Add support for VK_EXT_descriptor_heap

[GloriousPtr]

Version/Branch of Dear ImGui:

Version 1.92.7, Branch: (master/docking)

Back-ends:

imgui_impl_vulkan.cpp

Compiler, OS:

Windows 11 + MSVC 2022

Full config/build information:

No response

Details:

Despite VK_EXT_descriptor_heap being pretty new extension we can see where vulkan is going, we can add support for it to simplify application side quite a bit.
With this we can get rid of Add/RemoveTexture functions and Descriptor pool as well. ImGui TextureID will be index into descriptor heap and it will bring the backend closer to D3D12 as well.

Screenshots/Video:

No response

Minimal, Complete and Verifiable Example code:

No response

[GloriousPtr]

Additional information: https://docs.vulkan.org/features/latest/features/proposals/VK_EXT_descriptor_heap.html
Example implementation (I dont think any of this is even required to implement in the backend): https://github.com/SaschaWillems/Vulkan/blob/master/examples/descriptorheap/descriptorheap.cpp

[myleswortham]

As I am starting from scratch and using VK_EXT_descriptor_heap and just got to the, time to integrate ImGui part, I too would really appreciate this. If I get something up and running, I will post it, but it will likely be a naive implementation all things considered.

Of note, and apologies if this comes off as unnecessary explanation, but I think it's worth mentioning there are two ways to use Vulkan's descriptor heap. One is effectively backwards compatible and makes use of VkShaderDescriptorSetAndBindingMappingInfoEXT to enable the standard layout pattern and binding shaders explicitly.

Sascha Willem's example uses that. I am unsure if they have an example without it. Their corresponding slang shader also assumes its use. However, I'm unsure if I'd say Vulkan's calling that a "legacy" path, but they do note the alternative method for direct indexing:

"While descriptor heaps can be accessed directly through the SamplerHeapEXT and ResourceHeapEXT built-ins, shaders using the existing DescriptorSet and Binding decorations can map these to heap offsets. In place of descriptor set layouts and pipeline layouts, information can be provided at pipeline or shader creation time to indicate how these bindings are mapped, through a combination of constants, push data, and indirections through device addresses. This interface provides significantly more flexibility than descriptor set layouts, enabling applications to specify precisely where they expect each descriptor to be."

https://docs.vulkan.org/spec/latest/chapters/descriptorheaps.html#descriptorheaps-bindings

That said, at least in slang, there seems to be a variety of... fun... quirks as that all gets hammered out. Alternatively, I might just be bad at coding/reading and missed something obvious. All I know is, it REALLY didn't seem to like when I tried using ConstantBuffer or indexing into any buffer where the combined size of the elements describing the buffer element's data were more than 128 bytes (i.e., one of my Camera's projection float4x4s had to be removed).

IDK if that's useful, but just information I found over the last few weeks.

[myleswortham]

Side note, in case anyone finds this thread prior to this support being added, due to the heap's incompatibility with standard descriptors:

"When any heap state command is recorded to a command buffer, it immediately invalidates all descriptor set and descriptor buffer state set by vkCmdBindDescriptorSets2, vkCmdPushDescriptorSet2, vkCmdPushDescriptorSetWithTemplate2, vkCmdSetDescriptorBufferOffsets2EXT, vkCmdBindDescriptorBufferEmbeddedSamplers2EXT, vkCmdPushConstants2, vkCmdBindDescriptorSets, vkCmdPushDescriptorSet, vkCmdPushDescriptorSetWithTemplate, vkCmdBindDescriptorBuffersEXT, vkCmdSetDescriptorBufferOffsetsEXT, vkCmdBindDescriptorBufferEmbeddedSamplersEXT, or vkCmdPushConstants."

the simple workaround that I used was to define the DescriptorPool for ImGui, use it in the InitInfo and use a separate Command Buffer from the Heap command buffer and link the two command buffers by the same internal timeline semaphore. My likely highly inefficient and unsafe methods below with the vulkan-hpp API; hopefully it will save you a couple hours. Any non-defined variables are defined elsewhere, but are presumably straightforward enough in terms of function.

Initializing Descriptor Pool/InitInfo

void Initialize() {
    std::array<vk::DescriptorPoolSize, 2> poolSizes{
            vk::DescriptorPoolSize{
                .type            = vk::DescriptorType::eSampledImage,
                .descriptorCount = IMGUI_DESCRIPTOR_POOL_SIZE,
            },
            vk::DescriptorPoolSize{
                .type            = vk::DescriptorType::eSampler,
                .descriptorCount = IMGUI_IMPL_VULKAN_MINIMUM_IMAGE_SAMPLER_POOL_SIZE,
            },
        };
        vk::DescriptorPoolCreateInfo poolInfo{
            .flags         = vk::DescriptorPoolCreateFlagBits::eFreeDescriptorSet,
            .maxSets       = IMGUI_DESCRIPTOR_POOL_SIZE + IMGUI_IMPL_VULKAN_MINIMUM_IMAGE_SAMPLER_POOL_SIZE,
            .poolSizeCount = static_cast<uint32_t>(poolSizes.size()),
            .pPoolSizes    = poolSizes.data(),
        };
        SharedImGuiDescriptorPool = GlobalDevice.createDescriptorPool(poolInfo);

        vk::Format colorFormat = SharedSwapchainData.Format;
        vk::PipelineRenderingCreateInfo dynRender{
            .colorAttachmentCount    = 1,
            .pColorAttachmentFormats = &colorFormat,
        };

        ImGui_ImplVulkan_InitInfo initInfo{
            .ApiVersion         = VK_API_VERSION_1_4,
            .Instance           = *GlobalInstance,
            .PhysicalDevice     = *GlobalPhysDevice,
            .Device             = *GlobalDevice,
            .QueueFamily        = SharedQueueData[Queues::Graphics].familyIndex, //Grouped data about the Queue
            .Queue              = *GlobalQueues[Queues::Graphics].Queue,                  //Array of queues/timelinevalues. Using the same one as the main frames
            .DescriptorPool     = *SharedImGuiDescriptorPool,
            .MinImageCount      = SharedSwapchainSpec.info.minImageCount,
            .ImageCount         = SharedSwapchainData.ImageCount,
            .PipelineInfoMain   = { .PipelineRenderingCreateInfo = dynRender },
            .UseDynamicRendering= true,
        };

        ImGui_ImplVulkan_Init(&initInfo);
}