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= Patch control point
ifdef::site-gen-antora[]
TIP: The source for this sample can be found in the https://github.com/KhronosGroup/Vulkan-Samples/tree/main/samples/extensions/patch_control_points[Khronos Vulkan samples github repository].
endif::[]
image::./images/patch_control_point_screenshot.png[Sample]
== Overview
This sample demonstrates how to use `VK_EXT_extended_dynamic_state2` extension, which eliminates the need to create multiple pipelines in case of specific different parameters.
This extension changes how *Patch Control Points* are managed.
Instead of static description during pipeline creation, this extension allows developers to change the parameter by using a function before every draw.
Below is a comparison of common Vulkan static and dynamic setting of patch control points in the tessellation.
|===
| Static/Non-dynamic | Dynamic State 2
| dynamic_state = {VK_DYNAMIC_STATE_VIEWPORT, + VK_DYNAMIC_STATE_SCISSOR}
| dynamic_state = {VK_DYNAMIC_STATE_VIEWPORT, + VK_DYNAMIC_STATE_SCISSOR, + *VK_DYNAMIC_STATE_PATCH_CONTROL_POINTS_EXT*}
| VkPipelineTessellationStateCreateInfo tessellation_state{} + tessellation_state.sType = VK_STRUCTURE_TYPE_PIPELINE_TESSELLATION_STATE_CREATE_INFO + *tessellation_state.patchControlPoints = patchControlPoints* + ...
+ vkCreateGraphicsPipelines(pipeline)
| vkCreateGraphicsPipelines(pipeline)
| draw(model, pipeline)
| *vkCmdSetPatchControlPointsEXT(commandBuffer, patchControlPoints)* + draw(model, pipeline)
|===
More details are provided in the sections that follow.
== Pipelines
In the static approach to pipeline creation patch control points have to be defined while creating the pipeline.
This is illustrated in a static/non-dynamic pipeline creation.
[,C++]
----
VkPipelineInputAssemblyStateCreateInfo input_assembly_state =
vkb::initializers::pipeline_input_assembly_state_create_info(
VK_PRIMITIVE_TOPOLOGY_PATCH_LIST,
0,
VK_FALSE);
VkPipelineTessellationStateCreateInfo tessellation_state =
vkb::initializers::pipeline_tessellation_state_create_info(3);
std::vector<VkDynamicState> dynamic_state_enables = {
VK_DYNAMIC_STATE_VIEWPORT,
VK_DYNAMIC_STATE_SCISSOR
};
VkPipelineDynamicStateCreateInfo dynamic_state =
vkb::initializers::pipeline_dynamic_state_create_info(
dynamic_state_enables.data(),
static_cast<uint32_t>(dynamic_state_enables.size()),
0);
VkGraphicsPipelineCreateInfo graphics_create{VK_STRUCTURE_TYPE_GRAPHICS_PIPELINE_CREATE_INFO};
graphics_create.pInputAssemblyState = &input_assembly_state;
graphics_create.pDynamicState = &dynamic_state;
graphics_create.pTessellationState = &tessellation_state;
graphics_create.layout = pipeline_layouts.statically_tessellation;
...
VK_CHECK(vkCreateGraphicsPipelines(get_device().get_handle(), pipeline_cache, 1, &graphics_create, VK_NULL_HANDLE, &pipeline.statically_tessellation));
----
In the approach if developer would like to change the patch control points number, then for each different number a new pipeline would be required.
However, with `VK_EXT_extended_dynamic_state2` the number of pipelines can be reduced by the possibility to change parameter Patch Control Points by calling `vkCmdSetPatchControlPointsEXT` before calling the `draw_model` method.
With the usage of above function we can reduce the number of pipelines.
Required dynamic states must be enabled and passed to the `VkGraphicsPipelineCreateInfo` structure.
By setting `VK_PRIMITIVE_TOPOLOGY_PATCH_LIST` in the `VkPipelineInputAssemblyStateCreateInfo` structure, the pipeline can use the `patch control points` functionality.
[,C+]
----
VkPipelineInputAssemblyStateCreateInfo input_assembly_state =
vkb::initializers::pipeline_input_assembly_state_create_info(
VK_PRIMITIVE_TOPOLOGY_PATCH_LIST,
0,
VK_FALSE);
std::vector<VkDynamicState> dynamic_state_enables = {
VK_DYNAMIC_STATE_VIEWPORT,
VK_DYNAMIC_STATE_SCISSOR,
VK_DYNAMIC_STATE_PATCH_CONTROL_POINTS_EXT,
};
VkPipelineDynamicStateCreateInfo dynamic_state =
vkb::initializers::pipeline_dynamic_state_create_info(
dynamic_state_enables.data(),
static_cast<uint32_t>(dynamic_state_enables.size()),
0);
VkGraphicsPipelineCreateInfo graphics_create{VK_STRUCTURE_TYPE_GRAPHICS_PIPELINE_CREATE_INFO};
graphics_create.pInputAssemblyState = &input_assembly_state;
graphics_create.pDynamicState = &dynamic_state;
graphics_create.layout = pipeline_layouts.dynamically_tessellation;
...
VK_CHECK(vkCreateGraphicsPipelines(get_device().get_handle(), pipeline_cache, 1, &graphics_create, VK_NULL_HANDLE, &pipeline.dynamically_tessellation));
----
And now, thanks to `VK_EXT_extended_dynamic_state2`, we can change parameters before each corresponding draw call.
[,C++]
----
VK_CHECK(vkBeginCommandBuffer(draw_cmd_buffer, &command_begin));
...
/* Binding dynamically_tessellation pipeline and descriptor sets */
vkCmdBindDescriptorSets(draw_cmd_buffer,
VK_PIPELINE_BIND_POINT_GRAPHICS,
pipeline_layouts.dynamically_tessellation,
0,
1,
&descriptor_sets.dynamically_tessellation,
0,
nullptr);
vkCmdBindPipeline(draw_cmd_buffer, VK_PIPELINE_BIND_POINT_GRAPHICS, pipeline.dynamically_tessellation);
/* Set patch control points value */
vkCmdSetPatchControlPointsEXT(draw_cmd_buffer, patch_control_points_triangle);
/* Drawing scene with objects using tessellation feature */
draw_model(model, draw_cmd_buffer);
...
VK_CHECK(vkEndCommandBuffer(draw_cmd_buffer));
----
== Enabling the Extension
The extended dynamic state 2 API requires Vulkan 1.0 and the appropriate headers / SDK is required.
This extension has been https://registry.khronos.org/vulkan/specs/1.3-extensions/man/html/VK_EXT_extended_dynamic_state2.html#_promotion_to_vulkan_1_3[partially] promoted to Vulkan 1.3.
The device extension is provided by `VK_EXT_EXTENDED_DYNAMIC_STATE_2_EXTENSION_NAME`.
It also requires `VK_KHR_GET_PHYSICAL_DEVICE_PROPERTIES_2_EXTENSION_NAME` instance extension to be enabled.
[,C++]
----
add_instance_extension(VK_KHR_GET_PHYSICAL_DEVICE_PROPERTIES_2_EXTENSION_NAME);
add_device_extension(VK_EXT_EXTENDED_DYNAMIC_STATE_2_EXTENSION_NAME);
----
If the https://registry.khronos.org/vulkan/specs/1.3-extensions/man/html/VkPhysicalDeviceExtendedDynamicState2FeaturesEXT.html[`VkPhysicalDeviceExtendedDynamicState2FeaturesEXT`] structure is included in the pNext chain of the `VkPhysicalDeviceFeatures2` structure passed to vkGetPhysicalDeviceFeatures2, it is filled in to indicate whether each corresponding feature is supported.
`VkPhysicalDeviceExtendedDynamicState2FeaturesEXT` can also be used in the pNext chain of `VkDeviceCreateInfo` to selectively enable these features.
