[Public WebGL] questions about vertexAttribPointer / getVertexAttrib offsets

Benoit Jacob [email protected]
Mon Apr 23 13:23:34 PDT 2012


----- Original Message -----

> On Mon, Apr 23, 2012 at 12:56 PM, Benoit Jacob < [email protected] >
> wrote:

> > > On Mon, Apr 23, 2012 at 12:06 PM, Boris Zbarsky <
> > > [email protected]
> > > >
> > > wrote:
> > 
> 

> > > > On 4/23/12 2:43 PM, Gregg Tavares (勤) wrote:
> > > 
> > 
> 

> > > > > There's at least one negative offset test in
> > > > 
> > > 
> > 
> 
> > > > > https://cvs.khronos.org/svn/ repos/registry/trunk/public/
> > > > > webgl/sdk/tests/conformance/ more/functions/
> > > > > vertexAttribPointerBadArgs. html
> > > > 
> > > 
> > 
> 

> > > > Doesn't that call fail per current spec because the offset (-1)
> > > > is
> > > > not a multiple of sizeof(float), independently of what negative
> > > > offset handling looks like? My Gecko build with the negative
> > > > offset
> > > > check in vertexAttribPointer removed passes this test,
> > > > precisely
> > > > for
> > > > that reason:
> > > 
> > 
> 

> > > > VertexAttribPointer: byteOffset doesn't satisfy the alignment
> > > > requirement of given type @ https://cvs.khronos.org/svn/
> > > > repos/registry/trunk/public/ webgl/sdk/tests/conformance/
> > > > more/util.js:965
> > > 
> > 
> 

> > > > -Boris
> > > 
> > 
> 

> > > To be clear
> > 
> 

> > > 1) AFAICT OpenGL does not allow negative offsets
> > 
> 

> > > 2) WebGL should not allow them either.
> > 
> 

> > > So,
> > 
> 

> > > a) we should update the WebGL spec to make it clear negative
> > > offsets
> > > generate INVALID_VALUE
> > 
> 

> > > and
> > 
> 

> > > b) we should update the conformance tests to test that case.
> > 
> 

> > > Agree?
> > 
> 
> > I agree, except for 1): I don't see where the OpenGL spec forbids
> > negative offsets.
> 
> How do you get a negative offset in OpenGL? The function takes a
> pointer, not an offset. To generate an offset the spec says "offset
> = pointer - NULL" which is always positive.
OK, I understand now. The OpenGL function takes an actual void*, not a GLintptr. That's where it differs from WebGL. 

Thanks 
Benoit 

> > I agree that there doesn't seem to exist a valid use case for them.
> > I
> > would support forbidding negative offsets in WebGL for now, and
> > having a conversation on that topic with the OpenGL WG.
> 

> > Benoit
> 
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