On 28.09.2017 20:02, Roland Scheidegger wrote:
Am 28.09.2017 um 18:19 schrieb Jose Fonseca:
On 28/09/17 17:16, Roland Scheidegger wrote:
Am 28.09.2017 um 17:53 schrieb Jose Fonseca:
On 28/09/17 16:29, Roland Scheidegger wrote:
Am 28.09.2017 um 16:12 schrieb Jose Fonseca:
On 27/09/17 15:07, Roland Scheidegger wrote:
Am 27.09.2017 um 09:13 schrieb Olivier Lauffenburger:
Software rasterizer and LLVM contain code to enable clipping as
soon as
a vertex shader writes to gl_ClipDistance, even if the corresponding
clip planes are disabled.
GLSL specification states that "Values written into gl_ClipDistance
for
planes that are not enabled have no effect."
The actual behavior is thus non-conformant.
This patch removes the code that automagically enables user clipping
planes even if they are disabled.
Signed-off-by: Olivier Lauffenburger <o.lauffenbur...@topsolid.com>
FWIW that code is there because you can't disable clip distances with
d3d10 - if you write them in the shader, they're enabled (d3d9 didn't
have clip distances, just old user clip planes, which of course have
enable bits). They are very similar to cull distances there (which
you
can't disable with gl neither).
I suppose we cheated there a bit... I might even have realized it
wasn't
quite GL conformant when we did this, but it didn't cause piglit
regressions then (I guess it's very rare a shader actually declares
clip
distance outputs but does not enable them).
This was introduced back in June 2013:
https://lists.freedesktop.org/archives/mesa-dev/2013-June/040559.html
So with this removed, I suppose we need to add a workaround in our
code
(which is indeed rather unfortunate). But I don't see another
(reasonable) way to make it gl conformant.
If however there's still no piglit test exercising this, there
should be
one.
I'm still not following. Are we talking about
pipe_rasterizer_state::clip_plane_enable controlling
TGSI_SEMANTIC_CLIPDIST?
Yes.
I thought these have nothing to do with one another.
pipe_rasterizer_state::clip_plane_enable should control
legacy/fixed-fuction user clip planes.
Nope. Every single driver (except those using draw) assumes this also
enables clip dists - this includes svga which translates those away in
the shader which aren't enabled.
If the OpenGL state tracker needs to translate GLSL shaders that write
gl_ClipDistance but where the clip plane is disabled, the solution is
simple: just create a shader variant that omits the
TGSI_SEMANTIC_CLIPDIST in question, or writes an constant to it.
Well, it is easier to have an extra enable and having to add additional
rasterizer dependencies on state trackers which don't have separate
enable, rather than having to hack shaders with state trackers which
don't have them. Of course, svga does exactly that but it's a bit
annoying.
Like it was mentioned, it should be extremely rare for apps to emit
gl_ClipDistance yet disable the clip planes, hence the shader variants
should be extremely rare.
It's not just D3D10/11 that doesn't have flags to disable clip
distances: I can't find them in Vulkan spec neither. And while I know
few/none want to build Vulkan drivers atop Gallium interface I think
it's still useful to decide what deserves to be in Gallium
interface or
not.
Of course, newer apis won't have that - clearly the separate enables
just
carried over from legacy GL.
So in short, llvmpipe is fine, please let's fix the state tracker
instead.
Well, we're really the only ones caring about non-gl gallium state
tracker, so I'm not sure it makes sense to impose the d3d10 semantics
there. We just cheated in draw.
And actually, thinking about this, it's really not even possible easily
and cleanly doing this in the state tracker: you can pass those clip
dists to the next shader stage. If that's all pre-fragment stage
(vertex, geometry, tesselation), this is still doable just annoying
(must pass them unaltered between stages not the last stage before
rasterization), but it's impossible to pass them to fragment stage
(unless you'd use generic varying) if you rewrite the shader to not
include them (don't ask me if it works correctly in svga, it might have
code for this too).
So I believe there's really no other choice other than following GL
semantics there in gallium.
Ok..
So when implementing D3D10 we just need to ensure clip enable is always
set to all ones?
I don't think that will work - the (gl and presumably gallium) semantics
are that enabled clip planes must be written or the results are
undefined. I suppose we could cheat there too (in draw) and take the
insersection of enabled and written clip dists, but otherwise we'd have
to enable only these clip planes a shader actually writes...
I wouldn't call this "cheat", but merely do something sane, something
that's not merely undefined, something that works for all APIs.
Well, basically it amounts to using a undefined shader output, and such
things tend to give undefined results.
After all, when did gallium stop to be an abstraction to become
"whatever GL api does"?
Yes, we could do it differently.
Actually, there's a comment in p_state.h which even states this is
indeed the case.
" /**
* Enable bits for clipping half-spaces.
* This applies to both user clip planes and shader clip distances.
* Note that if the bound shader exports any clip distances, these
* replace all user clip planes, and clip half-spaces enabled here
* but not written by the shader count as disabled.
*/
unsigned clip_plane_enable:PIPE_MAX_CLIP_PLANES;
"
However, I was wrong saying this could work. I realized it cannot - this
is because there's no explicit switch between old user clip planes and
clip distances. If the shader writes (at least one) clip distance, then
those clip distances will be used as additional clip planes (when the
clip enable bits are set). However, if clip dist output does not exist,
then that means old user clip planes are enabled instead and the
corresponding clip math performed (using the user clip planes and either
ordinary position output or clipVertex output).
Therefore, always setting all clip enable bits to 1 and not writing clip
dist at all would perform user clip plane clipping... Would be fixable
with another rasterizer bit I suppose but I'm not sure it's worth it?
If you do care sufficiently, IMO it'd be cleaner to have separate bit
sets for user clip planes and clip distances. It's more bits, but I
think the disentangling of state would be worth it.
Cheers,
Nicolai
Roland
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