permalink: /Notes/004-3d-rendering/vulkan/chapters/descriptor_dynamic_offset.html ---

Descriptor Dynamic Offset

Vulkan offers two types of descriptors that allow adjusting the offset at bind time as defined in the spec.

  • dynamic uniform buffer (VK_DESCRIPTOR_TYPE_UNIFORM_BUFFER_DYNAMIC)

  • dynamic storage buffer (VK_DESCRIPTOR_TYPE_STORAGE_BUFFER_DYNAMIC)

1. Example

This example will have buffer of 32 bytes and 16 of the bytes will be set at vkUpdateDescriptorSets time. In this first example, we will not add any dynamic offset.

VkDescriptorSet descriptorSet; // allocated
VkBuffer buffer; // size of 32 bytes

VkDescriptorBufferInfo bufferInfo = {
    buffer,
    4,      // offset
    16      // range
};

VkWriteDescriptorSet writeInfo = {
    .dstSet = descriptorSet,
    .descriptorType = VK_DESCRIPTOR_TYPE_STORAGE_BUFFER_DYNAMIC,
    .pBufferInfo = bufferInfo
};

vkUpdateDescriptorSets(
    1,         // descriptorWriteCount,
    &writeInfo // pDescriptorWrites,
);

// No dynamic offset
vkCmdBindDescriptorSets(
    1,              // descriptorSetCount,
    &descriptorSet, // pDescriptorSets,
    0,              // dynamicOffsetCount
    NULL            // pDynamicOffsets
);

Our buffer now currently looks like the following:

descriptor_dynamic_offset_example_a.png

Next, a 8 byte dynamic offset will applied at bind time.

uint32_t offsets[1] = { 8 };
vkCmdBindDescriptorSets(
    1,              // descriptorSetCount,
    &descriptorSet, // pDescriptorSets,
    1,              // dynamicOffsetCount
    offsets         // pDynamicOffsets
);

Our buffer currently looks like the following:

descriptor_dynamic_offset_example_b.png

2. Example with VK_WHOLE_SIZE

This time the VK_WHOLE_SIZE value will be used for the range. Everything looks the same as the above example except the VkDescriptorBufferInfo::range

VkDescriptorSet descriptorSet; // allocated
VkBuffer buffer; // size of 32 bytes

VkDescriptorBufferInfo info = {
    buffer,
    4,             // offset
    VK_WHOLE_SIZE  // range
};

VkWriteDescriptorSet writeInfo = {
    .dstSet = descriptorSet,
    .descriptorType = VK_DESCRIPTOR_TYPE_STORAGE_BUFFER_DYNAMIC,
    .pBufferInfo = bufferInfo
};

vkUpdateDescriptorSets(
    1,         // descriptorWriteCount,
    &writeInfo // pDescriptorWrites,
);

// No dynamic offset
vkCmdBindDescriptorSets(
    1,              // descriptorSetCount,
    &descriptorSet, // pDescriptorSets,
    0,              // dynamicOffsetCount
    NULL            // pDynamicOffsets
);

Our buffer currently looks like the following:

descriptor_dynamic_offset_example_c.png

This time, if we attempt to apply a dynamic offset it will be met with undefined behavior and the validation layers will give an error

// Invalid
uint32_t offsets[1] = { 8 };
vkCmdBindDescriptorSets(
    1,              // descriptorSetCount,
    &descriptorSet, // pDescriptorSets,
    1,              // dynamicOffsetCount
    offsets         // pDynamicOffsets
);

This is what it looks like with the invalid dynamic offset

descriptor_dynamic_offset_example_d.png

3. Limits

It is important to also check the minUniformBufferOffsetAlignment and minStorageBufferOffsetAlignment as both the base offset and dynamic offset must be multiples of these limits.