Re: llvmpipe MSAA (Was: Fwd: [Mesa-users] Issues with removal of classic OSMesa)

It looks like llvmpipe has real MSAA with NIR.

Marek


On Wed, Jan 6, 2021 at 5:06 AM Jose Fonseca <[email protected]> wrote:

> That's an interesting idea!
>
> llvmpipe rasterization is complicated and very optimized, so changing
> llvmpipe's rasterizer to spit out MSAA coverages is very hard.  I think
> that a good way to approach this is to:
>
> 1) continue to do single sample rasterization, but adjust the line coeffs
> of the triangles edges (in llvmpipe rasterizer code known as *plane
> coefficients*) to do conservative rasterization (ie, so that any fragment
> that intersects a triangle is covered) when MSAA is enabled.  See
> https://developer.nvidia.com/content/dont-be-conservative-conservative-rasterization
>
> 2) once inside the fragment shader, compute SampleMaskIn from the
> unadjusted vertex positions, using the desired number of samples (and
> corresponding sample pattern)
>
> None of this would be throwaway work: the SampleMaskIn are correct and could
> be used for full MSAA support in the future too, and the conservative
> rasterization could be a feature on its own right too eventually.
>
> Jose
>
> ------------------------------
> *From:* mesa-dev <[email protected]> on behalf of
> Marek Olšák <[email protected]>
> *Sent:* Wednesday, January 6, 2021 05:57
> *To:* Brian Paul <[email protected]>
> *Cc:* [email protected] <[email protected]>;
> [email protected] <[email protected]>
> *Subject:* Re: [Mesa-dev] Fwd: [Mesa-users] Issues with removal of
> classic OSMesa
>
> Hi,
>
> llvmpipe could implement line and polygon smoothing by rasterizing in MSAA
> and passing the coverage to SampleMaskIn in the fragment shader, but doing
> Z/S tests and color writes and everything else single-sampled. Then,
> FragColor.a *= bitcount(SampleMaskIn) / (float)num_samples. It's roughly
> what OpenGL requires. There is at least one other gallium driver that does
> that.
>
> Marek
>
> On Mon, Jan 4, 2021 at 3:02 PM Brian Paul <[email protected]> wrote:
>
> Hi Andreas,
>
> I'm forwarding your message to the mesa-dev list for better visibility.
>
> BTW, when you say "antialiasing" below, what exactly do you mean?
>
> -Brian
>
>
> -------- Forwarded Message --------
> Subject:        [Mesa-users] Issues with removal of classic OSMesa
> Date:   Thu, 31 Dec 2020 12:56:04 +0100
> From:   Andreas Fänger <[email protected]>
> To:     [email protected]
>
> Hi,
>
> I've just seen that classic OSMesa has been removed (again) from Mesa3D
> a few weeks ago with this commit "mesa: Retire classic OSMesa".
>
> We are still actively using classical OSMesa for high quality rendering
> of still images in a headless environment with no GPU support
> (server-based rendering on windows and linux)
>
> Unfortunately, none of the alternative software renderers provide all
> the features that we require, which is antialiasing and anisotropic
> filtering. The current state is (correct me if I'm wrong)
>
> * softpipe: anisotropic filtering is supported, no antialiasing
>
> * llvmpipe: no anisotropic filtering, has MSAA
>
> * openswr: no anisotropic filtering, has MSAA, no OSMesa interface (?)
>
> We had hoped that classical OSMesa is only removed when there is a full
> replacement after the discussions in 2016 when OSMesa was about to be
> removed for the first time
>
> https://lists.freedesktop.org/archives/mesa-dev/2016-March/109665.html
> <https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.freedesktop.org%2Farchives%2Fmesa-dev%2F2016-March%2F109665.html&data=04%7C01%7Cjfonseca%40vmware.com%7C38811048f7d4434b32cc08d8b208137f%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C637455095057094583%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=HjIWy%2B4Mip6TkKYWUEuix41bPSgHUzZqgzwnBnjtBWA%3D&reserved=0>
>
> https://lists.freedesktop.org/archives/mesa-users/2016-March/001132.html
> <https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.freedesktop.org%2Farchives%2Fmesa-users%2F2016-March%2F001132.html&data=04%7C01%7Cjfonseca%40vmware.com%7C38811048f7d4434b32cc08d8b208137f%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C637455095057094583%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=nSZ3ZSfFRt8Z06avVzILOg4TipMkZ9CLyGMONcOkZxk%3D&reserved=0>
>
> and the commit that reverted the removal
>
>
> http://cgit.freedesktop.org/mesa/mesa/commit/?id=9601815b4be886f4d92bf74916de98f3bdb7275c
> <https://nam04.safelinks.protection.outlook.com/?url=http%3A%2F%2Fcgit.freedesktop.org%2Fmesa%2Fmesa%2Fcommit%2F%3Fid%3D9601815b4be886f4d92bf74916de98f3bdb7275c&data=04%7C01%7Cjfonseca%40vmware.com%7C38811048f7d4434b32cc08d8b208137f%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C637455095057104572%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=DufgWCKsN0v1nvTnT9VmyRkTSWiksioEYKIabLO%2BDvI%3D&reserved=0>
>
> Are there any plans to enhance the renderers so that at least one of
> them is providing both anisotropic filtering and antialiasing?
>
> As far as I know, anisotropic texture filtering is also one of the
> OpenGL 4.6 requirements.
>
> In 2016 I was told that there are only very few developers involved in
> llvmpipe and that chances are not high that someone is going to port the
> softpipe anisotropic filtering implementation as llvmpipe is much more
> complex. Is there any change in that situation?
>
> If there are no such plans, is there any chance of reverting this commit
> again so that classical OSMesa is available for windows and linux in
> mesa >20?
>
> Regards,
>
> Andreas Fänger
>
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