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Re: [Public WebGL] ARB_robustness and array bounds checking

On Fri, Oct 21, 2011 at 10:02 AM, Gregg Tavares (wrk) <gman@google.com> wrote:
> On Fri, Oct 21, 2011 at 8:20 AM, Chris Marrin <cmarrin@apple.com> wrote:
>> Currently the WebKit implementation of WebGL keeps around the current
>> index array so it can be used to do bounds checking on draw calls. This is
>> mandated in section 6.4 of the spec. The ARB_robustness extension requires
>> that out-of-bounds array accesses be forbidden. But by my reading it doesn't
>> require the behavior mandated in the WebGL spec. They simply guarantee that
>> no fetches outside the array will happen. Compliant behavior would be to
>> simply return 0 values for these accesses. So bounds checking on the WebGL
>> side would still be required.
>> If this is the case, I think we should make changes so drivers
>> implementing this extension can avoid the WebGL side bounds checking. I'm
>> not sure it's practical to change the ARB_isolation extension at this point.
>> So I would be in favor of changing section 6.4 of the spec to match the
>> behavior required in the extension.
> If I understand correctly the issue with ARB_isolation is there is no
> guarantee the driver is obeying anything. It doesn't report errors for out
> of bounds access so there is no way to test that it's actually working. For
> WebGL we wanted something testable as far as I remember.

This is my recollection as well. There have been discussions in some
of the working groups about strengthening the guarantees in
ARB_robustness' robust buffer access: for example, either clamping the
access to within the buffer, or returning a specified constant value,
so that the extension's behavior is testable. Otherwise the best test
that can be written is one which accesses wildly out-of-range values
and passes if it doesn't crash, which isn't very good.


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