[Docs] (Diligent): add Diligent GL3.2 backend handoff document

2026-09-08 04:08:32 +09:00 · 2026-08-23 08:36:57 +08:00
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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`:
+
+```bash
+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.