VK_KHR_vertex_attribute_divisor.proposal

Promoted from VK_EXT_vertex_attribute_divisor, this extension allows instance-rate vertex attributes to be repeated for certain number of instances instead of advancing for every instance when instanced rendering is enabled.

Problem Statement

In other APIs such as OpenGL, it is possible to use the same values of vertex attributes for a number of successive instances when instanced rendering is enabled. This is commonly referred to as "vertex attribute divisor". For example, if the divisor is N, the same vertex attribute will be applied to N successive instances before moving on to the next vertex attribute. A value of 0 allows the same vertex attribute to be applied to all instances.

This functionality was originally introduced in Vulkan with the VK_EXT_vertex_attribute_divisor extension.

Proposal

The API in VK_EXT_vertex_attribute_divisor is promoted to KHR with the following exception:

The VkPhysicalDeviceVertexAttributeDivisorPropertiesKHR struct contains an additional property supportsNonZeroFirstInstance, compared to VkPhysicalDeviceVertexAttributeDivisorPropertiesEXT.

The vertex attribute divisor can be specified with:

typedef struct VkVertexInputBindingDivisorDescriptionKHR {
    uint32_t    binding;
    uint32_t    divisor;
} VkVertexInputBindingDivisorDescriptionKHR;

which specifies a divisor value for the given vertex binding. divisor is the number of successive instances that will use the same value of the vertex attribute when instanced rendering is enabled. For example, if the divisor is N, the same vertex attribute will be applied to N successive instances before moving on to the next vertex attribute.

The above is specified once per vertex binding in:

typedef struct VkPipelineVertexInputDivisorStateCreateInfoKHR {
    VkStructureType                                     sType;
    const void*                                         pNext;
    uint32_t                                            vertexBindingDivisorCount;
    const VkVertexInputBindingDivisorDescriptionKHR*    pVertexBindingDivisors;
} VkPipelineVertexInputDivisorStateCreateInfoKHR;

The maximum allowed divisor value can be queried with the following struct when chained to VkPhysicalDeviceProperties2:

typedef struct VkPhysicalDeviceVertexAttributeDivisorPropertiesKHR {
    VkStructureType    sType;
    void*              pNext;
    uint32_t           maxVertexAttribDivisor;
    VkBool32           supportsNonZeroFirstInstance;
} VkPhysicalDeviceVertexAttributeDivisorPropertiesKHR;

In the above, supportsNonZeroFirstInstance specifies whether a non-zero value for the firstInstance parameter of drawing commands is supported when VkVertexInputBindingDivisorDescriptionKHR::divisor (or the equivalent dynamically set state) is not 1. Note that this property is new compared to VkPhysicalDeviceVertexAttributeDivisorPropertiesEXT.

The VK_EXT_vertex_input_dynamic_state and VK_EXT_shader_object extensions provide the vkCmdSetVertexInputEXT function, allowing the vertex attribute divisor state to be dynamically set.

The functionality of this extension is controlled by the following features:

typedef struct VkPhysicalDeviceVertexAttributeDivisorFeaturesKHR {
    VkStructureType    sType;
    void*              pNext;
    VkBool32           vertexAttributeInstanceRateDivisor;
    VkBool32           vertexAttributeInstanceRateZeroDivisor;
} VkPhysicalDeviceVertexAttributeDivisorFeaturesKHR;

• vertexAttributeInstanceRateDivisor specifies whether the functionality in the extension is supported.
• vertexAttributeInstanceRateZeroDivisor specifies whether a zero value for VkVertexInputBindingDivisorDescriptionKHR::divisor is supported.

Examples

The vertex attribute divisor can be specified when creating a graphics pipeline as such:

const VkVertexInputBindingDivisorDescriptionKHR divisorDesc =
{
    .binding = 0,
    .divisor = 4,
};

const VkPipelineVertexInputDivisorStateCreateInfoKHR divisorInfo =
{
    .sType = VK_STRUCTURE_TYPE_PIPELINE_VERTEX_INPUT_DIVISOR_STATE_CREATE_INFO_KHR,
    .vertexBindingDivisorCount = 1,
    .pVertexBindingDivisors = &divisorDesc,
}

const VkVertexInputBindingDescription binding =
{
    .binding = 0,
    .stride = sizeof(Vertex),
    .inputRate = VK_VERTEX_INPUT_RATE_INSTANCE,
};

const VkPipelineVertexInputStateCreateInfo viInfo =
{
    .sType = VK_STRUCTURE_TYPE_PIPELINE_VERTEX_INPUT_CREATE_INFO,
    .pNext = &divisorInfo,
    ...
};

Issues

1) What is the effect of a non-zero value for firstInstance?

RESOLVED: The Vulkan API should follow the OpenGL convention and offset attribute fetching by firstInstance while computing vertex attribute offsets.