Phase 4 step 1

docs/PHASE_4_RENDER_THREAD_OWNERSHIP_DESIGN.md

@@ -7,12 +7,12 @@ Phase 1 named the subsystems. Phase 2 added the typed event substrate. Phase 3 m
 ## Status
 
 - Phase 4 design package: proposed.
-- Phase 4 implementation: not started.
+- Phase 4 implementation: Step 1 started. The existing synchronous `RenderEngine` entrypoints now delegate their GL bodies to named `...OnRenderThread(...)` helpers, but no command queue or dedicated render thread exists yet.
 - Current alignment: the repo has a named frame-state contract and cleaner render-state preparation, but GL work is still entered through multiple paths protected by one shared `CRITICAL_SECTION`.
 
 Current GL ownership footholds:
 
-- `RenderEngine` owns GL resources and the current context-binding helpers.
+- `RenderEngine` owns GL resources, the current context-binding compatibility shims, and named render-thread helper methods.
 - `RenderFrameInput` / `RenderFrameState` provide the frame-state contract that a render thread can consume.
 - `RenderFrameStateResolver` prepares the render-facing layer state before drawing.
 - `OpenGLVideoIOBridge` still calls `RenderEngine::TryUploadInputFrame(...)` from the input path and `RenderEngine::RenderOutputFrame(...)` from the output path.
@@ -64,10 +64,10 @@ The current code paths that matter most are:
 
 | Entry point | Current behavior | Phase 4 direction |
 | --- | --- | --- |
-| `RenderEngine::TryUploadInputFrame(...)` | attempts to take the GL lock, binds the context, uploads input texture | enqueue latest input frame; render thread uploads |
-| `RenderEngine::RenderOutputFrame(...)` | takes the GL lock, binds the context, renders, packs/readbacks output | render thread executes output frame production |
-| `RenderEngine::TryPresentPreview(...)` | attempts to take the GL lock and presents preview | render thread or preview presenter consumes latest completed frame |
-| `RenderEngine::CaptureOutputFrameRgbaTopDown(...)` | takes the GL lock and reads output pixels | screenshot request becomes render-thread command |
+| `RenderEngine::TryUploadInputFrame(...)` | attempts to take the GL lock, binds the context, delegates upload to `UploadInputFrameOnRenderThread(...)` | enqueue latest input frame; render thread uploads |
+| `RenderEngine::RenderOutputFrame(...)` | takes the GL lock, binds the context, delegates render/readback to `RenderOutputFrameOnRenderThread(...)` | render thread executes output frame production |
+| `RenderEngine::TryPresentPreview(...)` | attempts to take the GL lock and delegates presentation to `PresentPreviewOnRenderThread(...)` | render thread or preview presenter consumes latest completed frame |
+| `RenderEngine::CaptureOutputFrameRgbaTopDown(...)` | takes the GL lock, binds the context, delegates readback to `CaptureOutputFrameRgbaTopDownOnRenderThread(...)` | screenshot request becomes render-thread command |
 | `OpenGLVideoIOBridge::UploadInputFrame(...)` | calls render upload directly | push input frame into render queue/mailbox |
 | `OpenGLVideoIOBridge::RenderScheduledFrame(...)` | calls render output directly from backend path | request/consume render-produced output without callback-owned GL |
 
@@ -184,7 +184,7 @@ Render-thread-only methods should be private or clearly named:
 
 - `RenderEngine::UploadInputFrameOnRenderThread(...)`
 - `RenderEngine::RenderOutputFrameOnRenderThread(...)`
-- `RenderEngine::CaptureOutputFrameOnRenderThread(...)`
+- `RenderEngine::CaptureOutputFrameRgbaTopDownOnRenderThread(...)`
 
 The current `TryUploadInputFrame`, `RenderOutputFrame`, `TryPresentPreview`, and `CaptureOutputFrameRgbaTopDown` methods can remain as compatibility shims during migration, but their implementations should move toward enqueue-and-wait or enqueue-and-return behavior instead of binding GL directly from the caller's thread.
 
@@ -214,9 +214,9 @@ Split existing direct GL methods into public request methods and private render-
 
 Initial target:
 
-- keep current synchronous behavior where callers need a result
-- move GL bodies into clearly render-thread-owned helpers
-- make future queue migration mechanical
+- [x] keep current synchronous behavior where callers need a result
+- [x] move GL bodies into clearly render-thread-owned helpers for upload, output render, preview presentation, and screenshot readback
+- [x] make future queue migration mechanical
 
 ### Step 2. Add Render Command Queue