vk-bootstrap/docs/getting_started.md

Getting Started

vk-bootstrap reduces the complexity of setting up a vulkan application by simplifying the three initial steps; instance creation, Physical device selection, and device creation.

Instance Creation

Creating an instance with vk-bootstrap uses the vkb::InstanceBuilder class.

Simply create a builder variable and call the build() member function.

vkb::InstanceBuilder instance_builder;
auto instance_builder_return = instance_builder.build();

Because creating an instance may fail, the builder returns an 'Result' type. This contains either a valid vkb::Instance struct, which includes a VkInstance handle, or contains an vkb::InstanceError.

if (!instance_builder_return) {
    printf("Failed to create Vulkan instance. Cause %s\n", 
        instance_builder_return.error().message());
    return -1;
} 

Once any possible errors have been dealt with, we can pull the vkb::Instance struct out of the Result.

vkb::Instance vkb_instance = instance_builder_return.value();

This is enough to create a usable VkInstance handle but many will want to customize it a bit. To configure instance creation, simply call the member functions on the vkb::InstanceBuilder object before build() is called.

The most common customization to instance creation is enabling the "Validation Layers", an invaluable tool for any vulkan application developer.

instance_builder.request_validation_layers ();

The other common customization point is setting up the Debug Messenger Callback, the mechanism in which an application can control what and where the "Validation Layers" log its output.

instance_builder.set_debug_callback (
    [] (VkDebugUtilsMessageSeverityFlagBitsEXT messageSeverity,
	    VkDebugUtilsMessageTypeFlagsEXT messageType,
	    const VkDebugUtilsMessengerCallbackDataEXT* pCallbackData,
	    void *pUserData) 
        -> VkBool32 {
			auto severity = vkb::to_string_message_severity (messageSeverity);
			auto type = vkb::to_string_message_type (messageType);
			printf ("[%s: %s] %s\n", severity, type, pCallbackData->pMessage);
			return VK_FALSE;
		}
    );

Alternatively, vk-bootstrap provides a 'default debug messenger' that prints to standard output.

instance_builder.use_default_debug_messenger();

Configuration can be chained together and done inline with building, like so.

auto inst_builder_ret = instance_builder 
        .set_app_name ("Awesome Vulkan Application")
        .set_engine_name("Excellent Game Engine")
        .require_api_version(1,0,0)
        .build();

To query the available layers and extensions, use the get_system_info() function of vkb::InstanceBuilder to get a SystemInfo struct. It contains a is_layer_available() and is_extension_available() function to check for a layer or extensions before enabling it. It also has booleans for if the validation layers are present and if the VK_EXT_debug_utils extension is available.

auto system_info = instance_builder.get_system_info();
if (system_info.is_layer_available("VK_LAYER_LUNARG_api_dump")) {
    instance_builder.enable_layer("VK_LAYER_LUNARG_api_dump");
}
if (system_info.validation_layers_available){
    instance_builder.enable_validation_layers();
}

The vkb::Instance struct is meant to hold all the necessary instance level data to enable proper Physical Device selection. It also is meant for easy destructuring into custom classes if so desired.

struct CustomVulkanWrapper {
    VkInstance instance;
    //...
};
CustomVulkanWrapper custom_vk_class;
custom_vk_class.instance = vkb_instance.instance;

When the application is finished with the vulkan, call vkb::destroy_instance() to dispose of the instance and associated data.

// cleanup 
vkb::destroy_instance(vkb_instance);

Instance Creation Summary

vkb::InstanceBuilder instance_builder;
auto instance_builder_return = instance_builder
        // Instance creation configuration
        .request_validation_layers()
        .use_default_debug_messenger()
        .build ();
if (!instance_builder_return) {
    // Handle error
} 
vkb::Instance vkb_instance = instance_builder_return.value ();

// at program end
vkb::destroy_instance(vkb_instance);

Surface Creation

Presenting images to the screen Vulkan requires creating a surface, encapsulated in a VkSurfaceKHR handle. Creating a surface is the responsibility of the windowing system, thus is out of scope for vk-bootstrap. However, vk-bootstrap does try to make the process as painless as possible by automatically enabling the correct windowing extensions in VkInstance creation.

Windowing libraries which support Vulkan usually provide a way of getting the VkSurfaceKHR handle for the window. These methods require a valid Vulkan instance, thus must be done after instance creation.

Examples for GLFW and SDL2 are listed below.

vkb::Instance vkb_instance; //valid vkb::Instance
VkSurfaceKHR surface = nullptr;
// window is a valid library specific Window handle

// GLFW
VkResult err = glfwCreateWindowSurface (vkb_instance.instance, window, NULL, &surface);
if (err != VK_SUCCESS) { /* handle error */ }

// SDL2
SDL_bool err = SDL_Vulkan_CreateSurface(window, vkb_instance.instance, &surface);
if (!err){ /* handle error */ }

Physical Device Selection

Once a Vulkan instance has been created, the next step is to find a suitable GPU for the application to use. vk-bootstrap provide the vkb::PhysicalDeviceSelector class to streamline this process.

Creating a vkb::PhysicalDeviceSelector requires a valid vkb::Instance to construct.

It follows the same pattern laid out by vkb::InstanceBuilder.

vkb::PhysicalDeviceSelector phys_device_selector (vkb_instance); 
auto physical_device_selector_return = phys_device_selector
        .set_surface(surface_handle)
        .select ();
if (!physical_device_selector_return) {
    // Handle error
}
auto phys_device = phys_device_ret.value ();

To select a physical device, call select() on the vkb::PhysicalDeviceSelector object. By default, this will prefer a discrete GPU.

No cleanup is required for vkb::PhysicalDevice.

The vkb::PhysicalDeviceSelector will look for the first device in the list that satisfied all the specified criteria, and if none is found, will return the first device that partially satisfies the criteria.

The various "require" and "desire" pairs of functions indicate to vk-bootstrap what features and capabilities are necessary for an application and what are simply preferred. A "require" function will fail any VkPhysicalDevice that doesn't satisfy the constraint, while any criteria that doesn't satisfy the "desire" functions will make the VkPhysicalDevice only 'partially satisfy'.

// Application cannot function without this extension
phys_device_selector.add_required_extension("VK_KHR_timeline_semaphore");

// Application can deal with the lack of this extension
phys_device_selector.add_desired_extension("VK_KHR_imageless_framebuffer");

Note:

Because vk-bootstrap does not manage creating a VkSurfaceKHR handle, it is explicitly passed into the vkb::PhysicalDeviceSelector for proper querying of surface support details. Unless the vkb::InstanceBuilder::set_headless() function was called, the physical device selector will emit no_surface_provided error. If an application does intend to present but cannot create a VkSurfaceKHR handle before physical device selection, use defer_surface_initialization() to disable the no_surface_provided error.

Device Creation

Once a VkPhysicalDevice has been selected, a VkDevice can be created. Facilitating that is the vkb::DeviceBuilder. Creation and usage follows the forms laid out by vkb::InstanceBuilder.

vkb::DeviceBuilder device_builder{ phys_device};
auto dev_ret = device_builder.build ();
if (!dev_ret) {
    // error
}
vkb::Device vkb_device = dev_ret.value();

The features and extensions used as selection criteria in vkb::PhysicalDeviceSelector automatically propagate into vkb::DeviceBuilder. Because of this, there is no way to enable features or extensions that were not specified during vkb::PhysicalDeviceSelector. This is by design as any feature or extension enabled in a device must have support from the VkPhysicalDevice it is created with.

The common method to extend Vulkan functionality in existing API calls is to use the pNext chain. This is accounted for VkDevice creation with the add_pNext member function of vkb::DeviceBuilder. Note: Any structures added to the pNext chain must remain valid until build() is called.

VkPhysicalDeviceDescriptorIndexingFeatures descriptor_indexing_features{};

auto dev_ret = device_builder.add_pNext(&descriptor_indexing_features)
                             .build ();

To destroy a vkb::Device, call vkb::destroy_device().

vkb::destroy_device(vkb_device);

Queues

By default, vkb::DeviceBuilder will enable one queue from each queue family available on the VkPhysicalDevice. This is done because in practice, most use cases only need a single queue from each family.

To get a VkQueue or the index of a VkQueue, use the get_queue(QueueType type) and get_queue_index(QueueType type) functions of vkb::Device. These will return the appropriate VkQueue or uint32_t if they exist and were enabled, else they will return an error.

auto queue_ret = vkb_device.get_queue (vkb::QueueType::graphics);
if (!queue_ret) {
    // handle error
}
graphics_queue = queue_ret.value ();

Queue families represent a set of queues with similar operations, such as graphics, transfer, and compute. Because not all Vulkan hardware has queue families for each operation category, an application should be able to handle the presence or lack of certain queue families. For this reason the get_dedicated_queue and get_dedicated_queue_index functions of vkb::Device exist to allow applications to easily know if there is a queue dedicated to a particular operation, such as compute or transfer operations.

Custom queue setup

If an application wishes to have more fine grained control over their queue setup, they should create a std::vector of vkb::CustomQueueDescription which describe the index, count and a std::vector<float> of priorities. To build up such a vector, use the get_queue_families function in vkb::PhysicalDevice to get a std::vector<VkQueueFamilyProperties>

For example

std::vector<vkb::CustomQueueDescription> queue_descriptions;
auto queue_families = phys_device.get_queue_families ();
for (uint32_t i = 0; i < static_cast<uint32_t>(queue_families.size ()); i++) {
    if (queue_families[i].queueFlags & VK_QUEUE_GRAPHICS_BIT) {
        // Find the first queue family with graphics operations supported
        queue_descriptions.push_back (vkb::CustomQueueDescription (
            i, queue_families[i].queueCount, 
            std::vector<float> (queue_families[i].queueCount, 1.0f)));
    }
}

Swapchain

Creating a swapchain follows the same form outlined by vkb::InstanceBuilder and vkb::DeviceBuilder. Create the vkb::SwapchainBuilder, provide vkb::Device, call the appropriate builder functions, and call build().

vkb::SwapchainBuilder swapchain_builder{ device };
auto swap_ret = swapchain_builder.build ();
if !(swap_ret){

}
vkb::swapchain swapchain = swap_ret.value();

By default, the swapchain will use the VK_FORMAT_B8G8R8A8_SRGB or VK_FORMAT_R8G8B8A8_SRGB image format with the color space VK_COLOR_SPACE_SRGB_NONLINEAR_KHR. The present mode will default to VK_PRESENT_MODE_MAILBOX_KHR if available and fallback to VK_PRESENT_MODE_FIFO_KHR. The image usage default flag is VK_IMAGE_USAGE_COLOR_ATTACHMENT_BIT.

Recreating the swapchain is equivalent to creating a new swapchain but providing the old swapchain as a source. Be sure to not use the same VkSwapchainKHR again as it expires when it is recycled after trying to create a new swapchain.

vkb::SwapchainBuilder swapchain_builder{ device };
auto swap_ret = swapchain_builder.set_old_swapchain (vkb_swapchain)
                                 .build ();
if !(swap_ret){
    // If it failed to create a swapchain, the old swapchain handle is invalid.
    vkb_swapchain.swapchain = VK_NULL_HANDLE;
}
// Even though we recycled the previous swapchain, we need to free its resources.
vkb::destroy_swapchain(vkb_swapchain);
// Get the new swapchain and place it in our variable
vkb_swapchain = swap_ret.value();

To destroy the swapchain, call vkb::destroy_swapchain(). This is null out the VkSwapchainHandle inside of it.

vkb::destroy_swapchain(vkb_swapchain);