feat: LOD 网格分级——meshopt 解析期简化 + 按距离/级联选级

vendored meshoptimizer 1.2（submodule 环境故障改直接 vendor）。Mesh
多 LOD：各级索引连续拼进同一 index buffer（共享顶点，LodRange 区间
选择），解析期 meshopt_simplify 生成链（LOD0=原始，每级目标减半，
相对误差 1e-2，小网格复用上级）；都在 worker 线程，加载零额外卡顿。

SceneRenderer 按 (mesh, LOD) 聚合 batch（lookup key = 指针<<8|lod）：
主视图按屏占比近似 radius/dist 选级（阈值 10/20 倍半径）；阴影级联
按级联序降级（越远越简）。Draw 按 batch.lod 选索引区间。

A/B 对比（100 背包 10×10，RTX 5060 Laptop，vsync off）：
- 纯背包：LOD 开 93-95 fps / 关 90-107 fps —— 无显著差异
- 加 SunTemple：LOD 开 52-56 / 关 51-55 —— 无显著差异
结论：当前场景瓶颈不在顶点数（5060 顶点吞吐富余），LOD 基础设施
保留（几何复杂资产/低端 GPU/更大场景生效），帧率问题转向 Tracy
GPU 时间线定位真实瓶颈。

坑：LodRange 记录"LOD 不足复用最后一级"时 first 不得继续累加
（buffer 无重复数据，累加必越界——VUID-08798 抓的）。

Engine/src/Renderer/Mesh.cpp

@@ -17,13 +17,13 @@ namespace Engine
 {
     Mesh::Mesh(Device& device, MemoryAllocator& allocator,
                const std::vector<Vertex>& vertices,
-               const std::vector<u32>& indices,
+               const std::vector<std::vector<u32>>& lods,
                Material* material)
         : m_Material(material)
     {
-        if (vertices.empty() || indices.empty())
+        if (vertices.empty() || lods.empty() || lods[0].empty())
         {
-            ENGINE_ERROR("Mesh: empty vertices or indices");
+            ENGINE_ERROR("Mesh: empty vertices or LOD0 indices");
             return;
         }
 
@@ -35,15 +35,31 @@ namespace Engine
             vk::MemoryPropertyFlagBits::eHostCoherent);
         m_VertexBuffer->Upload(vertices.data(), vbSize);
 
-        u64 ibSize = sizeof(u32) * indices.size();
+        // 各级索引连续拼进同一 index buffer（共享顶点，只换索引区间）
+        std::vector<u32> allIndices;
+        for (const auto& idx : lods) {
+            allIndices.insert(allIndices.end(), idx.begin(), idx.end());
+        }
+
+        u64 ibSize = sizeof(u32) * allIndices.size();
         m_IndexBuffer = CreateScope<Buffer>(
             device, allocator, ibSize,
             vk::BufferUsageFlagBits::eIndexBuffer,
             vk::MemoryPropertyFlagBits::eHostVisible |
             vk::MemoryPropertyFlagBits::eHostCoherent);
-        m_IndexBuffer->Upload(indices.data(), ibSize);
+        m_IndexBuffer->Upload(allIndices.data(), ibSize);
 
-        m_IndexCount = static_cast<u32>(indices.size());
+        // 记录各级区间。LOD 不足的级复用最后一级的"已有区间"
+        //（first 不得继续累加——buffer 里没有重复数据，累加必越界）
+        u32 first = 0;
+        for (u32 lod = 0; lod < kMeshLodCount; ++lod) {
+            if (lod < lods.size()) {
+                m_Lods[lod] = {first, static_cast<u32>(lods[lod].size())};
+                first += m_Lods[lod].indexCount;
+            } else {
+                m_Lods[lod] = m_Lods[lods.size() - 1];
+            }
+        }
 
         // 局部空间包围球：顶点包围盒 → 中心 + 最大半径（视锥剔除用）
         {
@@ -61,10 +77,12 @@ namespace Engine
             }
         }
 
-        ENGINE_INFO("Mesh created: {} vertices, {} indices", vertices.size(), indices.size());
+        ENGINE_INFO("Mesh created: {} vertices, LOD0 {} indices", vertices.size(), lods[0].size());
 
         m_VertexBytes = sizeof(Vertex) * vertices.size();
-        m_IndexBytes = sizeof(u32) * indices.size();
+        for (const auto& idx : lods) {
+            m_IndexBytes += sizeof(u32) * idx.size();
+        }
         STATS.AddMesh(m_VertexBytes, m_IndexBytes);
     }
 
@@ -73,10 +91,14 @@ namespace Engine
         STATS.RemoveMesh(m_VertexBytes, m_IndexBytes);
     }
 
-    void Mesh::Draw(vk::CommandBuffer cmd, u32 instanceCount, u32 firstInstance) const
+    void Mesh::Draw(vk::CommandBuffer cmd, u32 instanceCount, u32 firstInstance,
+                    u32 lod) const
     {
+        const LodRange& range =
+            m_Lods[lod < kMeshLodCount ? lod : kMeshLodCount - 1];
         cmd.bindVertexBuffers(0, m_VertexBuffer->Get(), {0});
         cmd.bindIndexBuffer(m_IndexBuffer->Get(), 0, vk::IndexType::eUint32);
-        cmd.drawIndexed(m_IndexCount, instanceCount, 0, 0, firstInstance);
+        cmd.drawIndexed(range.indexCount, instanceCount, range.firstIndex,
+                        0, firstInstance);
     }
 } // namespace Engine

Engine/src/Renderer/Model.cpp

@@ -14,6 +14,8 @@
 #include <assimp/scene.h>
 #include <assimp/postprocess.h>
 
+#include <meshoptimizer.h>
+
 #include <filesystem>
 #include <algorithm>
 
@@ -81,13 +83,14 @@ Model::Model(Device& device, MemoryAllocator& allocator,
         {{ h, 0.0f, -h}, {0, 1, 0}, {0.0f, 0.0f}},
     };
     const std::vector<u32> indices = {0, 1, 2, 0, 2, 3};
+    const std::vector<std::vector<u32>> lodIndices = {indices};
 
     // 材质/纹理生命周期随模型（缓存持有）
     std::string paths[Material::kSlotCount] = {"", "", "", ""};
     Material* mat = GetOrCreateMaterial(paths, MaterialParams{});   // 全默认材质
 
     m_Meshes.push_back(CreateScope<Mesh>(
-        m_Device, m_Allocator, vertices, indices, mat));
+        m_Device, m_Allocator, vertices, lodIndices, mat));
 }
 
 Scope<Model> Model::CreatePlane(Device& device, MemoryAllocator& allocator,
@@ -158,7 +161,7 @@ Scope<Model> Model::FromParsed(Device& device, MemoryAllocator& allocator,
     for (auto& pm : parsed.meshes) {
         Material* mat = model->GetOrCreateMaterial(pm.texPaths.data(), pm.params);
         model->m_Meshes.push_back(CreateScope<Mesh>(
-            device, allocator, std::move(pm.vertices), std::move(pm.indices), mat));
+            device, allocator, pm.vertices, pm.lods, mat));
     }
     ENGINE_INFO("Model loaded: {} ({} meshes, {} textures)",
                 parsed.path, model->m_Meshes.size(), model->m_TextureCache.size());
@@ -177,7 +180,7 @@ void Model::LoadModel(const std::string& path)
     for (auto& pm : parsed.meshes) {
         Material* mat = GetOrCreateMaterial(pm.texPaths.data(), pm.params);
         m_Meshes.push_back(CreateScope<Mesh>(
-            m_Device, m_Allocator, std::move(pm.vertices), std::move(pm.indices), mat));
+            m_Device, m_Allocator, pm.vertices, pm.lods, mat));
     }
     ENGINE_INFO("Model loaded: {} ({} meshes, {} textures)",
                 path, m_Meshes.size(), m_TextureCache.size());
@@ -256,7 +259,7 @@ ParsedMesh Model::ProcessMesh(const aiMesh* mesh, const aiScene* scene,
 {
     ParsedMesh out;
     std::vector<Vertex>& vertices = out.vertices;
-    std::vector<u32>& indices = out.indices;
+    std::vector<u32> indices;
 
     vertices.reserve(mesh->mNumVertices);
 
@@ -386,6 +389,32 @@ ParsedMesh Model::ProcessMesh(const aiMesh* mesh, const aiScene* scene,
         out.texPaths[slot] = std::move(texPaths[slot]);
     }
     out.params = params;
+
+    // ---- LOD 链（解析期生成，worker 线程免费时间）：----
+    // LOD0 = 原始；每级目标索引数减半，相对误差 1e-2。顶点共享，
+    // 简化只重排索引（meshopt_simplify 从 LOD0 单次起点逐级向下）
+    out.lods.push_back(std::move(indices));
+    for (u32 lod = 1; lod < kMeshLodCount; ++lod) {
+        const size_t target = out.lods[0].size() >> lod;
+        // 太小的网格不简化：简化收益低于误差风险，直接复用上一级
+        if (target < 96 || out.lods[0].size() < 192) {
+            out.lods.push_back(out.lods.back());
+            continue;
+        }
+        std::vector<u32> dest(out.lods[0].size());
+        float error = 0.0f;
+        const size_t count = meshopt_simplify(
+            dest.data(),
+            out.lods[0].data(), out.lods[0].size(),
+            &out.vertices[0].position.x, out.vertices.size(), sizeof(Vertex),
+            target, 1e-2f, 0, &error);
+        if (count == 0) {
+            out.lods.push_back(out.lods.back());   // 简化失败兜底
+            continue;
+        }
+        dest.resize(count);
+        out.lods.push_back(std::move(dest));
+    }
     return out;
 }
 

Engine/src/Renderer/Passes/ForwardPass.cpp

@@ -139,7 +139,7 @@ namespace Engine::Passes
                 cmd.bindDescriptorSets(vk::PipelineBindPoint::eGraphics,
                                        m_Gpu.forwardLayout, 1,
                                        batch.mesh->GetMaterial()->GetDescriptorSet(), {});
-                batch.mesh->Draw(cmd, batch.count, batch.first);
+                batch.mesh->Draw(cmd, batch.count, batch.first, batch.lod);
             }
 
             cmd.endRendering();

Engine/src/Renderer/Passes/ShadowPass.cpp

@@ -101,7 +101,7 @@ namespace Engine::Passes
                                   &m_Frame.ubo.lightSpace[c]);
 
                 for (const auto& batch : m_Frame.shadowCascades[c].batches) {
-                    batch.mesh->Draw(cmd, batch.count, batch.first);
+                    batch.mesh->Draw(cmd, batch.count, batch.first, batch.lod);
                 }
 
                 cmd.endRendering();

Engine/src/Renderer/SceneRenderer.cpp

@@ -297,7 +297,19 @@ namespace Engine
                         glm::vec3(t.World() * glm::vec4(bounds.center, 1.0f));
                     const f32 worldRadius = bounds.radius * worldScale;
 
-                    auto tryAppend = [&](ViewScratch& s, const Frustum& frustum) {
+                    // LOD 选级：屏占比近似 = radius/距离（主视图按实例距离；
+                    // 阈值 10/20 倍半径，可调）
+                    const f32 dist =
+                        glm::length(worldCenter - camTransform->GetPosition());
+                    u32 mainLod = 0;
+                    if (dist >= worldRadius * 20.0f) {
+                        mainLod = 2;
+                    } else if (dist >= worldRadius * 10.0f) {
+                        mainLod = 1;
+                    }
+
+                    auto tryAppend = [&](ViewScratch& s, const Frustum& frustum,
+                                         u32 lod) {
                         if (!frustum.IntersectsSphere(worldCenter, worldRadius)) {
                             return;
                         }
@@ -309,10 +321,12 @@ namespace Engine
                         // 单实体时两序号恰好恒等，多实体共享 mesh 后分叉
                         // → batches[worldsIdx] 越界）。batch.first（GPU
                         // firstInstance 基址）才是 worlds 索引，语义别混
+                        const u64 key =
+                            (reinterpret_cast<u64>(mesh.get()) << 8) | lod;
                         const auto [it, inserted] = s.lookup.emplace(
-                            mesh.get(), static_cast<u32>(s.result.batches.size()));
+                            key, static_cast<u32>(s.result.batches.size()));
                         if (inserted) {
-                            s.result.batches.push_back({mesh.get(), 0, 0});
+                            s.result.batches.push_back({mesh.get(), 0, 0, lod});
                             s.batchWorlds.emplace_back();
                             s.batchPrevWorlds.emplace_back();
                         }
@@ -326,9 +340,12 @@ namespace Engine
                         ++totalInstances;
                     };
 
-                    tryAppend(m_MainScratch, mainFrustum);
+                    tryAppend(m_MainScratch, mainFrustum, mainLod);
                     for (u32 c = 0; c < kMaxCascades; ++c) {
-                        tryAppend(m_CascScratch[c], cascFrustum[c]);
+                        // 级联降级：越远的级联覆盖范围越大，用越低的 LOD
+                        //（阴影本来就是低频信息，误差可接受）
+                        tryAppend(m_CascScratch[c], cascFrustum[c],
+                                  std::min(c, kMeshLodCount - 1));
                     }
                 }
             });