 - 03:24 PM

You will use all the vertices.

 

The idea is that you don't use the vertices to describe your mesh.

You use indices to describe your mesh.

 

At the end, to actually render a rectangle, you have to give OpenGL two triangles, which are six vertices. So you can either give OpenGL the six vertices up front, or you can give it the four vertices that actually define the rectangle, and tell GL "I gave you four vertices, I want you to make two triangles out of them like this index buffer tells you".

 

When using indices, GL simply grabs the vertices from your vertex buffer. Your first value in the index buffer is 0? ok, let's grab the first value in the vertex buffer. The second value is 1? grab the second value in the vertex buffer. 2? grab the third. Now we grabbed three vertices, which form the first triangle.

Then it continues with indices 0, 2 and 3. Note that we reused two vertices - 0, and 2. Even though we sent them only once, we actually used them twice.






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