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Re: [Public WebGL] Index Validation and using the same buffer for both GL_ARRAY_BUFFER and GL_ELEMENT_ARRAY_BUFFER

On 2010-01-13 20:32, Kenneth Russell wrote:
On Tue, Jan 12, 2010 at 4:36 PM, Chris Marrin <cmarrin@apple.com <mailto:cmarrin@apple.com>> wrote:

On Jan 12, 2010, at 3:23 PM, Gregg Tavares wrote:

    > I was in the process of writing some index validation code
    before calling glDrawElements and I found out that apparently some
    WebGL implementations currently assume that the same buffer will
    never be used for both a GL_ARARY_BUFFER and a
    > as in
    >     buf = ctx.createBuffer();
    >     ctx.bindBuffer(ctx.ARRAY_BUFFER, buf);
    >     ctx.bindBuffer(ctx.ELEMENT_ARRAY_BUFFER, buf);
    > The spec does not disallow this.
    > My understanding is the reason some implementations assumed this
    would never happen was that to efficiently validate the indices,
    client side shadow copies of the buffer data needed to be kept. If
    there is an assumption that any buffer bound to
    GL_ELEMENT_ARRAY_BUFFER will only ever be bound to
    GL_ELEMENT_ARRAY_BUFFER then only the few buffers that are bound
    to GL_ELEMENT_ARRAY_BUFFER need to be shadowed.

    I will assume you're talking about the WebKit implementation. When
    I did that, I tried not to make an assumption that they would be
    different buffers. It tried to keep information about both types
    separate so a single buffer could represent both. If you use the
    same buffer for both you'd be shadowing much more than you needed
    to, but I think it should still work. Let me know what the case is
    where this will fail if the same buffer is used for both and we
    can see if it can be fixed.

The case that will fail is:

- Bind the buffer to ARRAY_BUFFER
- Use bufferData to upload unsigned byte or short data which will be treated as indices
- Unbind the buffer
- Bind the buffer to ELEMENT_ARRAY_BUFFER
- Call drawElements, referring to the previously uploaded data as indices

Because the buffer was bound to ARRAY_BUFFER when the data was uploaded, the WebKit index validation code won't keep a client-side copy.

Possible solutions include:

1. Keep a client-side copy of *all* data uploaded to buffer objects. Very wasteful of memory.
2. Lazily pull back data from buffer objects bound to ELEMENT_ARRAY_BUFFER using glMapBuffer when they are referenced from drawElements. Once client-side data exists for a buffer object, always keep it in sync during bufferData and bufferSubData calls.
3. Forbid binding the same buffer object to the ARRAY_BUFFER and ELEMENT_ARRAY_BUFFER points in the WebGL spec.

We should keep in mind that hardware and drivers are moving in the direction where index clamping will be done in hardware.

I'm not sure whether solution (3) would impact any real-world apps; I haven't seen any demos which would be affected.

Personally I would prefer (3), with (2) as an alternative, but not (1).

I prefer (3) as well. An added benefit with (3) is that it makes implementing WebGL on D3D9 easier since index and vertex buffers are different there.

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