Hydrogent: skip OIT passes if there are no translucent items

Author: TheMostDiligent

Hydrogent/interface/HnRenderPassState.hpp

@@ -47,6 +47,17 @@ namespace Diligent
 namespace USD
 {
 
+struct HnRenderPassStats
+{
+    Uint32 NumDrawItems = 0;
+
+    HnRenderPassStats& operator+=(const HnRenderPassStats& Stats)
+    {
+        NumDrawItems += Stats.NumDrawItems;
+        return *this;
+    }
+};
+
 /// Hydra render pass state implementation in Hydrogent.
 class HnRenderPassState final : public pxr::HdRenderPassState
 {
@@ -147,6 +158,14 @@ class HnRenderPassState final : public pxr::HdRenderPassState
         m_IsCommited = false;
     }
 
+
+    const HnRenderPassStats& GetStats() const { return m_Stats; }
+
+    void UpdateStats(const HnRenderPassStats& Stats)
+    {
+        m_Stats += Stats;
+    }
+
 private:
     Uint32                                         m_NumRenderTargets = 0;
     std::array<TEXTURE_FORMAT, MAX_RENDER_TARGETS> m_RTVFormats       = {};
@@ -163,6 +182,8 @@ class HnRenderPassState final : public pxr::HdRenderPassState
     bool                                          m_IsCommited  = false;
 
     bool m_FrontFaceCCW = false;
+
+    HnRenderPassStats m_Stats;
 };
 
 } // namespace USD

Hydrogent/interface/Tasks/HnBeginOITPassTask.hpp

@@ -89,6 +89,7 @@ class HnBeginOITPassTask final : public HnTask
     std::array<RefCntAutoPtr<IShaderResourceBinding>, 2> m_RWLayersSRBs;
 
     Uint32 m_BoundOITResourcesVersion = ~0u;
+    bool   m_OITLayersCleared         = false;
 };
 
 } // namespace USD

Hydrogent/src/HnRenderPass.cpp

@@ -120,6 +120,8 @@ struct HnRenderPass::RenderState
     const Uint32 ConstantBufferOffsetAlignment;
     const bool   NativeMultiDrawSupported;
 
+    HnRenderPassStats Stats;
+
     RenderState(const HnRenderPass&      _RenderPass,
                 const HnRenderPassState& _RPState) :
         RenderPass{_RenderPass},
@@ -778,6 +780,8 @@ HnRenderPass::EXECUTE_RESULT HnRenderPass::Execute(HnRenderPassState& RPState, c
         FlushPendingDraws();
     }
 
+    RPState.UpdateStats(State.Stats);
+
     return m_UseFallbackPSO ? EXECUTE_RESULT_FALLBACK : EXECUTE_RESULT_OK;
 }
 
@@ -1613,6 +1617,8 @@ void HnRenderPass::RenderPendingDrawItems(RenderState& State)
         item_idx += PendingItem.DrawCount;
     }
 
+    State.Stats.NumDrawItems += static_cast<Uint32>(m_PendingDrawItems.size());
+
     m_PendingDrawItems.clear();
 }
 

Hydrogent/src/HnRenderPassState.cpp

@@ -239,6 +239,7 @@ void HnRenderPassState::Begin(Uint32        NumRenderTargets,
     m_ClearDepth = ClearDepth;
 
     m_IsCommited = false;
+    m_Stats      = {};
 }
 
 } // namespace USD

Hydrogent/src/Tasks/HnBeginOITPassTask.cpp

@@ -103,6 +103,9 @@ void HnBeginOITPassTask::Prepare(pxr::HdTaskContext* TaskCtx,
     const USD_Renderer& Renderer = *RenderDelegate->GetUSDRenderer();
     VERIFY_EXPR(Renderer.GetSettings().OITLayerCount > 0);
 
+    // If no translucent draw items were rendered in the last frame, we can skip clearing the layers.
+    m_OITLayersCleared = static_cast<bool>(FrameTargets->OIT) && m_RenderPassState.GetStats().NumDrawItems == 0;
+
     if (!FrameTargets->OIT)
     {
         FrameTargets->OIT = Renderer.CreateOITResources(ColorTargetDesc.Width, ColorTargetDesc.Height);
@@ -229,13 +232,15 @@ void HnBeginOITPassTask::Execute(pxr::HdTaskContext* TaskCtx)
         }
         Renderer.CreateRWOITLayersSRB(m_FrameTargets->OIT.Layers, pDepthSRV, &RWLayersSRB);
     }
-    const TextureDesc& OITTailDesc = m_FrameTargets->OIT.Tail->GetDesc();
-    Renderer.ClearOITLayers(pCtx, m_ClearLayersSRB, OITTailDesc.Width, OITTailDesc.Height);
+    if (!m_OITLayersCleared)
+    {
+        const TextureDesc& OITTailDesc = m_FrameTargets->OIT.Tail->GetDesc();
+        Renderer.ClearOITLayers(pCtx, m_ClearLayersSRB, OITTailDesc.Width, OITTailDesc.Height);
+    }
 
     IShaderResourceBinding* pFrameAttribsSRB = RenderDelegate->GetFrameAttribsSRB(HnRenderDelegate::FrameAttribsSRBType::Opaque);
     m_RenderPassState.SetFrameAttribsSRB(pFrameAttribsSRB);
     m_RenderPassState.SetRWOITLayersSRB(RWLayersSRB);
-    m_RenderPassState.Commit(pCtx);
 }
 
 } // namespace USD

Hydrogent/src/Tasks/HnEndOITPassTask.cpp

@@ -30,6 +30,7 @@
 #include "HnTokens.hpp"
 #include "HnFrameRenderTargets.hpp"
 #include "HnRenderParam.hpp"
+#include "HnRenderPassState.hpp"
 
 namespace Diligent
 {
@@ -164,7 +165,12 @@ void HnEndOITPassTask::Execute(pxr::HdTaskContext* TaskCtx)
         };
     pCtx->TransitionResourceStates(_countof(Barriers), Barriers);
 
-    Renderer.ApplyOITAttenuation(pCtx, m_ApplyOITAttenuationPSO, m_ApplyOITAttenuationSRB);
+    HnRenderPassState* RenderPassState = GetRenderPassState(TaskCtx, HnRenderResourceTokens->renderPass_OITLayers);
+    VERIFY_EXPR(RenderPassState != nullptr);
+    if (RenderPassState != nullptr && RenderPassState->GetStats().NumDrawItems > 0)
+    {
+        Renderer.ApplyOITAttenuation(pCtx, m_ApplyOITAttenuationPSO, m_ApplyOITAttenuationSRB);
+    }
 }
 
 } // namespace USD