Vulkan-Sample-Note-Dynamic-Uniform-Buffers
Vulkan Sample Note: Dynamic uniform buffers
The sample is from Sascha Williems
TL;DR
- Allocate aligned large uniform buffer for multiple objects, and access each uniform buffer by indexing
- Avoid creating multiple same uniform buffer to reduce overhead
Notes
Uniform buffer dynamic
When set up descriptor set layout, need to use special descriptor type VK_DESCRIPTOR_TYPE_UNIFORM_BUFFER_DYNAMIC during binding.
Declare bug chunk of dynamic uniform buffer
// One big uniform buffer that contains all matrices
// Note that we need to manually allocate the data to cope for GPU-specific uniform buffer offset alignments
struct UboDataDynamic {
glm::mat4 *model = nullptr;
} uboDataDynamic;
// Allocate data for the dynamic uniform buffer object
// We allocate this manually as the alignment of the offset differs between GPUs
// Calculate required alignment based on minimum device offset alignment
size_t minUboAlignment = vulkanDevice->properties.limits.minUniformBufferOffsetAlignment;
dynamicAlignment = sizeof(glm::mat4);
if (minUboAlignment > 0) {
dynamicAlignment = (dynamicAlignment + minUboAlignment - 1) & ~(minUboAlignment - 1);
}
size_t bufferSize = OBJECT_INSTANCES * dynamicAlignment;
uboDataDynamic.model = (glm::mat4*)alignedAlloc(bufferSize, dynamicAlignment);
assert(uboDataDynamic.model);Aligned memory allocation
Normally, we can use std::aligned_alloc in C++17.
Allocate size bytes of uninitialized storage whose alignment is specified by alignment
In Windows, it could be used _aligned_malloc, otherwise posix_memalign.
Calculate the correct alignment
// Calculate required alignment based on minimum device offset alignment
size_t minUboAlignment = vulkanDevice->properties.limits.minUniformBufferOffsetAlignment;
dynamicAlignment = sizeof(glm::mat4);
if (minUboAlignment > 0) {
dynamicAlignment = (dynamicAlignment + minUboAlignment - 1) & ~(minUboAlignment - 1);
}Since the minUboAlignment dynamicAlignment will be power of 2, the operation dynamicAlignment = (dynamicAlignment + minUboAlignment - 1) & ~(minUboAlignment - 1); acts like max(dynamicAlignment, minUboAlignment ).