[Mesa-dev] Mesa (gallium-index-bias): gallium: Add index bias parameter to draw_elements and friends.

[Jerome Glisse]

On Mon, Apr 19, 2010 at 03:38:54PM +0200, José Fonseca wrote:
> On Mon, 2010-04-19 at 06:22 -0700, Keith Whitwell wrote:
> > On Mon, 2010-04-19 at 06:07 -0700, Marek Olšák wrote:
> > > Unfortunately, this is supported only on r500 (i.e. not r300 nor r400)
> > > and only when the DRM version is >=2.3.0 (i.e. kernel 2.6.34). Please,
> > > can we make it an optional feature?
> > 
> > Are you sure about this?  It seems to be a long-time DX9 (and earlier?)
> > requirement.  
> > 
> > I'm sure it can be made optional, though - if a state tracker absolutely
> > required it & didn't want to implement a workaround, it could just fail
> > to initialize on such drivers.
> 
> Positive index bias are trivial to implement on any hardware.
> 
> One way of implementing negative index bias would be by programing the
> hardware with an artificially lower offset:
> 
>   + - - - - - - - - - - +---------------------+
>   : index_bias*stride   | real vertex buffer  |
>   + - - - - - - - - - - +---------------------+
>   ^
>   |
>   pass this to hardware
> 
> but the hardware would need to have builtin support for vertex buffer
> max/min index checking, to ensure only vertices inside the real range
> are read. Also, the DRM would need to support these relocation with
> negative offsets, and make sure that the start offset doesn't wrap
> around vram start.
> 
> If the above doesn't work, then is just a matter of reading the index
> buffer, make a copy of with with the right indices and use it instead.
> 
> Actually, a mixture of the above can be used.
> 
> So there is really no excuse to not support this feature unless it is
> perceived to be useless.
> 
> A big user of this extension would be wine, but I've checked their
> source code and they don't use it yet. Actually, they just set negative
> vertex buffers offsets as a unsigned and pray that the two's complement
> arithmetic inside the GL driver just works.
> 
> This feature is really used by D3D software, e.g. Google Earth on D3D.
> 
> Jose
> 

The way we communicate command stream (CS) with the kernel, doesn't
allow for negative offset. The positive offset can't still be achieve.

Cheers,
Jerome