[Public WebGL] Index Validation and using the same buffer for both GL_ARRAY_BUFFER and GL_ELEMENT_ARRAY_BUFFER
Wed Jan 13 12:49:44 PST 2010
On 2010-01-13 20:32, Kenneth Russell wrote:
> On Tue, Jan 12, 2010 at 4:36 PM, Chris Marrin <[email protected]
> <mailto:[email protected]>> 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
> 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
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