[Public WebGL] questions about vertexAttribPointer / getVertexAttrib offsets

Gregg Tavares (勤) [email protected]
Mon Apr 23 16:32:18 PDT 2012


On Mon, Apr 23, 2012 at 1:23 PM, Benoit Jacob <[email protected]> wrote:

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> On Mon, Apr 23, 2012 at 12:56 PM, Benoit Jacob <[email protected]> wrote:
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>> On Mon, Apr 23, 2012 at 12:06 PM, Boris Zbarsky <[email protected]> wrote:
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>>> 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<https://cvs.khronos.org/svn/repos/registry/trunk/public/webgl/sdk/tests/conformance/more/functions/vertexAttribPointerBadArgs.html>
>>>
>>>
Update this test to use -4. I think that makes it check for negative
offsets instead of the offset not being the correct size of the type. In
fact I see that difference in the console error message reported by Chrome
20. Feel free to submit something more comprehensive if you think it's
needed.

-g


>
>>>>
>>> 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<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
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>> 2) WebGL should not allow them either.
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>> So,
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>> a) we should update the WebGL spec to make it clear negative offsets
>> generate INVALID_VALUE
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>> and
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>> b) we should update the conformance tests to test that case.
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>> Agree?
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>> I agree, except for 1): I don't see where the OpenGL spec forbids
>> negative offsets.
>>
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> 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
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>> 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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