Exposing VkSwapchainCreateInfoKHR::flags for VK_KHR_present_id2 / present_wait2

[FrancoisBerard]

Hi, and thanks for VSG.

Disclaimer: this text was drafted by Claude Code, not by me.

What I'm doing. I'm building a low-latency stereo HCI application. To synchronise LCD shutter
glasses with a 240 Hz display I need to know exactly when each frame reaches scan-out: right after
vkQueuePresentKHR I block the render thread on present-wait for that frame's id and, when it
returns, fire a hardware ping (a Teensy over serial).

What works today. Using the v1 extensions VK_KHR_present_id + VK_KHR_present_wait this works
nicely on stock VSG. I add the two device-extension names and the presentId / presentWait
features through WindowTraits (deviceExtensionNames + a vsg::DeviceFeatures with
VkPhysicalDevicePresentIdFeaturesKHR / VkPhysicalDevicePresentWaitFeaturesKHR), subclass
vsg::Viewer, and in my own present path chain a VkPresentIdKHR onto VkPresentInfoKHR and then
call vkWaitForPresentKHR. No VSG change needed, because the v1 extensions don't require any
swapchain creation flag.

What I can't do. I'd like to move to the successor extensions VK_KHR_present_id2 /
VK_KHR_present_wait2. The main reason is robust support detection: v2 adds the surface-capability
structures (VkSurfaceCapabilitiesPresentId2KHR / VkSurfaceCapabilitiesPresentWait2KHR) so I can
query whether the surface in use actually supports present-wait, rather than enabling the v1
device extension blindly — on some WSI/driver paths that distinction matters and getting it wrong is
undefined behaviour.

The blocker is that v2 requires the swapchain to be created with
VK_SWAPCHAIN_CREATE_PRESENT_ID_2_BIT_KHR / VK_SWAPCHAIN_CREATE_PRESENT_WAIT_2_BIT_KHR. Without
those flags, chaining VkPresentId2KHR and calling vkWaitForPresent2KHR are invalid uses of the
swapchain. VSG can already enable the device extensions and features via WindowTraits, but as far
as I can tell there's no way to set VkSwapchainCreateInfoKHR::flags: Swapchain builds the
create-info internally and leaves flags at 0, and SwapchainPreferences has no field for it.

Proposed change. Add a flags field to SwapchainPreferences and forward it to the create
info — a minimal, backward-compatible addition (defaults to 0, current behaviour unchanged):

// include/vsg/vk/Swapchain.h, in struct SwapchainPreferences
VkSwapchainCreateFlagsKHR flags = 0;

// src/vsg/vk/Swapchain.cpp, in Swapchain::Swapchain(...)
createInfo.flags = preferences.flags;

An app could then write
traits->swapchainPreferences.flags |= VK_SWAPCHAIN_CREATE_PRESENT_ID_2_BIT_KHR | VK_SWAPCHAIN_CREATE_PRESENT_WAIT_2_BIT_KHR;
and the rest (extension names, DeviceFeatures) goes through the existing WindowTraits API.

Did I miss an existing way to set swapchain create flags?

Thanks!

[robertosfield]

Your suggests sound sensible. Could you implement what you think will work and then submit a PR so I can review and test it, and provide feedback/make revisions if required.

[FrancoisBerard]

PR submitted #1717

thanks!