...but this will only be supported with Vulkan and OpenGL 4.0+ and
OpenGL ES 3.2+ for the time being.
Taking the Vulkan model as our standard, the situation is the
following:
- Vulkan is ok, qsb secretly accepts .tesc and .tese files as input
already (plus QShader already has the necessary plumbing when it
comes to enums and such) To switch the tessellation domain origin to
bottom left we require Vulkan 1.1 (don't bother with
VK_KHR_maintenance2 on top of 1.0 at this point since 1.1 or 1.2
implementations should be common by now). The change is essential to
allow the same evaluation shader to work with both OpenGL and
Vulkan: this way we can use the same shader source, declaring the
tessellation winding order as CCW, with both APIs.
- OpenGL 4.0 and OpenGL ES 3.2 (or ES 3.1 with the Android extension
pack, but we won't bother with checking that for now) can be made
working without much complications, though we need to be careful
when it comes to gathering and setting uniforms so that we do not
leave the new tessellation stages out. We will stick to the Vulkan
model in the sense that the inner and outer tessellation levels must
be specified from the control shader, and cannot be specified from
the host side, even though OpenGL would allow this. (basically the
same story as with point size in vertex shaders)
- D3D11 would be no problem API-wise, and we could likely implement
the support for hull and domain shader stages in the backend, but
SPIRV-Cross does not support translating tessellation shaders to
HLSL. Attempting to feed in a .tesc or .tese file to qsb with
--hlsl specified will always fail. One issue here is how hull
shaders are structured, with the patchconstantfunc attribute
specifying a separate function computing the patch constant
data. With GLSL there is a single entry point in the tessellation
control shader, which then performs both the calculations on the
control points as well as the constant data (such as, the inner and
outer tessellation factors). One option here is to inject
handwritten HLSL shaders in the .qsb files using qsb's replace (-r)
mode, but this is not exactly a viable universal solution.
- Metal uses a different tessellation pipeline involving compute
shaders. This needs more investigation but probably not something we
can prioritize in practice. SPIRV-Cross does support this,
generating a compute shader for control and a (post-)vertex shader
for evaluation, presumably in order to enable MoltenVK to function
when it comes to tessellation, but it is not clear yet how usable
this is for us.
Change-Id: Ic953c63850bda5bc912c7ac354425041b43157ef
Reviewed-by: Andy Nichols <andy.nichols@qt.io>
a325016aa9