D3D Shaders - how does it work?

[Józef Kucia]

On Mon, Jul 17, 2017 at 8:30 PM, Thomas Berger <loki at lokis-chaos.de> wrote:
> On Montag, 17. Juli 2017 16:26:18 CEST Henri Verbeet wrote:
>> Quite frankly, I think those bug reports are questionable to begin
>> with. The level of analysis seems to be "I saw a FIXME", without any
>> reasoning how that's related to any negative effect in the
>> application.
> There are some bugs, blocked by the missing opcodes right now. It also sound
> reasonable that, if the interpolation for shaders is not working as expected,
> it results in graphical artifacts.

For a lot of games implementing those opcodes won't make any,
noticeable or significant difference, but yes, it may fix some
graphics artifacts for other games.

>
> There are many other bugs, much more critical with more effect, yes. But:
> The wine source is something new to me. Implementing this two opcodes is a
> task where i know what the opcodes should do, and how to do that. As there is
> a very straight forward translation to GLSL, it is not much afford to write
> high performance translation functions. So, that's the reason why i decided to
> pick these two bugs.

If we want to implement this we potentially need two code paths:
* in GLSL 4.40+ you can just specify interpolation qualifiers in a
fragment shaders (i.e. interpolation qualifiers don't have too much
between shader stages),
* in older GLSL versions the implementation will be more involved, you
have to specify matching interpolation qualifiers in a fragment shader
and a previous shader stage. It's a bit problematic because you will
need to generate a shader variant for a previous shader stage
depending on interpolation qualifiers in a fragment shader.

I think we should implement the code path for older GLSL versions
first, so we do not require GLSL 4.40+ for SM4 shader support. The
GLSL 4.40+ code path is a nice addition because it doesn't need shader
variants.

FWIW, there were some patches submitted for interpolation qualifiers
in the past [1], [2]. You can potentially use them as a starting
point. The mentioned patches contain a simple test case for
interpolation qualifiers. You can also find some review comments on
wine-devel [3], [4].

[1] - https://www.winehq.org/pipermail/wine-patches/2016-November/155649.html
[2] - https://www.winehq.org/pipermail/wine-patches/2016-November/155843.html
[3] - https://www.winehq.org/pipermail/wine-devel/2016-November/115234.html
[4] - https://www.winehq.org/pipermail/wine-devel/2016-November/115338.html