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Handoff: Diligent/Vulkan GL3.2 Backend for MobileGL

Date: 2026-08-18
Branch: feat/diligent-vulkan-backend
Repo: ~/MobileGL-dev
Status: Active work-in-progress. Do not mark complete yet.

1. Goal

Implement a complete OpenGL 3.2 front-end emulation on a new Diligent/Vulkan backend inside MobileGL, instead of the DirectVulkan / DirectGLES backends.

Target state:

  • Fully wire MobileGL front-end MG_State (buffers, VAO, program, texture, sampler, framebuffer, render-state) into Diligent.
  • Implement all GL 3.2 core entry points through the Diligent backend.
  • Pass local non-Android GL3.2 tests on the Turnip Adreno 750 GPU.

2. Current Branch / Commits

Latest 12 commits on feat/diligent-vulkan-backend:

2f5abf83 test(diligent): verify indexed DrawElements path from real frontend state
c31b7381 feat(diligent): add basic texture binding and textured state-draw test
8945c507 feat(diligent): clear depth in GL Clear when GL_DEPTH_BUFFER_BIT set
558d3aea feat(diligent): add offscreen depth target and depth clear
7e2f0bc8 feat(diligent): wire stencil and color-mask state into state PSO
f99786f6 feat(diligent): wire viewport/scissor state into state draws
be4cc3ce feat(diligent): wire blend/depth/cull render state into state PSO
c855e6cf feat(diligent): verify state-driven draw with real MobileGL frontend state
02e60bfa feat(diligent): add state-driven draw path (VAO/buffer/program to Diligent)
a9515c92 feat(diligent): add dynamic vertex buffer upload path
2f57582a feat(diligent): wire Clear/Draw/Present into GLFunctionsTable
beb21123 feat(diligent): add real offscreen renderer with clear and triangle draw

Working tree is clean.

3. Key Files

Backend core

  • MobileGL/MG_Backend/Diligent/BackendObject_Diligent.h/.cpp
    • BackendObject_Diligent
    • Creates Diligent Vulkan device/context
    • Owns DiligentRenderer
    • Wires GLFunctionsTable:
      • Clear (color + depth)
      • DrawArrays
      • DrawElements
      • Present
  • MobileGL/MG_Backend/Diligent/DiligentVulkan.h/.cpp
    • Backend identity helper / translation unit
  • MobileGL/MG_Backend/Diligent/Renderer/DiligentRenderer.h/.cpp
    • Offscreen RGBA8 + D32F targets
    • Clear / ClearDepth / DrawTriangle / DrawVertices
    • CreateTestTexture (RGBA8 texture + SRV + sampler)
    • DrawFromState (main front-end emulation draw path)
    • CreatePipelineFromState:
      • SPIR-V → Diligent shaders via SPIRV-Reflect
      • VAO attributes → input layout
      • primitive topology from GL mode
      • blend / depth / cull / stencil / color-mask state
    • UploadVertexDataFromState:
      • packs enabled VAO attributes from BufferObject into interleaved vertex buffer
      • supports DrawArrays, DrawElements, triangle-fan and line-loop expansion
    • Static texture binding to g_Texture through PSO static variables + SRB

Integration changes

  • CMakeLists.txt
    • New option MOBILEGL_ENABLE_DILIGENT (default ON for local)
    • DiligentCore added after glslang/SPIRV-Cross/xxHash/Vulkan-Headers so it reuses existing CMake targets
    • Diligent static libraries linked into MobileGL / MobileGL_s
    • New Diligent backend sources added
  • MobileGL/MG_Backend/BackendObject.h
    • New BackendType::DiligentVulkan
  • MobileGL/MG_Backend/Init.cpp
    • New backend switch case
  • MobileGL/ConfigLoader.cpp
    • MOBILEGL_BACKEND_TYPE=DiligentVulkan accepted
  • MobileGL/MG_Test/CMakeLists.txt
    • New MobileGL/MG_Test/Backend/Diligent subdirectory
  • MobileGL/MG_Test/Backend/Diligent/
    • CMakeLists.txt
    • SanityTest.cpp

Local test files

  • MobileGL/MG_Test/Backend/Diligent/SanityTest.cpp
    • CreatesDiligentDeviceAndAdvertisesGL32
    • ClearsAndDrawsTriangleOffscreen
    • DrawsFromMobileGLState
    • DrawsTexturedFromMobileGLState
    • DrawsIndexedFromMobileGLState

4. What Works Today

Verified locally on Turnip Adreno 750:

  • Diligent device/context creation
  • GL 3.2 / GLSL 1.50 capability advertisement
  • Offscreen color + depth rendering
  • Clear color and depth
  • Real mobilegl front-end state-driven drawing:
    • Program SPIR-V → Diligent shaders
    • VAO attributes + bound GL buffer → interleaved vertex buffer
    • DrawArrays path
    • DrawElements path (index buffer)
  • Texture basics:
    • Offscreen texture creation
    • CPU → Diligent texture (CreateTestTexture)
    • Static sampler2D binding to g_Texture
    • Textured draw test passes
  • Render state:
    • Blend enable/factors/equations
    • Depth test enable/func/write mask
    • Cull face enable/mode/front-face winding
    • Stencil test enable/masks/ops/func/ref
    • Color write mask
    • Viewport
    • Scissor rect
  • Primitive expansion:
    • GL_TRIANGLE_FAN expanded to triangle list
    • GL_LINE_LOOP expanded to line strip
  • Local test result:
[  PASSED  ] 5 tests

5. How to Build and Run Locally

From repo root ~/MobileGL-dev:

cmake -S . -B build-diligent -G Ninja \
  -DCMAKE_BUILD_TYPE=Debug \
  -DMOBILEGL_ENABLE_DILIGENT=ON \
  -DMOBILEGL_BUILD_TEST=ON \
  -DMOBILEGL_BUILD_BENCHMARK=OFF \
  -DFETCHCONTENT_SOURCE_DIR_GOOGLETEST="$PWD/3rdparty/DiligentCore/ThirdParty/googletest"

cmake --build build-diligent --target DiligentVulkanSanityTest -j 4

./build-diligent/MobileGL/MG_Test/Backend/Diligent/DiligentVulkanSanityTest --gtest_color=no

Notes:

  • MOBILEGL_BUILD_BENCHMARK=OFF avoids network fetch of google/benchmark in this environment.
  • FETCHCONTENT_SOURCE_DIR_GOOGLETEST pins googletest to DiligentCore's bundled copy, avoiding flaky network clone.
  • Max 4 cores is intentional: use -j 4.

6. Environment Notes

  • Host: Linux aarch64, glibc 2.43 (Fedora container on Android/Droidspaces)
  • GPU: Turnip Adreno 750, Vulkan API 1.4.354
  • GPU nodes available:
    • /dev/dri/renderD128
    • /dev/kgsl-3d0
  • Android SDK/NDK: ~/android-sdk (aarch64 glibc)
    • NDK 27.3.13750724
    • CMake 3.22.1
  • JDK/Gradle for APK builds:
    • ~/android-build-tools/jdk17
    • ~/android-build-tools/gradle/gradle-8.10.2

7. Known Limitations / Not Yet Implemented

  • DrawArrays/DrawElements draw through real MG_State, but only the color offscreen target is used; no user-created framebuffer/renderbuffer mapping yet.
  • Textures: only a test texture path (CreateTestTexture) is wired; no automatic sync of TextureObject to Diligent resources, no texture unit bindings, no mipmap/sampler state from MG_State.
  • Uniforms / UBOs are not uploaded or bound yet.
  • No swapchain / EGL window surface presentation yet; Present() only flushes.
  • No transform feedback / queries / sync / readback of non-color resources.
  • Some GL 3.2 entry points are still not implemented in the backend (draw range, multi-draw, blit, buffer subdata paths, etc.).
  • The state PSO is recreated on every draw (no caching), which is slow but correct for now.
  • The GLFunctionsTable is only partially populated.

8. Recommended Next Steps

  1. Framebuffer / Renderbuffer mapping

    • Map MG_State::GLState::FramebufferObject attachments to Diligent ITextureView / ITexture.
    • Support default framebuffer as current offscreen target.
    • Support glBindFramebuffer, glFramebufferTexture2D, renderbuffer color/depth attachments.

  2. Texture / Sampler full integration

    • Translate MobileGL ITextureObject to Diligent ITexture and cache by GetLifetimeId().
    • Propagate texture unit bindings into the PSO SRB.
    • Translate SamplerObject state into Diligent SamplerDesc.

  3. Uniform / UBO support

    • Create Diligent buffer for ProgramObject::GetUBOData() / GetUBOSize().
    • Bind the global UBO as a static shader resource.
    • Handle per-program uniform block bindings.

  4. PSO / resource caching

    • Cache PSOs by program + VAO config + render state + topology.
    • Cache textures, buffers, and SRBs.

  5. More GL 3.2 entry points

    • DrawRangeElements
    • MultiDraw*
    • BlitFramebuffer
    • ReadPixels from non-default framebuffer
    • GetTexImage / CopyTexImage*

  6. Expand local test suite

    • Depth test visual test
    • Stencil test
    • Blend test
    • Scissor test
    • Texture filtering / sampler state test
    • framebuffer offscreen render-to-texture test

9. Handoff Notes for Next Agent

  • Do not reference origin/Deprecated/Feat/Diligent; that old implementation is intentionally ignored.
  • Work from this branch, keep tests green.
  • The command ./build-diligent/.../DiligentVulkanSanityTest runs all 5 Diligent tests.
  • If a new test crashes during shader resource binding, remember Diligent texture SRVs need a sampler attached via ITextureView::SetSampler() before InitializeStaticSRBResources().
  • When re-creating a PSO or buffer, call Release() (or assign nullptr) before the create call to avoid Diligent debug “Overwriting reference” assertions.
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