[Mesa-dev] [PATCH] i965: Mask the cut index based on the index buffer type in 3DSTATE_VF.

[Kenneth Graunke]

On 05/23/2013 04:14 PM, Jordan Justen wrote:
> On Thu, May 23, 2013 at 3:46 PM, Kenneth Graunke <kenneth at whitecape.org> wrote:
>> According to the documentation: "The Cut Index is compared to the
>> fetched (and possibly-sign-extended) vertex index, and if these values
>> are equal, the current primitive topology is terminated.  Note that,
>> for index buffers <32bpp, it is possible to set the Cut Index to a
>> (large) value that will never match a sign-extended vertex index."
>>
>> This suggests that we should not set the value to 0xFFFFFFFF for
>> unsigned byte or short index buffers, but rather 0xFF or 0xFFFF.
>
> I was wondering what the GL spec had to say about this situation. For
> example, what should happen if the index is 0x100, and bytes are used.
> Should it effectively disable prim-restart? Should it use 0xff, or
> 0x00? Unfortunately, I didn't find anything concrete.
>
> Reviewed-by: Jordan Justen <jordan.l.justen at intel.com>

You raise a good point.  If I set the cut index to 0x31337 and 
DrawElements with GL_UNSIGNED_BYTE, should it reset when it sees 37 or not?

I'll have to write Piglit tests and find out what other implementations do.

Thanks for the excellent review!