所有现代 API 现在都支持光线追踪。它可在 Windows（D3D12 和 VK）、Linux (VK) 和 macOS (Metal) 上使用。D3D12 和 VK 光线追踪没有区别。Metal API 仅支持光线查询，D3D12 和 VK API 也支持光线查询。主要的光线追踪概念是为几何体创建 BLAS 并将所有几何体组合在单个 TLAS 内。司机对引擎盖下的一切负责。
通过简单的修改，我们的测试应用程序就可以在光线追踪 (RT)、光线查询 (RQ) 和计算着色器 (CS) 渲染模式下工作。我们将渲染 490K 三角形的 81 个相同实例。但这一次，根本不会进行光栅化。每个像素将始终追踪主光线、阴影光线和反射光线。
简单的计算着色器光线查询模式将使用我们的通用 GPU BVH 构建器，每个叶子分区有一个三角形。每个BVH节点消耗12字节。整个模型需要 45 MB BVH 缓冲区。对于光栅化，该模型需要 13 MB 用于 32 字节顶点和 32 位索引缓冲区。
我们先看一下1600×900分辨率的结果：
| 实时D3D12 | RT VK | RQ D3D12 | RQVK | 需求量MTL | 质量控制系统 | BLAS尺寸 | BLAS构建 | |
|---|---|---|---|---|---|---|---|---|
| 英伟达精视3080 | 0.58毫秒 | 0.54 毫秒 | 0.60毫秒 | 0.55毫秒 | 4.92 毫秒 | 33 MB (15 MB) | 7 毫秒（18 毫秒） | |
| GeForce 2080 钛 | 1.06 毫秒 | 1.03毫秒 | 0.95毫秒 | 0.97 毫秒 | 7.35 毫秒 | 33 MB (15 MB) | 7 毫秒（18 毫秒） | |
| 英伟达精视1060M | 34.59 毫秒 | 33.81 毫秒 | 43 MB (37 MB) | 7 毫秒（18 毫秒） | ||||
| Radeon 6700 XT | 2.50 毫秒 | 2.55 毫秒 | 1.71 毫秒 | 2.06 毫秒 | 6.13 毫秒 | 76 MB (65 MB) | 15 毫秒（> 500 毫秒） | |
| Radeon Vega 56 (macOS) | 15.84 毫秒 | 15.48 毫秒 | 82MB | 30毫秒 | ||||
| 苹果 M1 (macOS) | 48.8 毫秒 | 46.4 毫秒 | 82MB | 100毫秒 | ||||
| 苹果 A14（iOS） | 98.0 毫秒 | 94.3 毫秒 | 82MB | |||||
| 肾上腺素 660（安卓） | 431 毫秒 |
- Nvidia RTX 系列 GPU 具有最佳性能，Ray 查询速度稍快。最棒的是，压缩后的 BLAS 大小仅为 15 MB，这比光栅化的模型表示大几 MB。计算着色器光线追踪比硬件加速慢 7.7 倍。
- AMD 光线查询 (D3D12) 和光线追踪 (VK) 之间存在 50% 的差异。看起来该驱动程序仅针对 D3D12 光线查询进行了优化。硬件光线追踪比计算着色器实现快 3.5 倍。但主要问题是 BLAS 的大小比其所能容纳的大四倍多。此外，即使启用了快速构建标志，第一代 BLAS 的速度也慢得令人痛苦。
- 有趣的事实：计算着色器光线追踪在 AMD 上比在 Nvidia 上更快。
- Metal 上没有硬件光线追踪。当前的实现比简单的计算着色器慢。BLAS 还需要两倍的内存量。有些三角形不相交，因此模型在所有硬件上渲染期间都有孔。
- 被动冷却的 Apple M1 仅比 GeForce 1060M GPU 慢 33%。
- 顶点缓存优化也提高了光线追踪性能。
现在，原生 4K 怎么样？
| 实时D3D12 | RT VK | RQ D3D12 | RQVK | 质量控制系统 | |
|---|---|---|---|---|---|
| 英伟达精视3080 | 3.16 毫秒 | 3.13 毫秒 | 3.25毫秒 | 3.13 毫秒 | 21.00 毫秒 |
| GeForce 2080 钛 | 4.78 毫秒 | 4.77 毫秒 | 4.92 毫秒 | 4.86 毫秒 | 32.21 毫秒 |
| Radeon 6700 XT | 12.89 毫秒 | 13.35 毫秒 | 8.48 毫秒 | 9.77 毫秒 | 27.94 毫秒 |
- AMD 显示光线查询和光线追踪管道之间存在 65% 的差异。AMD 的选择是显而易见的。
- 对于 Nvidia 来说，使用光线追踪比光线查询更好，但差异只有 3%。
- 最好的 Radeon 6700 XT 光线查询性能比 GeForce 2080 Ti 慢 70%。最大差距为280%。
有趣的是，拥有更多光线追踪核心的 AMD 将如何表现。AMD 的主要问题是加速结构的大小及其构建时间。特别是对于具有数千个实例的动态 TLAS。
这是计算着色器光线追踪的图像和热图图像。红色通道表示按 128 缩放的 BLAS BVH 相交步骤数。计算着色器光线追踪的主要瓶颈是 Ray-BVH 相交，由于发散和分散的内存访问，无法很好地优化该相交。相比之下，体积-BVH 交集在计算着色器上表现出色。但不幸的是，我们无法将硬件（API）光线追踪加速结构重复用于不同的目的。
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