完善体积云渲染：添加旋转和参数优化

新增功能：
- 添加云的旋转动画（绕 Y 轴）
- 修复 HDDA 在旋转坐标系中的坐标转换问题

参数优化：
- tmax: 1000 → 5000（修复远处云消失的问题）
- MaxSteps: 500 → 2000
- StepSize: 0.5 → 2.0
- DensityScale: 0.05 → 0.1
- 相机位置调整到 z=-1200

已知问题：
- 缺少光照计算（phase_function 和 volumetric_shadow）
- 渲染结果比 Unity 版本更亮

Res/Shader/volume.hlsl

@@ -11,6 +11,7 @@ cbuffer CB0 : register(b1)
     float _StepSize;
     float3 _BBoxMax;
     uint _MaxSteps;
+    float _RotationY;
 };
 
 StructuredBuffer<uint> buf : register(t1);
@@ -113,7 +114,7 @@ float4 MainPS(PSInput input) : SV_TARGET
     float3 rayDir = normalize(input.worldPos - _CameraPos);
     
     float tmin = 0.01;
-    float tmax = 1000.0;
+    float tmax = 5000.0;
     
     NanoVolume volume;
     initVolume(volume);
@@ -121,8 +122,19 @@ float4 MainPS(PSInput input) : SV_TARGET
     float3 color = float3(0, 0, 0);
     float transmittance = 1.0;
     
+    float cosR = cos(_RotationY);
+    float sinR = sin(_RotationY);
+    float3x3 invRotY = float3x3(
+        cosR, 0, sinR,
+        0, 1, 0,
+        -sinR, 0, cosR
+    );
+    
+    float3 localCameraPos = mul(invRotY, _CameraPos);
+    float3 localRayDir = mul(invRotY, rayDir);
+    
     float not_used;
-    bool hit = get_hdda_hit(volume.acc, tmin, _CameraPos, rayDir, tmax, not_used);
+    bool hit = get_hdda_hit(volume.acc, tmin, localCameraPos, localRayDir, tmax, not_used);
     if (!hit) { return float4(0, 0, 0, 0); }
     
     float skip = 0;
@@ -130,9 +142,9 @@ float4 MainPS(PSInput input) : SV_TARGET
     for (uint i = 0; i < _MaxSteps; i++) {
         if (tmin >= tmax || transmittance < 0.01) break;
         
-        float3 worldPos = _CameraPos + rayDir * tmin;
+        float3 localPos = localCameraPos + localRayDir * tmin;
         
-        uint dim = get_dim_coord(volume.acc, worldPos);
+        uint dim = get_dim_coord(volume.acc, localPos);
         if (dim > 1) {
             float skip_step = 15.0;
             tmin += skip_step;
@@ -140,7 +152,7 @@ float4 MainPS(PSInput input) : SV_TARGET
             continue;
         }
         
-        float density = get_value_coord(volume.acc, worldPos) * _DensityScale;
+        float density = get_value_coord(volume.acc, localPos) * _DensityScale;
         
         if (density < 0.001) {
             float skip_step = 5.0;

main.cpp

@@ -176,7 +176,7 @@ int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPWSTR lpCmdLi
 	DirectX::XMMATRIX projectionMatrix=DirectX::XMMatrixPerspectiveFovLH(
 		(45.0f*3.141592f)/180.0f,1280.0f/720.0f,0.1f,1000.0f);
 	DirectX::XMMATRIX viewMatrix = DirectX::XMMatrixLookAtLH(
-		DirectX::XMVectorSet(-10.0f, 73.0f, -800.0f, 1.0f),
+		DirectX::XMVectorSet(-10.0f, 73.0f, -1200.0f, 1.0f),
 		DirectX::XMVectorSet(-10.0f, 73.0f, 0.0f, 1.0f),
 		DirectX::XMVectorSet(0.0f, 1.0f, 0.0f, 1.0f));
 	DirectX::XMMATRIX modelMatrix = DirectX::XMMatrixTranslation(0.0f,0.0f,5.0f);
@@ -294,17 +294,18 @@ int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPWSTR lpCmdLi
 			memcpy(volumeCBData, &invViewProjMat, sizeof(float) * 16);
 			volumeCBData[16] = -10.0f;
 			volumeCBData[17] = 73.0f;
-			volumeCBData[18] = -800.0f;
-			volumeCBData[19] = 0.05f;  // DensityScale
+			volumeCBData[18] = -1200.0f;
+			volumeCBData[19] = 0.1f;  // DensityScale
 			volumeCBData[20] = (float)nanoVDBData.worldBBox[0];
 			volumeCBData[21] = (float)nanoVDBData.worldBBox[1];
 			volumeCBData[22] = (float)nanoVDBData.worldBBox[2];
-			volumeCBData[23] = 0.5f;  // StepSize
+			volumeCBData[23] = 2.0f;  // StepSize
 			volumeCBData[24] = (float)nanoVDBData.worldBBox[3];
 			volumeCBData[25] = (float)nanoVDBData.worldBBox[4];
 			volumeCBData[26] = (float)nanoVDBData.worldBBox[5];
-			volumeCBData[27] = 500;
-			UpdateConstantBuffer(volumeCB, volumeCBData, sizeof(float) * 28);
+			volumeCBData[27] = 2000;
+			volumeCBData[28] = timeSinceAppStartInSecond * 0.3f;  // RotationY
+			UpdateConstantBuffer(volumeCB, volumeCBData, sizeof(float) * 29);
 			if (frameCount == 1) {
 				printf("Volume BBox: [%.2f, %.2f, %.2f] - [%.2f, %.2f, %.2f]\n",
 					volumeCBData[20], volumeCBData[21], volumeCBData[22],