Ray-tracing: Extended features and dynamic objects

The source for this sample can be found in the Khronos Vulkan samples github repository.

This code sample demonstrates how to incorporate animations into a ray-traced scene, and shows how to incorporate different types of changing objects within the acceleration structures.

Acceleration structures

The ray tracing acceleration structures are separated into two types: bottom-level acceleration structures (BLAS) and top-level acceleration structures (TLAS). The BLAS contains information about each object’s geometry within its own coordinate system and is built using the vertex and index data stored in a GPU buffer. In contrast, the TLAS contains information about each instance of the geometry and its transformation (i.e. scaling, rotation, translation, etc.).

Each object must be represented in the BLAS, but can have any number of instances, each with its own transformation. This allows objects to be replicated without creating an acceleration structure for each instance.

Objects: Static, moving, and changing

There are three categories of objects to consider when building acceleration structures: static, moving, and changing geometry. Static geometry includes scene data. In this code sample, the Sponza scene has a single, non-moving instance. In contrast, dynamic objects can have a changing transformation, changing geometry, or both. An example of transformation-only dynamic objects in this code sample are given by the flame particle effect, which is achieved by adjusting only the location and rotation of a square billboard — the internal geometry (and thus the billboard’s BLAS) does not change. In contrast, the refraction effect is achieved by changing both the internal geometry each frame, and the rotation (so that it faces the viewer).

Vulkan offers methods of optimizing the acceleration structures for each type of geometry. The VkAccelerationStructureBuildGeometryInfoKHR struct has flags that can either toggle "fast trace", which optimizes run-time performance at the expense of build time, or "fast build", which optimizes build time. When constructing large, static objects such as the Sponza scene, for instance, the "fast trace" bit (VK_BUILD_ACCELERATION_STRUCTURE_PREFER_FAST_TRACE_BIT_KHR) is selected because the build will occur once and the model contains many points. When constructing dynamic objects such as the refraction model, which will need a BLAS update every frame, the "fast build" bit (VK_BUILD_ACCELERATION_STRUCTURE_PREFER_FAST_BUILD_BIT_KHR) is selected.

Further optimization methods can be used. For instance, the refraction model is updated every frame by the CPU and thus uses host-visible memory. However, because host-visible memory can incur a performance penalty, the Sponza and billboard models use a staging buffer to copy to device-exclusive memory. An alternative method would be to use a "compute shader" to generate the refraction model each frame, but that is outside the scope of this tutorial.

Reference Object Data from a Closest-Hit Shader

Though the ray-tracing pipeline uses an acceleration structure to traverse the scene’s geometry, the acceleration structures themselves do not store user-defined information about the geometry and instead give the developer the flexibility to define their own custom geometry information. This information can be encoded at the per-instance level, per-object level, or per-primitive level.

Per-instance level: The top-level acceleration structure allows instance information to encode a custom ID ( per-instance level).

Per-object level: In this code sample, this custom ID then references a struct at the per-object level containing the object ID , the index of the vertices in the vertex buffer, and the index of the (triangle) indices in the index buffer:

struct SceneInstanceData
    uint32_t vertex_index;
    uint32_t indices_index;
    uint32_t image_index;
    uint32_t object_type;

Per-primitive level In this sample, each vertex is encoded with a per-vertex normal and texture coordinate, though other applications may wish to provide other information at the per-vertex level. To allow the bottom-level acceleration structure to reference geometry data with a custom-defined layout, the VkAccelerationStructureGeometryKHR provides the ability to set geometry offsets and strides (i.e. vertexStride). In the code below, the struct acceleration_structure_geometry of type VkAccelerationStructureGeometryKHR references the data layout provided by NewVertex, which encodes the normal and texture coordinate:

acceleration_structure_geometry.geometry.triangles.vertexData    = vertex_data_device_address;
acceleration_structure_geometry.geometry.triangles.maxVertex     = model_buffer.num_vertices;
acceleration_structure_geometry.geometry.triangles.vertexStride  = sizeof(NewVertex);
acceleration_structure_geometry.geometry.triangles.indexType     = VK_INDEX_TYPE_UINT32;
acceleration_structure_geometry.geometry.triangles.indexData     = index_data_device_address;
acceleration_structure_geometry.geometry.triangles.transformData = transform_matrix_device_address;

This technique allows the closest-hit shader to access pre-calculated vertex information.

Texture Binding and Shaders

In a traditional raster pipeline, it is possible to render each object separately and bind its appropriate texture images during that pass. However, in a ray-tracing pipeline, each ray during a render pass could intersect with many objects within the scene, and thus all textures must be available to the shader. In this code sample, an array of textures (Sampler2D[]) is bound, and each object is associated with a given texture index. The texture ID information is stored in the object data.

Ambient Occlusion and Ray-Traced Shadows

This code sample explores two different ways to calculate lighting: ray-traced shadows and ambient occlusion, both of which are updated each frame and are triggered when a primary ray intersects a scene object (i.e. an element of the Sponza scene).

Ray-traced shadows are calculated by performing a test: a ray is shot from the object point in the direction of the light. If the returned distance is less than the distance to the light source, then the object point is in a shadow. In pseudocode:

direction = object_pt - light_pt
dist = trace_ray(object_pt, direction)
if (dist < distance(object_pt, light_pt)):
    color.rgb *= 0.2

The ambient occlusion effect is used to simulate the light diminishing effect of clustered geometry. It’s simulated by tracing rays distributed about a hemisphere centered at the intersection point with the object’s normal. The light-diminishing effect is estimated using the distance to the nearest ray intersection. In some implementations, a hard threshold is used. In pseudocode:

for theta,phi in angles:
    hard_threshold = 10.f
    direction = hemisphere_pt(object_pt, normal, theta, phi)
    dist = trace_ray(object_pt, direction)
    if (dist < hard_threshold):
        color.rgb *= 0.2

The code sample in this tutorial instead linearly interpolates up to the hard_threshold:

color.rgb *= min(dist, hard_threshold) / min_threshold

There are further optimizations that can be used. One common technique is to reduce the number of generated ambient occlusion rays at each point, often shooting just a single ray. The resulting image can then be de-noised using a separate de-noising pass, though this technique is outside the scope of this tutorial.