From 0dcaff90dba50200c8df6dfb1663d5020fc3272e Mon Sep 17 00:00:00 2001
From: rune stubbe <Rune.Stubbe@epicgames.com>
Date: Thu, 11 Mar 2021 10:07:26 -0400
Subject: [PATCH] Fixed GeometryCollections having uninitialized vertex color
 when the source mesh has no colors. Fixed issue in BitStreamWriter_Writer
 when NumBits=32. #jira UE-110362 #rb graham.wihlidal

#ROBOMERGE-OWNER: rune.stubbe
#ROBOMERGE-AUTHOR: rune.stubbe
#ROBOMERGE-SOURCE: CL 15675611 in //UE5/Release-5.0-EarlyAccess/...
#ROBOMERGE-BOT: STARSHIP (Release-5.0-EarlyAccess -> Main) (v781-15675533)
#ROBOMERGE-CONFLICT from-shelf

[CL 15676219 by rune stubbe in ue5-main branch]
---
 .../GeometryCollection/GeometryCollectionConversion.cpp  | 4 ++++
 Engine/Shaders/Private/BitPacking.ush                    | 9 ++++++++-
 .../Developer/NaniteBuilder/Private/NaniteBuilder.cpp    | 2 +-
 3 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/Engine/Plugins/Experimental/GeometryCollectionPlugin/Source/GeometryCollectionEditor/Private/GeometryCollection/GeometryCollectionConversion.cpp b/Engine/Plugins/Experimental/GeometryCollectionPlugin/Source/GeometryCollectionEditor/Private/GeometryCollection/GeometryCollectionConversion.cpp
index 5e7e70bd49e2..4ec9e94dad93 100644
--- a/Engine/Plugins/Experimental/GeometryCollectionPlugin/Source/GeometryCollectionEditor/Private/GeometryCollection/GeometryCollectionConversion.cpp
+++ b/Engine/Plugins/Experimental/GeometryCollectionPlugin/Source/GeometryCollectionEditor/Private/GeometryCollection/GeometryCollectionConversion.cpp
@@ -72,6 +72,8 @@ void FGeometryCollectionConversion::AppendStaticMesh(const UStaticMesh* StaticMe
 			UV[VertexOffset] = VertexBuffer.StaticMeshVertexBuffer.GetVertexUV(VertexIndex, 0);
 			if (VertexBuffer.ColorVertexBuffer.GetNumVertices() == VertexCount)
 				Color[VertexOffset] = VertexBuffer.ColorVertexBuffer.VertexColor(VertexIndex);
+			else
+				Color[VertexOffset] = FLinearColor::White;
 		}
 
 		// Triangle Indices
@@ -654,6 +656,8 @@ void FGeometryCollectionConversion::AppendSkeletalMesh(const USkeletalMesh* Skel
 							UV[VertexOffset] = VertexBuffers.StaticMeshVertexBuffer.GetVertexUV(VertexIndex, 0);
 							if (VertexBuffers.ColorVertexBuffer.GetNumVertices() == VertexCount)
 								Color[VertexOffset] = VertexBuffers.ColorVertexBuffer.VertexColor(VertexIndex);
+							else
+								Color[VertexOffset] = FLinearColor::White;
 						}
 
 						int32 InitialIndex = -1;
diff --git a/Engine/Shaders/Private/BitPacking.ush b/Engine/Shaders/Private/BitPacking.ush
index 7b44680054a3..a2424ccb5d39 100644
--- a/Engine/Shaders/Private/BitPacking.ush
+++ b/Engine/Shaders/Private/BitPacking.ush
@@ -278,6 +278,7 @@ void BitStreamWriter_Writer(inout FBitStreamWriterState State, uint Value, int N
 {
     State.BufferBits |= Value << State.BufferOffset;
 
+	// State.BufferOffset <= 31
     uint NextBufferOffset = State.BufferOffset + NumBits;
     
     if (NextBufferOffset >= 32)
@@ -285,7 +286,13 @@ void BitStreamWriter_Writer(inout FBitStreamWriterState State, uint Value, int N
         State.Output.InterlockedAnd(State.AlignedByteAddress, State.BufferMask);
         State.Output.InterlockedOr(State.AlignedByteAddress, State.BufferBits);
 		State.BufferMask = 0;
-        State.BufferBits = Value >> (32 - State.BufferOffset);
+		
+		// Shifts are mod 32, so we need special handling when shift could be >= 32.
+		// State.BufferOffset can only be 0 here if NumBits >= 32 and therefore CompileTimeMaxBits >= 32.
+		if(CompileTimeMaxBits >= 32)
+			State.BufferBits = State.BufferOffset ? (Value >> (32 - State.BufferOffset)) : 0u;
+		else
+			State.BufferBits = Value >> (32 - State.BufferOffset);
         State.AlignedByteAddress += 4;
     }
 
diff --git a/Engine/Source/Developer/NaniteBuilder/Private/NaniteBuilder.cpp b/Engine/Source/Developer/NaniteBuilder/Private/NaniteBuilder.cpp
index d07dc0c73871..155cec7485ea 100644
--- a/Engine/Source/Developer/NaniteBuilder/Private/NaniteBuilder.cpp
+++ b/Engine/Source/Developer/NaniteBuilder/Private/NaniteBuilder.cpp
@@ -24,7 +24,7 @@
 // differences, etc.) replace the version GUID below with a new one.
 // In case of merge conflicts with DDC versions, you MUST generate a new GUID
 // and set this new GUID as the version.
-#define NANITE_DERIVEDDATA_VER TEXT("3AAB0510-8F1D-59F4-B3C9-91FADBE10CB5")
+#define NANITE_DERIVEDDATA_VER TEXT("3ABB0516-8F1E-59F6-B3CA-91FDDBF10CB5")
 
 namespace Nanite
 {