Custom Engine Integration: ImGui not rendering to Filament Viewport (Vulkan)

[anantamengine]

20260714-0505-32.0310617.mp4

I am integrating Dear ImGui with Filament (Vulkan backend) in a custom engine. I have implemented a multi-primitive renderer that rebuilds the RenderableManager component per frame, effectively mapping each ImDrawCmd to a unique primitive slot with a dedicated MaterialInstance and scissor rect.

Current Behavior:

• ImDrawData contains valid vertex/index counts (UI Verts: 456 verified in the window title).
• The RenderableManager successfully builds the UI entity every frame.
• The UI geometry is being passed to the GPU (confirmed via "Red Box Test" where geometry renders as solid red blocks when alpha/textures are stripped).
• Issue: When using standard alpha blending (transparent or masked), the UI remains completely invisible, even though the mouse cursor reacts to UI element hover states.

Setup Details:

• Backend: Vulkan
• View: uiView set to TRANSLUCENT
• Projection: uiCamera set to ORTHO (mapped to window pixel dimensions).
• Diagnostics: - Verified vertex/index buffers are correctly allocated and filled.
• Verified the uiView is rendered in the correct pass (renderer->render(uiView)).
• Verified that standard PBR objects in the main view render perfectly.

Is there a specific configuration requirement for a secondary View in Filament to correctly composite over a main View when using custom geometry? Are there known issues with Renderable transparency compositing in this specific viewport configuration? sometimes it renders but the UI is Invisible, sometimes it renders like a glitch

https://github.com/user-attachments/assets/a5be466f-fa07-4d6b-84b7-89d1b032ad68

[poweifeng]

You can consider how this is done in filament here: https://github.com/google/filament/blob/main/libs/filagui/src/ImGuiHelper.cpp