[Mesa-dev] [PATCH 1/3] gallium: add TGSI_SEMANTIC_VERTEXID_ZEROBASE and TGSI_SEMANTIC_BASEVERTEX

[Roland Scheidegger]

Am 11.12.2014 um 17:49 schrieb Ilia Mirkin:
> On Thu, Dec 11, 2014 at 11:39 AM, Jose Fonseca <jfonseca at vmware.com> wrote:
>> Overall I think this is a great cleanup.
>>
>> Just a remark about the chosen names.
>>
>> On 10/12/14 21:22, sroland at vmware.com wrote:
>>>
>>> From: Roland Scheidegger <sroland at vmware.com>
>>
>>
>>
>>
>>> Plus a new PIPE_CAP_VERTEXID_NOOFFSET query.
>>
>>
>> "Vertex offset" and "base vertex" are difference concepts, so I'd rather not
>> mix them up.
>>
>> AFAICT, this is about base vertex, not offsets in vertex buffers, hence
>> NOOFFSET seems to be a misnomer.  Please pick "NOBASE" or "ZEROBASE" suffix
>> and stick with it.
>>
>> Personally, I prefer "NOBASE".  As "ZEROBASE" sounds like that base vertex
>> must some how be zero, which is not the case.
>>
> 
> Is all this really just about the base vertex? The situation on, e.g.,
> a3xx is that the vertex id does not include any offset, even the
> 'start' from glDrawArrays (which makes sense, since they use the same
> general logic).
> 
>   -ilia
> 

Well this is actually kind of unresolved. I thought that gl_BaseVertex
would include "first" parameter and similar of DrawArrays, so that
really gl_VertexID = gl_BaseVertex + "vertex id without basevertex
included as known from d3d10". Always. And not just for DrawElements
calls (which would be a pain - the ARB_shader_draw_parameters extension
sort of implies it could be used to translate between different APIs,
but it's mostly useless if it doesn't work for all kind of draw calls)!

The discussion about this was here:
http://lists.freedesktop.org/archives/mesa-dev/2014-August/065172.html

So, I have no idea if this is right. And you are right "BaseVertex" is
sort of ambigous there, I guess if it doesn't include drawArrays "first"
parameter the whole point of lowering is kinda lost. I've written this
under the assumption that gl_Basevertex does include "first" from
DrawArrays, so the ambigous name wasn't chosen by me in that case then :-).

Roland