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docs/asahi: Document clip distance varyings

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These implement gl_ClipDistance in hardware, avoiding the fragment shader
lowering. Unfortunately, they can't be disabled on a per-plane basis and they
can't be interpolated, so using them for OpenGL would still require a bunch of
extra lowering steps. Still, we should document the hardware and the caveats.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21097>
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Alyssa Rosenzweig 2023-02-02 22:27:02 -05:00 committed by Marge Bot
parent 13b25a6114
commit 897c47aa1c
1 changed files with 13 additions and 2 deletions
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15
docs/drivers/asahi.rst
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@ -60,7 +60,12 @@ consist of a single 32-bit value or an aligned 16-bit register pair, depending
on whether interpolation should happen at 32-bit or 16-bit. Vertex outputs are
indexed starting from 0, with the *vertex position* always coming first, the
32-bit user varyings coming next, then 16-bit user varyings, and finally *point
size* at the end if present.
size* and *clip distances* at the end if present. Note that *clip distances* are
not accessible from the fragment shader; if the fragment shader needs to read
the interpolated clip distance, the vertex shader must *also* write the clip
distance values to a user varying for the fragment shader to interpolate. Also
note there is no clip plane enable mask anywhere; that must lowered for APIs
that require this (OpenGL but not Vulkan).
.. list-table:: Ordering of vertex outputs with all outputs used
:widths: 25 75
@ -84,6 +89,12 @@ size* at the end if present.
- Packed pair of 16-bit varyings n
* - 4 + m + 1 + n + 1
- Point size
* - 4 + m + 1 + n + 2 + 0
- Clip distance for plane 0
* -
- ...
* - 4 + m + 1 + n + 2 + 15
- Clip distance for plane 15
Remapping
`````````
@ -92,7 +103,7 @@ Vertex outputs are remapped to varying slots to be interpolated.
The output of remapping consists of the following items: the *W* fragment
coordinate, the *Z* fragment coordinate, user varyings in the vertex
output order. *Z* may be omitted, but *W* may not be. This remapping is
configured by the "Linkage" packet.
configured by the "Output select" word.
.. list-table:: Ordering of remapped slots
:widths: 25 75