CODEWITHSUNDEEP
openglopengl-3
Lexi
Lexi

Reputation: 1730

Using more than one index list in a single VAO

I'm probably going about this all wrong, but hey.

I am rendering a large number of wall segments (for argument's sake, let's say 200). Every segment is one unit high, even and straight with no diagonals. All changes in direction are a change of 90 degrees.
I am representing each one as a four pointed triangle fan, AKA a quad. Each vertex has a three dimensional texture coordinate associated with it, such as 0,0,0, 0,1,7 or 10,1,129.

This all works fine, but I can't help but think it could be so much better. For instance, every point is duplicated at least twice (Every wall is a contiguous line of segments and there are some three & four way intersections) and the starting corner texture coordinates (0,0,X and 0,1,X) are going to be duplicated for every wall with texture number X on it. This could be compressed even further by moving the O coordinate into a third attribute and indexing the S and T coordinates separately.

The problem is, I can't seem to work out how to do this. VAOs only seem to allow one index, and taken as a lump, each position and texture coordinate form a unique snowflake never to be repeated. (Admittedly, this may not be true on certain corners, but that's a very edge case)

Is what I want to do possible, or am I going to have to stick with the (admittedly fine) method I currently use?

Upvotes: 2

Views: 365

Answers (2)

datenwolf
datenwolf

Reputation: 162317

each vertex and texture coordinate form a unique snowflake never to be repeated

A vertex is not just position, but the whole vector formed by position, texture coordinates and all the other attributes. It is, what you referred to as "snowflake".

And for only 200 walls, I'd not bother about memory consumption. It comes down to cache efficiency. And GPUs use vertices – and that means the whole position and other attributes vector – as caching key. Any "optimization" like you want to do them will probably have a negative effect on performance.

But having some duplicated vertices doesn't hurt so much, if they're not too far apart in the primitive index list. Today GPUs can hold about between 30 to 1000 vertices (that is after transformation, i.e. the shader stage) in their cache, depending on the number of vertex attributes are fed and the number of varying variables delivered to the fragment shader stage. So if a vertex (input key) has been cached, the shader won't be executed, but the cached result fed to fragment processing directly.

So the optimization you should really aim for is cache locality, i.e. batching things up in a way, that shared/duplicated vertices are sent to the GPU in quick succession.

Upvotes: 2

Nicol Bolas
Nicol Bolas

Reputation: 474376

It depends on how much work you want to do.

OpenGL does not directly allow you to use multiple indices. But you can get the same effect.

The most direct way is to use a Buffer Texture to access an index list (using gl_VertexID), which you then use to access a second Buffer Texture containing your positions or texture coordinates. Basically, you'd be manually doing what OpenGL automatically does. This will naturally be slower per-vertex, as attributes are designed to be fast to access. You also lose some of the compression features, as Buffer Textures don't support as many formats.

Upvotes: 2

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