[Mesa-dev] Gallium API questions

[Marek Olšák]

On Wed, Jun 23, 2010 at 11:51 PM, Corbin Simpson
<mostawesomedude at gmail.com>wrote:

> Yep, it's that time again. These are just things marked in the docs as
> unknown, so I'm not grinding any axes, just cleaning stuff up.
>
> ~ "If a surface includes several layers/slices (XXX: not yet...) then
> all layers will be cleared." From the information on
> pipe_context::clear(). Is this just wishful thinking, or do drivers
> need to implement this?
>

I believe this has something to do with geometry shaders where you can
select a layer (slice, face) you want to emit a primitive to. This way you
may e.g. render to a whole cubemap in one pass.


> ~ An explanation of dual-source blends and how they work would be nice.
>

http://www.opengl.org/registry/specs/ARB/blend_func_extended.txt


>
> ~ If logicop_enable and independent_blend_enable are set, are lops
> performed for all render targets, or only those that have blend_enable
> set?
>
> ~ PIPE_CAP_TEXTURE_SHADOW_MAP and PIPE_CAP_MAX_PREDICATE_REGISTERS
> aren't explained.
>

I think PIPE_CAP_TEXTURE_SHADOW_MAP means shadow samplers.


> ~ PIPE_CAP_GUARD_BAND_* obviously refers to the guard bands, but what
> exactly do they mean as capabilities? Maximum guard band settings?
> Minimum?
>
> ~ Could somebody explain exactly what's up with the RCC opcode? Its
> definition is kind of ridiculous: dst = (1 / src.x) > 0 ? clamp(1 /
> src.x, 5.42101e-020, 1.884467e+019) : clamp(1 / src.x, -1.884467e+019,
> -5.42101e-020)
>
> ~ Are we going to introduce new cap bits for parts of the TGSI ISA?
> (I'll understand if this is off the table for now.)
>
> ~ TGSI_SEMANTIC_POSITION is (x, y, z, w) where x, y, and z are
> position, and w is the homogeneous coordinate. Is w also for
> perspective divide? Is the perspective divide configurable, or is it
> always done in shaders?
>

The perspective divide is done per-pixel after the rasterization and before
the fragment shader, and there is no way to disable it in Gallium. It's
never done in shaders, it's rather a fixed functionality and imposing
additional restrictions on it would make it hard to implement in drivers.


> ~ "Position, if not specified, usually defaults to (0, 0, 0, 1), and
> can be partially specified as (x,  y, 0, 1) or (x, y, z,  1)."
> "Usually" is probably Corbin-speak for "I'm just guessing here." Is
> that actually how it goes, or is position mandatory?
>
> ~ Can somebody explain the format of TGSI_SEMANTIC_EDGEFLAG?
>
> ~ What are the components of textures in the UV or Z format when
> sampled? For UV, current candidates are (0, 0, 0, 1) and (u, v, 1, 1).
> The latter seems more reasonable given that Gallium is shaderful. For
> Z, it could be (z, z, z, 1), (0, z, 0, 1), (0, 0, 0, z), or (z, z, z,
> z) and is configurable in GL. Maybe we want to have this be
> configurable?
>
> Thanks!
>
> ~ C.
>
> --
> When the facts change, I change my mind. What do you do, sir? ~ Keynes
>
> Corbin Simpson
> <MostAwesomeDude at gmail.com>
> _______________________________________________
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> mesa-dev at lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/mesa-dev
>
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