Compatibility issue between xrCreateSwapchain and VK_KHR_imageless_framebuffer

[omigamedev]

Hi, I'm using the Vulkan API extension VK_KHR_imageless_framebuffer which requires the framebuffer object to be created with the usage flags that match the VkImage attachment when the VkRenderpass is started with vkCmdBeginRenderPass2 with a VkRenderPassAttachmentBeginInfo next struct.

The problem is that xrCreateSwapchain can add arbitratry flags that the app won't be able to know and it won't be able to succesfully forward to the framebuffer creation flag.

However the validation layer is able to catch this incompatibility when the VkImage is create within the same VkInstance. This doesn't happen when the image is created externally like on the Meta Quest, but it does happen on my desktop runtime using the Meta Quest Link.

This is how the swapchain is being created:

constexpr XrVulkanSwapchainCreateInfoMETA color_swapchain_info_meta{
    .type = XR_TYPE_VULKAN_SWAPCHAIN_CREATE_INFO_META,
    .additionalCreateFlags = VK_IMAGE_CREATE_MULTISAMPLED_RENDER_TO_SINGLE_SAMPLED_BIT_EXT
};
const XrSwapchainCreateInfo color_swapchain_info{
    .type = XR_TYPE_SWAPCHAIN_CREATE_INFO,
    .next = has_swapchain_create_info ? &color_swapchain_info_meta : nullptr,
    .usageFlags = XR_SWAPCHAIN_USAGE_COLOR_ATTACHMENT_BIT,
    .format = color_format,
    .sampleCount = view_configs[0].recommendedSwapchainSampleCount,
    .width = m_framebuffer_size.width,
    .height = m_framebuffer_size.height,
    .faceCount = 1,
    .arraySize = static_cast<uint32_t>(view_configs.size()),
    .mipCount = 1
};
if (const XrResult result = xrCreateSwapchain(m_session, &color_swapchain_info, &color_swapchain);
    !XR_SUCCEEDED(result))
{
    LOGE("xrCreateSwapchain failed: %s", to_string(result));
    return false;
}

The XrVulkanSwapchainCreateInfoMETA already adds the ability to forward custom flags, however it doesn't allow those to be retrieved back.
It would have been useful if XrSwapchainImageVulkanKHR had the VkImageUsageFlags field.

typedef struct XrSwapchainImageVulkanKHR {
    XrStructureType       type;
    void* XR_MAY_ALIAS    next;
    VkImage               image;
} XrSwapchainImageVulkanKHR;

Here's the validation error:

Validation Error: [ VUID-VkRenderPassBeginInfo-framebuffer-04627 ] | MessageID = 0x45125641
vkCmdBeginRenderPass2(): pRenderPassBegin->pNext<VkRenderPassAttachmentBeginInfo>.pAttachments[2].usage is (VK_IMAGE_USA
GE_TRANSFER_SRC_BIT|VK_IMAGE_USAGE_TRANSFER_DST_BIT|VK_IMAGE_USAGE_SAMPLED_BIT|VK_IMAGE_USAGE_COLOR_ATTACHMENT_BIT|VK_IM
AGE_USAGE_INPUT_ATTACHMENT_BIT), but the VkFramebuffer was created with vkFramebufferAttachmentsCreateInfo::pAttachmentI
mageInfos[2].usage = VK_IMAGE_USAGE_COLOR_ATTACHMENT_BIT.
The Vulkan spec states: If framebuffer was created with a VkFramebufferCreateInfo::flags value that included VK_FRAMEBUF
FER_CREATE_IMAGELESS_BIT, each element of the pAttachments member of a VkRenderPassAttachmentBeginInfo structure include
d in the pNext chain must be a VkImageView with an inherited usage equal to the usage member of the corresponding elemen
t of VkFramebufferAttachmentsCreateInfo::pAttachmentImageInfos used to create framebuffer (https://vulkan.lunarg.com/doc
/view/1.4.321.1/windows/antora/spec/latest/chapters/renderpass.html#VUID-VkRenderPassBeginInfo-framebuffer-04627)
Objects: 4
    [0] VkRenderPass 0x690000000069[ce_renderpass]
    [1] VkFramebuffer 0x6a000000006a[ce_framebuffer]
    [2] VkImageView 0x5f000000005f[swapchain_color_view[1]]
    [3] VkImage 0x490000000049[swapchain_color[1]]

For reference this happens here: https://github.com/omigamedev/choppy/blob/3e285b2c38c489105bb205ec5cee7299989701f4/modules/app/app.cppm#L291
You should be able to reproduce the issue by building and running the project.

Hope this helps improve the API.
Thanks, bye.

[rpavlik-bot]

An issue (number 2581) has been filed to correspond to this issue in the internal Khronos GitLab (Khronos members only: KHR:openxr/openxr#2581 ), to facilitate working group processes.

This GitHub issue will continue to be the main site of discussion.

[nv-jakob]

Hello @omigamedev, that is an annoying corner case. The working group is looking into it. One question, would being able to query the usage flags help in this case? Or would that be hard to deal with since you might need to create your VkFrameBuffers at a later time then normal?

[omigamedev]

@nv-jakob I think it would be totally fine to query right after xrCreateSwapchain. What do you mean by later time? Wouln't such info be available right after the swapchain have been created?

[nv-jakob]

Ah that's good to know, makes any potential extension that would let you query the flags easier.

What I meant was that maybe you needed to create your VkFrameBuffer objects before you call xrCreateSwapchain and having the usage flags be returned after create would be annoying. But if it's fine as you say then it's all good.