fixed GPU Lidar rendering issue

unreal/Blocks/Plugins/ProjectAirSim/Source/ProjectAirSim/Private/Sensors/LidarIntensitySceneViewExtension.cpp

@@ -5,6 +5,7 @@
 #include "SceneView.h"
 #include "RenderGraph.h"
 #include "ScreenPass.h"
+#include "RenderGraphUtils.h"
 #include "CommonRenderResources.h"
 #include "Containers/DynamicRHIResourceArray.h"
 #include "Engine/World.h"
@@ -116,7 +117,12 @@ FLidarIntensitySceneViewExtension::FLidarIntensitySceneViewExtension(
     const FAutoRegister& AutoRegister,
     TWeakObjectPtr<UTextureRenderTarget2D> InRenderTarget2D)
     : FSceneViewExtensionBase(AutoRegister),
-      RenderTarget2D(InRenderTarget2D) {}
+      RenderTarget2D(InRenderTarget2D) {
+  for (int i = 0; i < NumReadbackBuffers; ++i) {
+    ReadbackBuffers[i] = MakeUnique<FRHIGPUBufferReadback>(FName(*FString::Printf(TEXT("LidarReadback_%d"), i)));
+    ReadbackBuffersSizes[i] = 0;
+  }
+}
 
 void FLidarIntensitySceneViewExtension::PrePostProcessPass_RenderThread(
     FRDGBuilder& GraphBuilder, const FSceneView& View,
@@ -245,14 +251,40 @@ void FLidarIntensitySceneViewExtension::PrePostProcessPass_RenderThread(
       cachedParams.NumCams * cachedParams.HorizontalResolution * cachedParams.LaserNums;
   uint32 BufferSize = NumPoints * sizeof(float) * 4;
 
+  // Readback from previous frames if available
+  auto& CurrentReadback = ReadbackBuffers[CurrentReadbackIndex];
+  uint32 CurrentSize = ReadbackBuffersSizes[CurrentReadbackIndex];
+
+  if (CurrentSize > 0 && CurrentReadback && CurrentReadback->IsReady()) {
+      void* BufferData = CurrentReadback->Lock(CurrentSize);
+      if (BufferData) {
+          // Resize vector to match the data we are reading back
+          const int NumPointsRead = CurrentSize / (sizeof(float) * 4);
+          if (LidarPointCloudData.size() != NumPointsRead) {
+            LidarPointCloudData.resize(NumPointsRead);
+          }
+
+          FMemory::Memcpy(LidarPointCloudData.data(), BufferData, CurrentSize);
+          CurrentReadback->Unlock();
+      }
+  }
+
+  // Guard: nothing to dispatch if NumPoints is zero.
+  if (NumPoints == 0) {
+    return;
+  }
+
+  // Create an RDG structured buffer for the compute shader output.
+  // We do NOT upload initial data here (no nullptr crash); the shader writes
+  // every slot, and AddClearUAVFloatPass zeroes any slots it misses.
   FRDGBufferRef PointCloudBufferRDG =
-      CreateStructuredBuffer(GraphBuilder,
-                             TEXT("FLidarPointCloudCS_PointCloudBuffer"),
-                             sizeof(float), NumPoints * 4, nullptr,
-                             BufferSize);
+      GraphBuilder.CreateBuffer(
+          FRDGBufferDesc::CreateStructuredDesc(sizeof(float), NumPoints * 4),
+          TEXT("FLidarPointCloudCS_PointCloudBuffer"));
 
-  FRDGBufferUAVRef PointCloudBufferUAV = GraphBuilder.CreateUAV(
-      PointCloudBufferRDG, PF_FloatRGBA, ERDGUnorderedAccessViewFlags::None);
+  // Structured UAV — no pixel format; must match RWStructuredBuffer<float> in HLSL.
+  FRDGBufferUAVRef PointCloudBufferUAV = GraphBuilder.CreateUAV(PointCloudBufferRDG);
+  AddClearUAVFloatPass(GraphBuilder, PointCloudBufferUAV, -1.0f);
 
   FLidarPointCloudCS::FParameters* PassParameters =
       GraphBuilder.AllocParameters<FLidarPointCloudCS::FParameters>();
@@ -284,12 +316,17 @@ void FLidarIntensitySceneViewExtension::PrePostProcessPass_RenderThread(
       NUM_THREADS_PER_GROUP_DIMENSION_Y,
       NUM_THREADS_PER_GROUP_DIMENSION_Z);
 
-  LidarPointCloudData =
-      std::vector<FVector4>(NumPoints, FVector4(-1, -1, -1, -1));
-
   FComputeShaderUtils::AddPass(
       GraphBuilder, RDG_EVENT_NAME("LidarPointCloud Pass"),
       LidarPointCloudShader, PassParameters, GroupContext);
+
+  AddEnqueueCopyPass(GraphBuilder, CurrentReadback.Get(), PointCloudBufferRDG, BufferSize);
+
+  // Store the size for the next time we encounter this buffer slot
+  ReadbackBuffersSizes[CurrentReadbackIndex] = BufferSize;
+
+  // Advance index for next frame
+  CurrentReadbackIndex = (CurrentReadbackIndex + 1) % NumReadbackBuffers;
 }
 
 void FLidarIntensitySceneViewExtension::UpdateParameters(

unreal/Blocks/Plugins/ProjectAirSim/Source/ProjectAirSim/Private/Sensors/LidarIntensitySceneViewExtension.h

@@ -2,6 +2,7 @@
 
 #include "RHI.h"
 #include "RHIResources.h"
+#include "RHIGPUReadback.h"
 #include "SceneViewExtension.h"
 
 #include "LidarPointCloudCS.h"
@@ -45,6 +46,11 @@ class FLidarIntensitySceneViewExtension : public FSceneViewExtensionBase {
   std::vector<FVector4> LidarPointCloudData;
 
 private:
+  static const int32 NumReadbackBuffers = 4;
+  TUniquePtr<FRHIGPUBufferReadback> ReadbackBuffers[NumReadbackBuffers];
+  uint32 ReadbackBuffersSizes[NumReadbackBuffers];
+  int32 CurrentReadbackIndex = 0;
+
   std::queue<FLidarPointCloudCSParameters> CSParamsQ;
   TWeakObjectPtr<UTextureRenderTarget2D> RenderTarget2D;
 };