Vulkan-Sample-Note-Push-Constants
Vulkan Sample Note: Push constants
The sample is from Sascha Williems
TL;DR
- Using push constants to pass a small bit of static data to a shader
- Perfect for passing e.g. static per-object data or parameters without the need for descriptor sets
- The spec only requires a minimum of 128 bytes, so for passing larger blocks of data you’d use UBOs or SSBOs
Notes
The sample renders 16 different color and position spheres via pushing costant data.
struct SpherePushConstantData {
glm::vec4 color;
glm::vec4 position;
};
std::array<SpherePushConstantData, 16> spheres;Resources
setupDescriptorSetLayout
We create a VkPushConstantRange for pushing constants
void setupDescriptorSetLayout()
{
std::vector<VkDescriptorSetLayoutBinding> setLayoutBindings = {
// Binding 0 : Vertex shader uniform buffer
vks::initializers::descriptorSetLayoutBinding(
VK_DESCRIPTOR_TYPE_UNIFORM_BUFFER,
VK_SHADER_STAGE_VERTEX_BIT, 0),
};
VkDescriptorSetLayoutCreateInfo descriptorLayout = vks::initializers::descriptorSetLayoutCreateInfo(setLayoutBindings);
VK_CHECK_RESULT(vkCreateDescriptorSetLayout(device, &descriptorLayout, nullptr, &descriptorSetLayout));
// Define the push constant range used by the pipeline layout
// Note that the spec only requires a minimum of 128 bytes, so for passing larger blocks of data you'd use UBOs or SSBOs
VkPushConstantRange pushConstantRange{};
pushConstantRange.stageFlags = VK_SHADER_STAGE_VERTEX_BIT;
pushConstantRange.offset = 0;
pushConstantRange.size = sizeof(SpherePushConstantData);
VkPipelineLayoutCreateInfo pipelineLayoutCreateInfo = vks::initializers::pipelineLayoutCreateInfo(&descriptorSetLayout, 1);
pipelineLayoutCreateInfo.pushConstantRangeCount = 1;
pipelineLayoutCreateInfo.pPushConstantRanges = &pushConstantRange;
VK_CHECK_RESULT(vkCreatePipelineLayout(device, &pipelineLayoutCreateInfo, nullptr, &pipelineLayout));
}buildCommandBuffers
void buildCommandBuffers()
{
// Set target frame buffer
...
// [POI] Render the spheres passing color and position via push constants
uint32_t spherecount = static_cast<uint32_t>(spheres.size());
for (uint32_t j = 0; j < spherecount; j++) {
// [POI] Pass static sphere data as push constants
vkCmdPushConstants(
drawCmdBuffers[i],
pipelineLayout,
VK_SHADER_STAGE_VERTEX_BIT,
0,
sizeof(SpherePushConstantData),
&spheres[j]);
model.draw(drawCmdBuffers[i]);
}
...
}