From 8fdbf885a8c688ade0ad7ce429863971ddfe57d4 Mon Sep 17 00:00:00 2001
From: assiduous <assiduous@diligentgraphics.com>
Date: Sat, 9 Nov 2024 09:29:18 -0800
Subject: [PATCH] Hydrogent: use line strip for mesh edges

---
 Hydrogent/include/HnMeshUtils.hpp |  4 ++-
 Hydrogent/src/HnMesh.cpp          |  4 +--
 Hydrogent/src/HnMeshUtils.cpp     | 56 +++++++++++++++++++++----------
 Hydrogent/src/HnRenderPass.cpp    |  2 +-
 4 files changed, 45 insertions(+), 21 deletions(-)

diff --git a/Hydrogent/include/HnMeshUtils.hpp b/Hydrogent/include/HnMeshUtils.hpp
index fc5224c2..6bd43243 100644
--- a/Hydrogent/include/HnMeshUtils.hpp
+++ b/Hydrogent/include/HnMeshUtils.hpp
@@ -82,6 +82,8 @@ class HnMeshUtils final
     /// Computes the edge indices.
     ///
     /// \param[in] UseFaceVertexIndices - Whether to use face vertex indices.
+    /// \param[in] UseLineStrip         - Whether to use line strip topology.
+    ///                                   If false, line list will be used.
     /// \return The edge indices.
     ///
     /// Example:
@@ -102,7 +104,7 @@ class HnMeshUtils final
     ///
     ///         UseFaceVertexIndices == true
     /// 		    EdgeIndices = {0, 1,  1, 2,  2, 3,  3, 0,  3, 2,  2, 4,  4, 5,  5, 3}
-    pxr::VtVec2iArray ComputeEdgeIndices(bool UseFaceVertexIndices) const;
+    pxr::VtIntArray ComputeEdgeIndices(bool UseFaceVertexIndices, bool UseLineStrip = false) const;
 
 
     /// Computes the point indices.
diff --git a/Hydrogent/src/HnMesh.cpp b/Hydrogent/src/HnMesh.cpp
index e18be0f3..ba8481de 100644
--- a/Hydrogent/src/HnMesh.cpp
+++ b/Hydrogent/src/HnMesh.cpp
@@ -320,7 +320,7 @@ void HnMesh::Invalidate()
 struct HnMesh::StagingIndexData
 {
     pxr::VtVec3iArray FaceIndices;
-    pxr::VtVec2iArray EdgeIndices;
+    pxr::VtIntArray   EdgeIndices;
     pxr::VtIntArray   PointIndices;
 };
 
@@ -1085,7 +1085,7 @@ void HnMesh::UpdateIndexData(StagingIndexData& StagingInds, const pxr::VtValue&
         m_IndexData.Subsets.clear();
     }
 
-    StagingInds.EdgeIndices  = MeshUtils.ComputeEdgeIndices(!m_HasFaceVaryingPrimvars);
+    StagingInds.EdgeIndices  = MeshUtils.ComputeEdgeIndices(!m_HasFaceVaryingPrimvars, true);
     StagingInds.PointIndices = MeshUtils.ComputePointIndices(m_HasFaceVaryingPrimvars);
 }
 
diff --git a/Hydrogent/src/HnMeshUtils.cpp b/Hydrogent/src/HnMeshUtils.cpp
index 1541d3b4..dc5766b6 100644
--- a/Hydrogent/src/HnMeshUtils.cpp
+++ b/Hydrogent/src/HnMeshUtils.cpp
@@ -233,36 +233,58 @@ void HnMeshUtils::Triangulate(bool                UseFaceVertexIndices,
     }
 }
 
-pxr::VtVec2iArray HnMeshUtils::ComputeEdgeIndices(bool UseFaceVertexIndices) const
+pxr::VtIntArray HnMeshUtils::ComputeEdgeIndices(bool UseFaceVertexIndices, bool UseLineStrip) const
 {
     size_t NumEdges = 0;
+    size_t NumFaces = 0; // Count the actual number of faces
     ProcessFaces(
         [&](size_t FaceId, int StartVertex, int VertCount) {
             NumEdges += VertCount;
+            ++NumFaces;
         });
 
-    pxr::VtVec2iArray EdgeIndices;
-    EdgeIndices.reserve(NumEdges);
-
-    ProcessFaces(
-        [&](size_t FaceId, int StartVertex, int VertCount) {
-            for (int v = 0; v < VertCount - 1; ++v)
-                EdgeIndices.push_back({StartVertex + v, StartVertex + (v + 1)});
-            EdgeIndices.push_back({StartVertex + VertCount - 1, StartVertex});
-        });
-    VERIFY_EXPR(EdgeIndices.size() == NumEdges);
+    pxr::VtIntArray EdgeIndices;
+    if (UseLineStrip)
+    {
+        EdgeIndices.reserve(NumEdges + NumFaces * 2);
+        ProcessFaces(
+            [&](size_t FaceId, int StartVertex, int VertCount) {
+                for (int v = 0; v < VertCount; ++v)
+                {
+                    EdgeIndices.push_back(StartVertex + v);
+                }
+                EdgeIndices.push_back(StartVertex);
+                EdgeIndices.push_back(-1);
+            });
+        VERIFY_EXPR(EdgeIndices.size() == NumEdges + NumFaces * 2);
+    }
+    else
+    {
+        EdgeIndices.reserve(NumEdges * 2);
+        ProcessFaces(
+            [&](size_t FaceId, int StartVertex, int VertCount) {
+                for (int v = 0; v < VertCount - 1; ++v)
+                {
+                    EdgeIndices.push_back(StartVertex + v);
+                    EdgeIndices.push_back(StartVertex + (v + 1));
+                }
+                EdgeIndices.push_back(StartVertex + VertCount - 1);
+                EdgeIndices.push_back(StartVertex);
+            });
+        VERIFY_EXPR(EdgeIndices.size() == NumEdges * 2);
+    }
 
     if (UseFaceVertexIndices)
     {
         const int*   FaceVertexIndices = m_Topology.GetFaceVertexIndices().cdata();
         const size_t NumVertexIndices  = m_Topology.GetFaceVertexIndices().size();
-        for (pxr::GfVec2i& Edge : EdgeIndices)
+        for (int& Idx : EdgeIndices)
         {
-            int& Idx0 = Edge[0];
-            int& Idx1 = Edge[1];
-            VERIFY_EXPR(Idx0 < NumVertexIndices && Idx1 < NumVertexIndices);
-            Idx0 = FaceVertexIndices[Idx0];
-            Idx1 = FaceVertexIndices[Idx1];
+            if (Idx < 0)
+                continue;
+
+            VERIFY_EXPR(Idx < NumVertexIndices);
+            Idx = FaceVertexIndices[Idx];
         }
     }
 
diff --git a/Hydrogent/src/HnRenderPass.cpp b/Hydrogent/src/HnRenderPass.cpp
index 2d654e5f..a5bdd435 100644
--- a/Hydrogent/src/HnRenderPass.cpp
+++ b/Hydrogent/src/HnRenderPass.cpp
@@ -237,7 +237,7 @@ GraphicsPipelineDesc HnRenderPass::GetGraphicsDesc(const HnRenderPassState& RPSt
             break;
 
         case HN_RENDER_MODE_MESH_EDGES:
-            GraphicsDesc.PrimitiveTopology = PRIMITIVE_TOPOLOGY_LINE_LIST;
+            GraphicsDesc.PrimitiveTopology = PRIMITIVE_TOPOLOGY_LINE_STRIP;
             break;
 
         case HN_RENDER_MODE_POINTS: