Like the manual says, this is technically undefined behaviour. See:
https://msdn.microsoft.com/en-us/library/windows/desktop/ff476085.aspx
In particular, MSDN says texture arrays created with the BIND_DECODER
flag cannot be used with CreateShaderResourceView, which means they
can't be sampled through SRVs like normal Direct3D textures. However,
some programs (Google Chrome included) do this anyway for performance
and power-usage reasons, and it appears to work with most drivers.
Older AMD drivers had a "bug" with zero-copy decoding, but this appears
to have been fixed. See #3255, #3464 and http://crbug.com/623029.
This is a new RA/vo_gpu backend that uses Direct3D 11. The GLSL
generated by vo_gpu is cross-compiled to HLSL with SPIRV-Cross.
What works:
- All of mpv's internal shaders should work, including compute shaders.
- Some external shaders have been tested and work, including RAVU and
adaptive-sharpen.
- Non-dumb mode works, even on very old hardware. Most features work at
feature level 9_3 and all features work at feature level 10_0. Some
features also work at feature level 9_1 and 9_2, but without high-bit-
depth FBOs, it's not very useful. (Hardware this old is probably not
fast enough for advanced features anyway.)
Note: This is more compatible than ANGLE, which requires 9_3 to work
at all (GLES 2.0,) and 10_1 for non-dumb-mode (GLES 3.0.)
- Hardware decoding with D3D11VA, including decoding of 10-bit formats
without truncation to 8-bit.
What doesn't work / can be improved:
- PBO upload and direct rendering does not work yet. Direct rendering
requires persistent-mapped PBOs because the decoder needs to be able
to read data from images that have already been decoded and uploaded.
Unfortunately, it seems like persistent-mapped PBOs are fundamentally
incompatible with D3D11, which requires all resources to use driver-
managed memory and requires memory to be unmapped (and hence pointers
to be invalidated) when a resource is used in a draw or copy
operation.
However it might be possible to use D3D11's limited multithreading
capabilities to emulate some features of PBOs, like asynchronous
texture uploading.
- The blit() and clear() operations don't have equivalents in the D3D11
API that handle all cases, so in most cases, they have to be emulated
with a shader. This is currently done inside ra_d3d11, but ideally it
would be done in generic code, so it can take advantage of mpv's
shader generation utilities.
- SPIRV-Cross is used through a NIH C-compatible wrapper library, since
it does not expose a C interface itself.
The library is available here: https://github.com/rossy/crossc
- The D3D11 context could be made to support more modern DXGI features
in future. For example, it should be possible to add support for
high-bit-depth and HDR output with DXGI 1.5/1.6.
James Ross-Gowan