Add option to provide custom header file with vulkan function prototypes

[adalsteinnh]

When using a custom Vulkan function loader the current solution can cause symbol conflicts and runtime errors as discussed in issue #4854. This adds an option to replace the Vulkan header file with a custom header file by defining IMGUI_IMPL_VULKAN_CUSTOM_HEADER to resolve these issues.

[ocornut]

Hello,

What is the reason for also adding all those checks next to the VK_NO_PROTOTYPES checks?
If you have fine-tuned control over our loaded why defining VK_NO_PROTOTYPES at all ?

[adalsteinnh]

What is the reason for also adding all those checks next to the VK_NO_PROTOTYPES checks? If you have fine-tuned control over our loaded why defining VK_NO_PROTOTYPES at all ?

This was written with Volk in mind which does define VK_NO_PROTOTYPES before including some of the Vulkan headers for everything except the function declarations and I would imagine most custom loaders would do something similar.

The proposed solution in issue #4854 where you load the function pointers separately for ImGui would also work and maybe it's not a big deal to have to do that if you want to have a custom loader. However given that Volk currently ships with the Vulkan SDK I think it's reasonable to include this simpler alternative which only requires 1 define from the usage code pointing to the Volk header file or some other custom loader.

[ocornut]

Thank you for the clarification. I have installed volk and toying with it now.
I can confirm that adding #undef VK_NO_PROTOTYPES right under #include <Volk/volk.h> works fine as well.

Basically what I am not liking is to assume that IMGUI_IMPL_VULKAN_CUSTOM_HEADER imply that those block shouldn't be compiled. It seems like a bold assumption to make when the introduced define aims to be generic.

Given that everyone who posted in those thread are using Volk, I am tempted instead to add a define explicitly named after VOLK, and if other cases manifests we can later consider to introduce a more generic version.

[ocornut]

I have pushed b720c0f which adds support for #define IMGUI_IMPL_VULKAN_USE_VOLK
Both GLFW+Vulkan and SDL+Vulkan also now supports Volk optionally, which will make it easier to test forward.

Thanks for your help!

[mihaly-sisak]

With the addition of VOLK this can be easily achieved.

You can actually use custom vulkan headers with VOLK, without touching ImGui sources. Volk has a compile time define VOLK_VULKAN_H_PATH we can use! I used the example_sdl2_vulkan with cmake as my starting point.

1. Generate the interface with GLAD2 generator, I used vulkan 1.0 + VK_KHR_surface + VK_KHR_swapchain + VK_EXT_debug_report: https://gen.glad.sh/#generator=c&api=gl%3D4.3%2Cvulkan%3D1.0&profile=gl%3Dcore%2Cgles1%3Dcommon&extensions=VK_EXT_debug_report%2CVK_KHR_surface%2CVK_KHR_swapchain
2. Add to cmake target_include_directories so it includes the unzipped GLAD2 include folder
3. Download VOLK and add to target_include_directories so it includes volk.h. You actually have to edit the ImGui sources here a bit, but just replace <Volk/volk.h> to "volk.h" or use the specified directory name for the repo and include that, or whatever, but this should not be a problem here.
4. Add to cmake target_compile_definitions so it defines -DIMGUI_IMPL_VULKAN_USE_VOLK and -DVOLK_VULKAN_H_PATH="glad/vulkan.h"
5. Build with cmake, the example works without including the system <vulkan/vulkan.h>!