|
Wwise SDK 2023.1.8
|
はじめに
レイトレイシングは、n次のリフレクション（反射）やディフラクション（回折）を効率的に評価するテクニックです。リスナーからランダムにキャストされたレイ（ray）を追って、リフレクションやディフラクションを続けて受けたときのパスを、確認するというのが基本的な考え方です。この技術の発想は、グラフィックレンダリング技術からきています。Maximum Reflection order [1-4] and diffraction order [1-8] values are defined in the AkSpatialAudioInitSettings structure of AkSpatialAudio.h. 詳細は AkSpatialAudioInitSettings を参照してください。
概念
- Primary rays: リスナーから直接キャストされたレイ
- Reflection: サウンドの、面からの跳ね返り
- Diffraction: サウンドの、物体周りの回折
- Paths: リスナーからエミッタまでの、一連のリフレクションまたはディフラクション
設定
In order to tweak spatial audio CPU performance and quality, the raytracing engine uses several parameters. For a detailed description of the spatial audio settings, see AkSpatialAudioInitSettings().
CPU Limit Mode
Tweaking the number of primary rays can be tricky as it depends on the complexity of the scene (for example, the number of triangles and diffraction edges) and the number of emitters. CPU Limit Modeを有効にすれば、Primary rayの数はレイトレイシングエンジンで自動調整され、CPU使用量はユーザーが事前に定義した目標値付近で維持されます。このモードを使うと、CPU使用量の急上昇は抑えられますが、単発で急なピークを完全に排除することはできません。 目標値を高く設定すると、品質（リフレクションパスやディフラクションパスの認知数）は上がりますが、パフォーマンスは下がってしまいます。一方、目標値を低く設定すると、パフォーマンスは向上しますが品質が犠牲になります。 When CPU Limit Mode is active, the number of primary rays is capped at the user-defined number of primary rays (see AkSpatialAudioInitSettings). When the number of primary rays drops to 0, the raytracing engine stops scanning the environment. ただし、すでにある音の伝播パスは、引き続き検証され更新されます。その結果、レイトレイシングエンジンによるCPUの消費は続きます。 Note that CPU Limit Mode doesn't have a significant effect on portal ray tracing. Portal ray tracing increases the CPU load and might therefore force the CPU Limit Mode to temporarily reduce the number of primary rays.
Load Balancing
Load balancing provides a way to flatten CPU peaks by distributing spatial audio task computation over several frames. Spatial audio executes different types of tasks, such as ray casting and path validation, to compute the sound propagation paths (reflections and diffraction). Whenever one of these tasks needs to be performed, it is placed in a priority queue for further processing instead of being executed immediately. The tasks in the queue are then executed over the number of frames specified in the load balancing spread setting.
Load Balancing Use Cases
There are several ways to tweak spatial audio performance: you can apply a CPU limit, change the order of reflection, enable or disable diffraction and diffraction on reflections, and adjust the motion threshold setting. You can combine load balancing with the other settings to achieve the desired balance between quality and performance.
Although load balancing can be used in any scenario, it is most appropriate for scenes that involve multiple active emitters. Load balancing behavior is controlled through the AkSpatialAudioInitSettings::uLoadBalancingSpread setting (see AkSpatialAudioInitSettings). A spread of 1 frame is equivalent to disabling the feature, because every task is executed immediately. Higher values increase CPU performance, but can lead to less accurate results. For example, a spread of 4 frames for a frame buffer of 1024 samples (around 21ms at 48kHz) executes the tasks over a period of 84 ms. Spread values under 10 should not noticeably affect quality.
Load Balancing Example
The following example compares the performance in the same scene with three different spread values: 1 (disable), 4, and 10. The scene has 944 triangles and 649 diffraction edges. The number of emitters is 4. Reflection order is set to 4 and diffraction is enabled.
The following table shows the effect of different load balancing values on the scene.
| Spread | Average CPU % | Max CPU % |
|---|---|---|
| Disabled | 4.8 | 52.76 |
| 4 | 3.16 | 23.26 |
| 10 | 1.71 | 10.22 |
Load Balancing FAQ
-
Does load balancing cause delays?
Not exactly. It can introduce a "delay" in updating path information, but does not add latency to the audio signal. For instance, the same rays might be used until the next update. The paths are still computed at each frame, but are not validated at each frame. This means that some paths might be less accurate. For instance, specular reflection positions might be slightly inaccurate.
-
Does the load balancing improve the average CPU performance?
Although load balancing primarily focuses on decreasing CPU peaks, it can also decrease CPU averages because it prevents the same type of task from appearing in the queue multiple times. If a task is in the queue, it must be executed before another task of the same type can be added to the queue.
-
Does load balancing improve CPU performance when there is only one emitter?
Not significantly. Path validation is the most CPU-intensive spatial audio task. Given that one path validation task is executed for each emitter, load balancing is more appropriate for situations that involve several emitters. However, it can slightly improve single-emitter scenarios by preventing redundant computations.
Load Balancing in Depth
Whenever a spatial audio task needs to be performed, it is placed in a priority queue for further processing instead of being executed immediately. If the task is already in the queue, it is not placed in the queue again. This prevents redundant computation and reduces CPU consumption. At each frame, the number of tasks to process (N) is computed based on the number of elements in the queue and the load balancing spread settings. The first N priority tasks are pulled from the queue and executed.
In general, task priority is determined by the time at which the task was created (inserted): older tasks are executed first. For some tasks, such as the path validation tasks, the priority might be different. Whenever an emitter becomes active, the path validation task priority is increased to force the validation of the new paths. This is important to prevent delays in path creation.
制限
レイトレイシングエンジンのためにジオメトリを定義する際に、考慮すべき制限事項がいくつかあります。パフォーマンスと結果の質の両方に関する制限事項です。
ジオメトリの見える角度
三角形がサンプリング密度よりも小さいと、それをレイトレイシングエンジンが見つけづらくなります。
ジオメトリの見える角度α（アルファ）とは、リスナー視点からジオメトリが見える角度のことです。Primary rayの数によって、2つのレイの間の平均的な角度 γ（ガンマ）は異なります。αとγの相対関係によって、オブジェクトとの交点（リフレクションまたはディフラクション）が見つかる確率が変わります。γがαより小さければ、交点が見つかる確率が上がります。γがαより大きければ、交点が見つかる確率が下がります。
三角形の個数
ジオメトリ内の三角形の数が、エンジンのCPU消費量と直接関係し、三角形が増えれば、それだけCPUの消費量が増えます。これは、オブジェクトに必要となる交点テストが増えるからです。一般的に音の伝播に高度なジオメトリは必要ありません。そこで三角形の数を減らせば、品質を犠牲にすることなくパフォーマンスを向上できます。
ジオメトリの形状
ジオメトリ形状は、処理しやすいものと、しにくいものがあります。一般的に、平面やボックスなどは簡単に処理でき、音の伝播の観点では一番良い結果を出します。球体や円柱は、エラーが発生しやすくなります。球体や円柱には曲面が伴うからです。場合によって、見落とされる回折エッジが出てしまい、そうすると一部の回折パスが抜けてしまいます。アルゴリズムにヒューリスティックをいくつか導入しているので、多くのケースでこの問題に対処できます。Primary rayの数を増やしたり、ジオメトリを簡略化したりすることでも、この問題を解消できます。
Performance Tweaking
The following table provides information you can use to tweak parameters for different use cases.
| Symptoms | Possible solutions |
|---|---|
| Spatial Audio - General CPU issues |
|
| Spatial Audio - Raytracing CPU is high |
|
| Spatial Audio - Path Validation CPU is high and there is only one emitter |
|
| Spatial Audio - Path Validation CPU is high and there are multiple emitters |
|
| Spatial Audio - CPU is high (path validation and ray tracing are both involved) and there is only one emitter |
|