283 lines
9.6 KiB
Markdown
283 lines
9.6 KiB
Markdown
# Specialization Constants
|
|
|
|
|
|
|
|
In Vulkans, shaders are compiled to Spir-V, but the driver completes optimization during pipeline creation.
|
|
Having specialization constants in a shader, is like having #defines that can be changed when pipeline creation is submitted.
|
|
|
|
In this example, we will add three specialization constants with all possible permutations.
|
|
|
|
* USE_DIFFUSE: to add or not the diffuse contribution
|
|
* USE_SPECULAR: to add or not the specular contribution
|
|
* TRACE_SHADOW: to trace or not a shadow ray
|
|
|
|
## Shader
|
|
|
|
In the closest hit shader (`raytrace.chit`), we will add the three constants. Note that we are using `int` only because the tiny helper class later is made for `int` values.
|
|
|
|
~~~~ C
|
|
layout(constant_id = 0) const int USE_DIFFUSE = 1;
|
|
layout(constant_id = 1) const int USE_SPECULAR = 1;
|
|
layout(constant_id = 2) const int TRACE_SHADOW = 1;
|
|
~~~~
|
|
|
|
And later in the code, we branch based on the values. The modification of the shader will look like this.
|
|
|
|
~~~~ C
|
|
// Diffuse
|
|
vec3 diffuse = vec3(0);
|
|
if(USE_DIFFUSE == 1)
|
|
{
|
|
diffuse = computeDiffuse(mat, L, normal);
|
|
if(mat.textureId >= 0)
|
|
{
|
|
uint txtId = mat.textureId + scnDesc.i[gl_InstanceCustomIndexEXT].txtOffset;
|
|
vec2 texCoord = v0.texCoord * barycentrics.x + v1.texCoord * barycentrics.y
|
|
+ v2.texCoord * barycentrics.z;
|
|
diffuse *= texture(textureSamplers[nonuniformEXT(txtId)], texCoord).xyz;
|
|
}
|
|
}
|
|
|
|
vec3 specular = vec3(0);
|
|
float attenuation = 1;
|
|
|
|
// Tracing shadow ray only if the light is visible from the surface
|
|
if(dot(normal, L) > 0)
|
|
{
|
|
if(TRACE_SHADOW == 1)
|
|
{
|
|
float tMin = 0.001;
|
|
float tMax = lightDistance;
|
|
vec3 origin = gl_WorldRayOriginEXT + gl_WorldRayDirectionEXT * gl_HitTEXT;
|
|
vec3 rayDir = L;
|
|
uint flags = gl_RayFlagsTerminateOnFirstHitEXT | gl_RayFlagsOpaqueEXT
|
|
| gl_RayFlagsSkipClosestHitShaderEXT;
|
|
isShadowed = true;
|
|
traceRayEXT(topLevelAS, // acceleration structure
|
|
flags, // rayFlags
|
|
0xFF, // cullMask
|
|
0, // sbtRecordOffset
|
|
0, // sbtRecordStride
|
|
1, // missIndex
|
|
origin, // ray origin
|
|
tMin, // ray min range
|
|
rayDir, // ray direction
|
|
tMax, // ray max range
|
|
1 // payload (location = 1)
|
|
);
|
|
}
|
|
else
|
|
isShadowed = false;
|
|
|
|
if(isShadowed)
|
|
{
|
|
attenuation = 0.3;
|
|
}
|
|
else
|
|
{
|
|
// Specular
|
|
if(USE_SPECULAR == 1)
|
|
{
|
|
specular = computeSpecular(mat, gl_WorldRayDirectionEXT, L, normal);
|
|
}
|
|
}
|
|
}
|
|
~~~~
|
|
|
|
Running the example will not change anything, since by default it will do everything as it usually does.
|
|
|
|
## Pipeline
|
|
|
|
The specialization constants must be attached to their stages described by `VkPipelineShaderStageCreateInfo` structs.
|
|
See [specialization contants reference](https://www.khronos.org/registry/vulkan/specs/1.1-khr-extensions/html/chap10.html#pipelines-specialization-constants)
|
|
|
|
In our example, we will have only integers for constant data. There are various ways to do this, but here is the helper class we will be using to add as many constant IDs and values as we want. We will add specialization constants by calling this class's add function with a constant ID and a value, or with a vector of pairs of IDs and values.
|
|
|
|
~~~~ C
|
|
//////////////////////////////////////////////////////////////////////////
|
|
// Helper to generate specialization info
|
|
//
|
|
class Specialization
|
|
{
|
|
public:
|
|
void add(uint32_t constantID, int32_t value)
|
|
{
|
|
spec_values.push_back(value);
|
|
VkSpecializationMapEntry entry;
|
|
entry.constantID = constantID;
|
|
entry.size = sizeof(int32_t);
|
|
entry.offset = static_cast<uint32_t>(spec_entries.size() * sizeof(int32_t));
|
|
spec_entries.emplace_back(entry);
|
|
}
|
|
|
|
void add(const std::vector<std::pair<uint32_t, int32_t>>& const_values)
|
|
{
|
|
for(const auto& v : const_values)
|
|
add(v.first, v.second);
|
|
}
|
|
|
|
VkSpecializationInfo* getSpecialization()
|
|
{
|
|
spec_info.dataSize = static_cast<uint32_t>(spec_values.size() * sizeof(int32_t));
|
|
spec_info.pData = spec_values.data();
|
|
spec_info.mapEntryCount = static_cast<uint32_t>(spec_entries.size());
|
|
spec_info.pMapEntries = spec_entries.data();
|
|
return &spec_info;
|
|
}
|
|
|
|
private:
|
|
std::vector<int32_t> spec_values;
|
|
std::vector<VkSpecializationMapEntry> spec_entries;
|
|
VkSpecializationInfo spec_info;
|
|
};
|
|
~~~~
|
|
|
|
In `HelloVulkan::createRtPipeline()`, we will create 8 specialization of the closest hit shader.
|
|
So the number of stages, will be 11 instead of 4.
|
|
|
|
~~~~ C
|
|
enum StageIndices
|
|
{
|
|
eRaygen,
|
|
eMiss,
|
|
eMiss2,
|
|
eClosestHit, // <---- 8 specialization of this one
|
|
eShaderGroupCount = 11
|
|
};
|
|
~~~~
|
|
|
|
Then create a `Specialization` for each of the 8 on/off permutations of the 3 constants.
|
|
|
|
~~~~ C
|
|
// Specialization
|
|
std::vector<Specialization> specializations(8);
|
|
for(int i = 0; i < 8; i++)
|
|
{
|
|
int a = ((i >> 2) % 2) == 1;
|
|
int b = ((i >> 1) % 2) == 1;
|
|
int c = ((i >> 0) % 2) == 1;
|
|
specializations[i].add({{0, a}, {1, b}, {2, c}});
|
|
}
|
|
~~~~
|
|
|
|
Now the shader group will be created 8 times, each with a different specialization.
|
|
|
|
~~~~ C
|
|
// Hit Group - Closest Hit
|
|
// Create many variation of the closest hit
|
|
for(uint32_t s = 0; s < (uint32_t)specializations.size(); s++)
|
|
{
|
|
stage.module = nvvk::createShaderModule(m_device, nvh::loadFile("spv/raytrace.rchit.spv", true, defaultSearchPaths, true));
|
|
stage.stage = VK_SHADER_STAGE_CLOSEST_HIT_BIT_KHR;
|
|
stage.pSpecializationInfo = specializations[s].getSpecialization();
|
|
stages[eClosestHit + s] = stage;
|
|
}
|
|
~~~~
|
|
|
|
**Tip** : We can avoid to create 8 shader modules, but we would have to properly deal with the
|
|
deletion of them at the end of the function.
|
|
|
|
|
|
We will also modify the creation of the hit group to create as many HIT shader groups as we have specializations. This will give us the ability later to choose which 'specialization' we want to use.
|
|
|
|
~~~~ C
|
|
// Hit Group - Closest Hit + AnyHit
|
|
// Creating many Hit groups, one for each specialization
|
|
for(uint32_t s = 0; s < (uint32_t)specializations.size(); s++)
|
|
{
|
|
group.type = VK_RAY_TRACING_SHADER_GROUP_TYPE_TRIANGLES_HIT_GROUP_KHR;
|
|
group.generalShader = VK_SHADER_UNUSED_KHR;
|
|
group.closestHitShader = eClosestHit + s; // Using variation of the closest hit
|
|
m_rtShaderGroups.push_back(group);
|
|
}
|
|
~~~~
|
|
|
|
**Note**, it is important that the data and structures are not created on the stack inside the loop,
|
|
because we are passing the data address and specialization information, so all this would become
|
|
invalid when the pipeline is created.
|
|
|
|
# Using Specialization
|
|
|
|
If you would run the sample, nothing would have changed. This is because each TLAS's `hitGroupId` is set to `0`.
|
|
A quick test would be to change the value to `4`, corresponding to only using diffuse.
|
|
|
|
~~~~ C
|
|
rayInst.hitGroupId = 4; // We will use the same hit group for all objects
|
|
~~~~
|
|
|
|
Knowing the type of material each object is using, it would be possible to choose the appropriate
|
|
specialization for each object.
|
|
|
|
## Interactive Change
|
|
|
|
In our example, we will allow to choose globally the specialization for all objects. To do this, we will add
|
|
a new entry to the push constants structure, for both `ObjPushConstant` and `RtPushConstant`.
|
|
|
|
At the end of both structures, add
|
|
|
|
~~~~ C
|
|
int specialization{7}; // All in use
|
|
~~~~
|
|
|
|
In `raytrace.rgen`, we will use this new value to offset the hit group. Instead of always taking the hit group 0, it will
|
|
use the one we choose.
|
|
|
|
Add the `specialization` to the push constant layout.
|
|
|
|
~~~~ C
|
|
layout(push_constant) uniform Constants
|
|
{
|
|
vec4 clearColor;
|
|
vec3 lightPosition;
|
|
float lightIntensity;
|
|
int lightType;
|
|
int specialization;
|
|
}
|
|
pushC;
|
|
~~~~
|
|
|
|
Then where we trace, we will use the specialization value to change the SBT offset.
|
|
|
|
~~~~ C
|
|
traceRayEXT(topLevelAS, // acceleration structure
|
|
rayFlags, // rayFlags
|
|
0xFF, // cullMask
|
|
pushC.specialization, // sbtRecordOffset
|
|
0, // sbtRecordStride
|
|
0, // missIndex
|
|
origin.xyz, // ray origin
|
|
tMin, // ray min range
|
|
direction.xyz, // ray direction
|
|
tMax, // ray max range
|
|
0 // payload (location = 0)
|
|
);
|
|
~~~~
|
|
|
|
Now we only need UI to change interactively the value.
|
|
|
|
In main.cpp `renderUI()`, add the following code.
|
|
|
|
~~~~ C
|
|
// Specialization
|
|
ImGui::SliderInt("Specialization", &helloVk.m_pushConstant.specialization, 0, 7);
|
|
int s = helloVk.m_pushConstant.specialization;
|
|
int a = ((s >> 2) % 2) == 1;
|
|
int b = ((s >> 1) % 2) == 1;
|
|
int c = ((s >> 0) % 2) == 1;
|
|
ImGui::Checkbox("Use Diffuse", (bool*)&a);
|
|
ImGui::Checkbox("Use Specular", (bool*)&b);
|
|
ImGui::Checkbox("Trace shadow", (bool*)&c);
|
|
helloVk.m_pushConstant.specialization = (a << 2) + (b << 1) + c;
|
|
~~~~
|
|
|
|
|
|
|
|
|
|
|
|
|
|
## References
|
|
|
|
* Pipelines [Specialization Constants](https://www.khronos.org/registry/vulkan/specs/1.1-khr-extensions/html/chap10.html#pipelines-specialization-constants)
|
|
* [VkSpecializationInfo](https://www.khronos.org/registry/vulkan/specs/1.2-extensions/man/html/VkSpecializationInfo.html)
|
|
* [VkSpecializationMapEntry](https://www.khronos.org/registry/vulkan/specs/1.2-extensions/man/html/VkSpecializationMapEntry.html)
|
|
|