rhi: Add the basic infrastructure for tessellation support

...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>

1 files changed, 14 insertions, 4 deletions

diff --git a/src/gui/rhi/qrhi_p.h b/src/gui/rhi/qrhi_p.h
index dc788dd1e8..8c38b6383d 100644
--- a/src/gui/rhi/qrhi_p.h
+++ b/src/gui/rhi/qrhi_p.h
@@ -295,6 +295,8 @@ class Q_GUI_EXPORT QRhiShaderStage
 public:
     enum Type {
         Vertex,
+        TessellationControl,
+        TessellationEvaluation,
         Fragment,
         Compute
     };
@@ -347,8 +349,10 @@ public:
 
     enum StageFlag {
         VertexStage = 1 << 0,
-        FragmentStage = 1 << 1,
-        ComputeStage = 1 << 2
+        TessellationControlStage = 1 << 1,
+        TessellationEvaluationStage = 1 << 2,
+        FragmentStage = 1 << 3,
+        ComputeStage = 1 << 4
     };
     Q_DECLARE_FLAGS(StageFlags, StageFlag)
 
@@ -1126,7 +1130,8 @@ public:
         TriangleFan,
         Lines,
         LineStrip,
-        Points
+        Points,
+        Patches
     };
 
     enum CullMode {
@@ -1297,6 +1302,9 @@ public:
     QRhiRenderPassDescriptor *renderPassDescriptor() const { return m_renderPassDesc; }
     void setRenderPassDescriptor(QRhiRenderPassDescriptor *desc) { m_renderPassDesc = desc; }
 
+    int patchControlPointCount() const { return m_patchControlPointCount; }
+    void setPatchControlPointCount(int count) { m_patchControlPointCount = count; }
+
     virtual bool create() = 0;
 
 protected:
@@ -1318,6 +1326,7 @@ protected:
     float m_lineWidth = 1.0f;
     int m_depthBias = 0;
     float m_slopeScaledDepthBias = 0.0f;
+    int m_patchControlPointCount = 3;
     QVarLengthArray<QRhiShaderStage, 4> m_shaderStages;
     QRhiVertexInputLayout m_vertexInputLayout;
     QRhiShaderResourceBindings *m_shaderResourceBindings = nullptr;
@@ -1654,7 +1663,8 @@ public:
         RenderBufferImport,
         ThreeDimensionalTextures,
         RenderTo3DTextureSlice,
-        TextureArrays
+        TextureArrays,
+        Tessellation
     };
 
     enum BeginFrameFlag {