- The optional nice-to-haves DebugMarkers, Timestamps, PipelineCache
are not yet implemented (features reported as false, to be
implemented later, although buffer/texture resource name setting
already works as-is, regardless of DebugMarkers).
- Mipmap generation for 3D textures is missing. Won't matter much
given that 3D textures are not used in Qt for anything atm. For
generating mipmaps for 2D (or 2D array) textures, the MiniEngine
compute shader and approach is used. 3D support for the mipmap
generator may be added later. 1D textures / arrays are supported
except for mipmap generation, and so the
OneDimensionalTextureMipmaps feature is reported as false.
- Qt Quick and Qt Quick 3D are expected to be fully functional.
(unforeseen issues are not impossible, of course)
- Uses minimum feature level 11.0 when requesting the device. It is
expected to be functional on resource binding tier 1 hardware even,
although this has not been verified in practice.
- 2 frames in flight with the usual resource buffering
(QRhiBuffer::Dynamic is host visible (UPLOAD) and always mapped and
slotted, other buffers and textures are device local (DEFAULT).
Requests 3 swapchain buffers. Swapchains are mostly like with D3D11
(e.g. FLIP_DISCARD and SCALING_NONE).
- The root signature generation is somewhat limited by the SPIR-V
binding model and that we need to map every binding point using the
nativeResourceBindingMap from the QShader. Thus the root signature
is laid out so each stage has its own set of resources, with shader
register clashes being prevented by setting the visibility to a
given stage.
Sampler handling is somewhat suboptimal but we are tied by the
binding model and existing API design. It is in a fairly special
situation due to the 2048 limit on a shader visible sampler heap, as
opposed to 1000000 for SRVs and UAVS, so the approach we use for
textures (just stage the CPU SRVs on the (per-frame slot) shader
visible heap as they are encountered, effectively treating the heap
as a ring buffer) would quickly lead to having to switch heaps many
times with scenes with many draw calls and sampledTexture/sampler
bindings in the srb.
Whereas static samplers, which would be beautiful, are impossible to
utilize safely since we do not have that concept (i.e. samplers
specified upfront, tied to the graphics/compute pipeline) in the
QRhi API, and an srb used at pipeline creation may change its
associated resources, such as the QRhiSampler reference, by the time
the shader resources are set for the draw call (or another,
compatible srb may get used altogether), so specifying the samplers
at root signature creation time is impossible.
Rather, the current approach is to treat each sampler as a separate
root parameter (per stage) having a descriptor table with a single
entry. The shader visible sampler heap has exactly one instance of
each unique sampler encountered during the lifetime of the QRhi.
- Shader-wise no different from D3D11, works with HLSL/DXBC 5.0
(i.e. existing .qsb files with DXBC in them work as-is). But unlike
D3D11, this one will try to pick 6.7, 6.6, ..., down to 5.0 from the
QShader, in that order.
- Uses D3D12MA for suballocating. As a result it can report vmem
allocation statistics like the Vulkan backend, and it does more
since the DXGI memory usage (incl. implicit resources) is also
reported. This is optional technically, so we also have the option
of going straight with the heavyweight CreateCommittedResource()
instead. That is what we do if the adapter chosen reports it's
software-based or when QT_D3D_NO_SUBALLOC=1 is set.
- PreferSoftwareRenderer (picking the WARP device) and the env.var.
QT_D3D_ADAPTER_INDEX work as with the D3D11 backend.
- It is not unexpected that with large scenes that generate lots of
draw calls with multiple textures/samplers per call the performance
may be slightly below D3D11 (probably mostly due to descriptor
management). Similarly, the reported memory usage will be higher,
which is partly natural due to creating heaps, descriptor pools,
staging areas, etc. upfront. Will need to be evaluated later how
these can be tuned.
Change-Id: I5a42580bb65f391ebceaf81adc6ae673cceacb74
Reviewed-by: Andy Nichols <andy.nichols@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
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