mirror of
https://github.com/Qortal/Brooklyn.git
synced 2025-01-31 23:32:17 +00:00
75 lines
3.6 KiB
ReStructuredText
75 lines
3.6 KiB
ReStructuredText
Hierarchical Depth (HiZ)
|
||
========================
|
||
|
||
TODO: Add detailed docs like we have for CCS
|
||
|
||
HiZ/stencil on Sandy Bridge
|
||
---------------------------
|
||
|
||
Properly enabling HiZ on Sandy Bridge requires certain special considerations.
|
||
From the Sandy Bridge PRM Vol. 2, Pt. 1, 7.5.3 "Hierarchical Depth Buffer" (p.
|
||
312):
|
||
|
||
The hierarchical depth buffer does not support the LOD field, it is assumed
|
||
by hardware to be zero. A separate hierarachical depth buffer is required
|
||
for each LOD used, and the corresponding buffer’s state delivered to
|
||
hardware each time a new depth buffer state with modified LOD is delivered.
|
||
|
||
The ``3DSTATE_STENCIL_BUFFER`` packet for separate stencil (required for HiZ)
|
||
on sandy bridge also lacks an LOD field. Empirically, the hardware doesn't
|
||
pull the stencil LOD from ``3DSTATE_DEPTH_BUFFER``, it's just always 0 like
|
||
with HiZ.
|
||
|
||
As stated in the PRM, this means we need a separate HiZ or stencil buffer for
|
||
each LOD. However, it's not quite as simple as that. If you ignore layered
|
||
rendering, things are pretty straightforward: you need one HiZ surface for each
|
||
main surface slice With layered, rendering, however, we have to be a bit more
|
||
clever because we need a "real" array surface at each LOD. ISL solves this
|
||
with a special miptree layout for layered rendering
|
||
:cpp:enumerator:`isl_dim_layout::ISL_DIM_LAYOUT_GFX6_STENCIL_HIZ` which lays
|
||
out the surface as a miptree of layered images instead of an array of miptrees.
|
||
See the docs for
|
||
:cpp:enumerator:`isl_dim_layout::ISL_DIM_LAYOUT_GFX6_STENCIL_HIZ` for a nice
|
||
description along with an ASCII art diagram of the layout.
|
||
|
||
Also, neither ``3DSTATE_STENCIL_BUFFER`` nor ``3DSTATE_HIER_DEPTH_BUFFER`` have
|
||
their own surface dimensions or layout information on Sandy Bridge. They're
|
||
just an address and a surface pitch. Instead, all that other information is
|
||
pulled from ``3DSTATE_DEPTH_BUFFER``. When you combine this with the lack of
|
||
LOD, this means that, technically, we have a full-sized single-LOD stencil or
|
||
HiZ surface at each miplevel of which only the upper left-hand corner of each
|
||
array slice ever gets used. The net effect of this is that, in
|
||
:cpp:enumerator:`isl_dim_layout::ISL_DIM_LAYOUT_GFX6_STENCIL_HIZ`, all LODs
|
||
share the same QPitch even though it's horribly wasteful. This is actually
|
||
pretty convenient for ISL because we only have the one
|
||
:cpp:member:`isl_surf::array_pitch_el_rows` field.
|
||
|
||
Due to difficulties with plumbing relocation deltas through ISL's
|
||
depth/stencil/hiz emit interface, we can't handle this all automatically in
|
||
ISL. Instead, it's left up to the driver to do this offsetting. ISL does
|
||
provide helpers for computing the offsets and they work fine with
|
||
:cpp:enumerator:`isl_dim_layout::ISL_DIM_LAYOUT_GFX6_STENCIL_HIZ` so all that's
|
||
really required is to call the ISL helper and add the computed offset to the
|
||
HiZ or stencil buffer address. The following is an excerpt from BLORP where we
|
||
do this as an example:
|
||
|
||
.. code-block:: c
|
||
|
||
struct blorp_address hiz_address = params->depth.aux_addr;
|
||
#if GFX_VER == 6
|
||
/* Sandy bridge hardware does not technically support mipmapped HiZ.
|
||
* However, we have a special layout that allows us to make it work
|
||
* anyway by manually offsetting to the specified miplevel.
|
||
*/
|
||
assert(info.hiz_surf->dim_layout == ISL_DIM_LAYOUT_GFX6_STENCIL_HIZ);
|
||
uint32_t offset_B;
|
||
isl_surf_get_image_offset_B_tile_sa(info.hiz_surf,
|
||
info.view->base_level, 0, 0,
|
||
&offset_B, NULL, NULL);
|
||
hiz_address.offset += offset_B;
|
||
#endif
|
||
|
||
info.hiz_address =
|
||
blorp_emit_reloc(batch, dw + isl_dev->ds.hiz_offset / 4,
|
||
hiz_address, 0);
|