[Mesa-dev] Revert "gallium: add point size clamp to implementation limits in vertex shader"
maraeo at gmail.com
Mon Jan 23 10:55:05 PST 2012
On Mon, Jan 23, 2012 at 7:29 PM, Roland Scheidegger <sroland at vmware.com> wrote:
> Am 23.01.2012 19:07, schrieb Marek Olšák:
>> On Mon, Jan 23, 2012 at 6:34 PM, Roland Scheidegger <sroland at vmware.com> wrote:
>>> Am 23.01.2012 15:44, schrieb Marek Olšák:
>>>> I will have to (at least partially) revert this commit, as well as the
>>>> forked code in glsl_to_tgsi:
>>>>> commit 2c326e72664e65166c68b027b26aaf373f3be36d
>>>>> Author: Roland Scheidegger <sroland at vmware.com>
>>>>> Date: Thu Feb 4 19:23:09 2010 +0100
>>>>> gallium: add point size clamp to implementation limits in vertex shader
>>>>> The point size min/max registers (unused by mesa state tracker) were removed
>>>>> since most hardware couldn't do much with them. However, we don't want to have
>>>>> to rely on hw to do point size clamping correctly to implementation
>>>>> dependent limits, hence have to do that in the vertex shader. This should also
>>>>> solve a potential problem with (non-AA) points smaller than 1.0 which according
>>>>> to OGL still have size 1.0.
>>>>> Note that OGL point rendering is odd, in particular point sprites are rasterized
>>>>> differently to points. Some hardware might support those different modes, but in
>>>>> any case the different clamping values used for smooth/multisampled/sprite
>>>>> enabled points might help a bit for hw which rasterizes points the same as point
>>>>> Also tweak mesa's ff to vertex shader translation so don't have to clamp twice in
>>>>> case of point attenuation.
>>>> I mean the part which rewrites vertex shaders to clamp the point size
>>>> in st/mesa. We can't do that if transform feedback is enabled. The
>>>> point size must be unmodified when writing it into a transform
>>>> feedback buffer. Also it's unclear to me why we clamp the point size
>>>> at all.
>>>> If we want to do any clamping, we should do it after transform
>>>> feedback, that is after the vertex and geometry shaders. Hardware
>>>> drivers would have to do it in hardware somehow or fallback to Draw.
>>>> BTW all Radeons can do point size clamping, so adding
>>>> pipe_context::set_point_size_range(float min, float max) would be okay
>>>> for them.
>>>> BTW I think the only operation which can be done before transform
>>>> feedback is vertex color clamping (as per the EXT_transform_feedback
>>> I probably didn't think of transform feedback when doing this, so this
>>> looked like a good idea. IIRC most drivers didn't deal correctly (or at
>>> all) with point size limits - e.g. point size of 0 would disappear
>>> (instead of showing point size 1 with non-aa points), max/min point size
>>> ignored etc. and it looked like not all drivers actually could deal with
>>> it correctly (without adjusting the vertex shader themselves).
>>> But you're right this can't work correctly with transform feedback (as
>>> clamping should happen at rasterization time), so feel free to implement
>>> a solution which works.
>>> You can't fallback to Draw however (or rather you shouldn't) since you'd
>>> always need to fallback if points are drawn, because you don't know if
>>> the actual point size is larger than either the API or implementation
>>> dependent limit. So you'd need to fix the drivers to handle it themselves.
>> The only requirement so far seems to be:
>> If non-aa points, then minimum point size = 1.
>> I can fix Radeons and Draw. I guess Christoph wants to go with
>> out-of-spec behavior as usual or do what NVIDIA do. That leaves us
>> with nvfx and svga. It would be good to know if nvfx can force the
>> minimum point size to 1 for non-aa points (cc'ing Lucas Stach). Do you
>> have any idea what we should do for svga?
> Probably need to fix up the shader. d3d (and svga protocol) point size
> of 0.1 means the point will vanish (as the point quad rasterization
> state isn't handled), whereas ogl requires this to be rounded up to 1.0.
> While the legacy OGL point rasterization rules are significantly
> different to d3d points (which are the same as ogl point sprites mostly)
> this is really the difference you will notice.
OK, but this will get pretty ugly when you start supporting geometry
or tessellation shaders or streamout, assuming you ever want to
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