Skip rendering first vulkan frame when elements off screen

[RoryO]

Apologies as I'm rather new to imgui. I ran into this trying my first integration. There is probably a better solution than this work around but I don't know the depths of imgui yet. I'm hoping this PR gets the conversation started and towards the correct fix.

There's a bug with imgui_impl_vulkan where if the GUI elements are initially off screen on the first frame, the TotalIdxCount and TotalVtxCount are 0. When this happens later on we try and allocate a vulkan buffer with size 0, which Vulkan gives us an error code -2, VK_ERROR_OUT_OF_DEVICE_MEMORY. (terrible error since size 0 isn't out of memory, it's invalid).

Interestingly if we just skip rendering frames where the vertex count is 0 GetDrawData has vertices again. It looks like imgui repositions off screen elements back into the viewable area? This leads me to the belief that there may be a bug within GetDrawData and this hack is covering it up.

We can easily reproduce the behavior by placing this ini alongside a built Vulkan example. The check_vk_result on line 355/356 fails because of the 0 size passed into vkMapMemory

[Window][Debug##Default]
Pos=60,60
Size=400,400
Collapsed=0

[Window][Hello, world!]
Pos=45,56
Size=345,169
Collapsed=0

[Window][Dear ImGui Demo]
Pos=516,129
Size=550,680
Collapsed=0

[ibrahimsag]

I see the same error at the second run of an example. The changes helped.

Maybe not very helpful but removing the [Dear Imgui Demo] part from imgui.ini removes the error. Which can't be done before every execution of course.

[ocornut]

Thanks for the report!

I have pushed the fix for Vulkan renderer (surprisingly it reproduced immediately here, I think VulkanSDK probably made a change that made this more apparent).

This leads me to the belief that there may be a bug within GetDrawData and this hack is covering it up.

For various reasons many windows are going to be hidden in their first frame, leading to this. There's no plan to fix/improve this soon, if a major issue in your app best to skip framebuffer swapping the first time.

[ocornut]

I think VulkanSDK probably made a change that made this more apparent).

Never mind, this was due to 73c30aa ! so very recent.
Your fix was technically incorrect as it defeat the point of 73c30aa.