Monday, August 10, 2015

Luminance – Vertex Arrays

I’ve been up working on vertex arrays in my work-in-progress graphics framework, luminance, for several days. I’m a bit slow, because I’ve been through a very hard breakup and have been struggling to recover and focus. But here I am!

So, what’s new?

OpenGL allows programmers to send vertices to the GPU through what is called a vertex array. Vertex specification is performed through several functions, operating on several objects. You need, for instance, a vertex buffer object, an index buffer object and a vertex array object. The vertex buffer stores the vertices data.



For instance, you could imagine a teapot as a set of vertices. Those vertices have several attributes. We could use, for instance, a position, a normal and a bone index. The vertex buffer would be responsible of storing those positions, normals and bone indices. There’re two ways to store them:

  1. interleaved arrays ;
  2. deinterleaved arrays.

I’ll explain those later on. The index buffer stores integral numbers – mainly set to unsigned int – that index the vertices, so that we can connect them and create lines, triangles or more complex shapes.

Finally, the vertex array object is a state object that stores links to the two buffers and makes a connection between pointers in the buffer and attribute indices. Once everything is set up, we might only use the vertex array object. The exception is when we need to change the geometry of an object. We need to access the vertex buffer and the index buffer and upload new data. However, for now, that feature is disabled so that the buffers are not exposed to the programmer. If people think that feature should be implemented, I’ll create specialized code for that very purpose.

Interleaved and deinterleaved arrays

Interleaved arrays might be the most simple to picture, because you use such arrays every day when programming. Let’s imagine you have the following type in Haskell:

data Vertex = Vertex {
    vertPos    :: X
  , vertNor    :: Y
  , vertBoneID :: Z
  } deriving (Eq,Show)

Now, the teapot would have several vertices. Approximately, let’s state the teapot has five vertices – yeah, ugly teapot. We can represent such vertices in an interleaved array by simply recording them in a list or an array:



As you can see, the attributes are interleaved in memory, and the whole pattern is cycling. That’s the common way to represent an array of struct in a lot of languages, and it’s very natural for a machine to do things like that.

The deinterleaved version is:



As you can see, with deinterleaved arrays, all attributes are extracted and grouped. If you want to access the third vertex, you need to read the third X, the third Y and the third Z.

Both the methods have advantages and drawbacks. The cool thing about deinterleaved arrays is that we can copy huge regions of typed memory at once whilst we cannot with interleaved arrays. However, interleaved arrays store continuous structures, so writing and reading a structure back might be faster.

An important point to keep in mind: because we plan to pass those arrays to OpenGL, there’s no alignment restriction on the structure. That is, everything is packed, and we’ll have to pass extra information to OpenGL to tell it how to advance in memory to correctly build vertices back.

Generalized tuple

I think I haven’t told you yet. I have a cool type in luminance: the (:.) type. No, you don’t have to know how to pronounce that. I like to call it the gtuple type, because it’s a generalized tuple. You can encode (a,b), (a,b,c) and all kind of tuples with (:.). You can even encode single-typed infinite tuple! – a very special kind of list, indeed.

data a :. b = a :. b

infixr 6 :.

-- a :. b is isomorphic to (a,b)
-- a :. b :. c is isomorphic to (a,b,c)

newtype Fix f = Fix (f (Fix f)) -- from Control.Monad.Fix
type Inf a = Fix ((:.) a) -- infinite tuple!

Pretty simple, but way more powerful than the regular, monomorphic tuples. As you can see, (:.) is a right-associative. That means that a :. b :. c = a :. (b :. c).

That type will be heavily used in luminance, thus you should get your fet wet with it. There’s actually nothing much to know about it. It’s a Functor. I might add other features to it later on.

The Storable trick

The cool thing about (:.) is that we can provide a Storable instance for packed memory, as OpenGL requires it. Currently, the Storable instance is implemented like this:

instance (Storable a,Storable b) => Storable (a :. b) where
  sizeOf (a :. b) = sizeOf a + sizeOf b
  alignment _ = 1 -- packed data
  peek p = do
    a <- peek $ castPtr p
    b <- peek . castPtr $ p `plusPtr` sizeOf (undefined :: a)
    pure $ a :. b
  poke p (a :. b) = do
    poke (castPtr p) a
    poke (castPtr $ p `plusPtr` sizeOf (undefined :: a)) b

As you can see, the alignment is set to 1 to express the fact the memory is packed. The peek and poke functions use the size of the head of the tuple to advance the pointer so that we effectively write the whole tuple in packed memory.

Then, let’s rewrite our Vertex type in terms of (:.) to see how it’s going on:

type Vertex = X :. Y :. Z

If X, Y and Z are in Storable, we can directly poke one of our Vertex into a luminance buffer! That is, directly into the GPU buffer!

Keep in mind that the Storable instance implements packed-memory uploads and reads, and won’t work with special kinds of buffers, like shader storage ones, which require specific memory alignment. To cover them, I’ll create specific typeclasses instances. No worries.

Creating a vertex array

Creating a vertex array is done through the function createVertexArray. I might change the name of that object – it’s ugly, right? Maybe Shape, or something cooler!

createVertexArray :: (Foldable f,MonadIO m,MonadResource m,Storable v,Traversable t,Vertex v)
                  => t v
                  -> f Word32
                  -> m VertexArray

As you can see, the type signature is highly polymorphic. t and f represent foldable structures storing the vertices and the indices. And that’s all. Nothing else to feed the function with! As you can see, there’s a typeclass constraint on v, the inner vertex type, Vertex. That constraint ensures the vertex type is representable on the OpenGL side and has a known vertex format.

Disclaimer: the Traversable constraint might be relaxed to be Foldable very soon.

Once tested, I’ll move all that code from the unstable branch to the master branch so that you guys can test it. :)

About OpenGL…

I eventually came to the realization that I needed to inform you about the OpenGL prerequisites. Because I want the framework to be as modern and well-designed as possible, you’ll need… OpenGL 4.5. The latest version, indeed. You might also need an extension, ARB_bindless_texture. That would enable the framework to pass textures to shader in a very stateless way, which is our objective!

I’ll let you know what I decide about that. I don’t want to use an extension that is not implemented almost everywhere.

What’s next?

Well, tests! I need to be sure everything is correctly done on the GPU side, especially the vertex format specification. I’m pretty confident though.

Once the vertex arrays are tested, I’ll start defining a render interface as stateless as I can. As always, I’ll keep you informed!

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