Add the renderer process

The renderer becomes two programs: the adapter (t5deRenderEngine.dll, 32-bit, in IMVU's process) and a 64-bit renderer process (t5de-renderer.exe) drawing with Diligent Engine, fetched with FetchContent and pinned. For now a spike: the adapter starts the renderer process for each engine and signals each frame through a shared memory channel, and the renderer process draws a box into a child window in the client's 3D view.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

CLAUDE.md

@@ -1,7 +1,9 @@
 # t5de-renderer
 
-A renderer plugin for IMVU Classic (see README.md and docs/interface.md). Part of T5DE; the patches and the patcher
-are in the `t5de` repository (git.t5de.dev/t5de/t5de, usually checked out beside this one).
+A new renderer for IMVU Classic: a 32-bit adapter DLL the client loads as a renderer plugin, and a 64-bit renderer
+process drawing with Diligent Engine (see README.md, docs/architecture.md, docs/interface.md). Part of T5DE; the
+patches and the patcher are in the `t5de` repository (git.t5de.dev/t5de/t5de, usually checked out beside this one).
+The user chose this architecture for ray tracing, mesh shaders and a possible browser client later.
 
 ## Rules
 
@@ -13,9 +15,12 @@ are in the `t5de` repository (git.t5de.dev/t5de/t5de, usually checked out beside
 
 ## Notes
 
-- The plugin must match IMVU's ABI: 32-bit, MSVC (cl or clang-cl; MinGW's C++ ABI differs), `__thiscall`
-  virtual methods in IMVU's vtable order, Boost 1.61's `shared_ptr` layout. Exceptions must not cross into IMVU's
-  code (a different C runtime); the CRT is linked statically.
+- The adapter must match IMVU's ABI: 32-bit, MSVC (cl or clang-cl; MinGW's C++ ABI differs), `__thiscall`
+  virtual methods in IMVU's vtable order (never overloaded: MSVC orders overloads unlike their declaration), Boost
+  1.61's `shared_ptr` layout. Exceptions must not cross into IMVU's code (a different C runtime); the CRT is linked
+  statically. Code shared with the 64-bit renderer process (`include/t5de/protocol/`) uses fixed-size fields only.
+- The development VM has no graphics driver (Microsoft Basic Render Driver): Direct3D 11 works in software, IMVU's
+  OpenGL renderer doesn't, and performance there means nothing.
 - Ghidra 12.2 (Java 25) is at `%LOCALAPPDATA%\Programs\ghidra_12.2_PUBLIC`; projects in
   `%LOCALAPPDATA%\t5de-dev\ghidra`, analysed headless (`support\analyzeHeadless.bat`). The client to analyse is the
   `t5de` checkout's `.cache/<version>/install`.

CMakeLists.txt

@@ -5,15 +5,18 @@ project(t5de-renderer
     DESCRIPTION "A new 3D renderer for the IMVU Classic client"
     LANGUAGES CXX)
 
-# The plugin is loaded into IMVU Classic, a 32-bit program built with Visual C++: it must use the same ABI.
+# Two programs, for two architectures (a preset for each, see CMakePresets.json):
+# - 32-bit: the adapter, t5deRenderEngine.dll, a renderer plugin loaded into IMVU Classic (a 32-bit Visual C++
+#   program, so it must match its ABI).
+# - 64-bit: the renderer process, t5de-renderer.exe, which draws.
 if(NOT MSVC) # cl or clang-cl
-    message(FATAL_ERROR "Build with MSVC or clang-cl: the plugin must match IMVU Classic's Visual C++ ABI.")
-endif()
-if(NOT CMAKE_SIZEOF_VOID_P EQUAL 4)
-    message(FATAL_ERROR "Build for 32-bit x86 (IMVU Classic is 32-bit): use a CMakePresets.json preset.")
+    message(FATAL_ERROR "Build with MSVC or clang-cl: the adapter must match IMVU Classic's Visual C++ ABI.")
 endif()
 
-# The C runtime is linked statically, so the plugin needs nothing installed beside it.
+# No C++ modules: skip scanning for them.
+set(CMAKE_CXX_SCAN_FOR_MODULES OFF)
+
+# The C runtime is linked statically, so the programs need nothing installed beside them.
 set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")
 
 # Warnings and conformance for our own targets.
@@ -23,25 +26,25 @@ target_compile_features(t5de_options INTERFACE cxx_std_20)
 target_compile_options(t5de_options INTERFACE /W4 /permissive- /utf-8 /Zc:__cplusplus)
 target_compile_definitions(t5de_options INTERFACE WIN32_LEAN_AND_MEAN NOMINMAX UNICODE _UNICODE)
 
-# Our declarations of IMVU's interfaces.
-add_library(t5de_imvu INTERFACE)
-add_library(t5de::imvu ALIAS t5de_imvu)
-target_include_directories(t5de_imvu INTERFACE
-    $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>)
-target_compile_features(t5de_imvu INTERFACE cxx_std_20)
-
-# The renderer plugin, t5deRenderEngine.dll.
-add_library(t5de_render_engine SHARED
-    src/log.cpp
-    src/passthrough_engine.cpp
-    src/plugin.cpp
-    src/plugin.def)
-set_target_properties(t5de_render_engine PROPERTIES OUTPUT_NAME t5deRenderEngine)
-target_link_libraries(t5de_render_engine PRIVATE t5de::imvu t5de::options)
-
-install(TARGETS t5de_render_engine RUNTIME DESTINATION .)
+# What the adapter and the renderer process share.
+add_library(t5de_protocol INTERFACE)
+add_library(t5de::protocol ALIAS t5de_protocol)
+target_include_directories(t5de_protocol INTERFACE $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>)
+target_compile_features(t5de_protocol INTERFACE cxx_std_20)
 
 include(CTest)
-if(BUILD_TESTING)
-    add_subdirectory(tests)
+
+if(CMAKE_SIZEOF_VOID_P EQUAL 4)
+    # Our declarations of IMVU's interfaces (32-bit layouts).
+    add_library(t5de_imvu INTERFACE)
+    add_library(t5de::imvu ALIAS t5de_imvu)
+    target_include_directories(t5de_imvu INTERFACE $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>)
+    target_compile_features(t5de_imvu INTERFACE cxx_std_20)
+
+    add_subdirectory(adapter)
+    if(BUILD_TESTING)
+        add_subdirectory(tests)
+    endif()
+else()
+    add_subdirectory(renderer)
 endif()

CMakePresets.json

@@ -3,29 +3,43 @@
   "cmakeMinimumRequired": { "major": 3, "minor": 28, "patch": 0 },
   "configurePresets": [
     {
-      "name": "windows-x86",
-      "displayName": "Windows, 32-bit (Visual Studio Build Tools)",
-      "description": "IMVU Classic is a 32-bit Visual C++ program: build for Win32 with MSVC.",
+      "name": "windows",
+      "hidden": true,
       "generator": "Visual Studio 18 2026",
-      "architecture": { "value": "Win32", "strategy": "set" },
       "binaryDir": "${sourceDir}/build/${presetName}",
-      "installDir": "${sourceDir}/build/${presetName}/install",
+      "installDir": "${sourceDir}/build/install",
       "condition": { "type": "equals", "lhs": "${hostSystemName}", "rhs": "Windows" }
+    },
+    {
+      "name": "windows-x86",
+      "inherits": "windows",
+      "displayName": "Adapter (32-bit)",
+      "description": "t5deRenderEngine.dll, loaded into IMVU Classic, a 32-bit Visual C++ program.",
+      "architecture": { "value": "Win32", "strategy": "set" }
+    },
+    {
+      "name": "windows-x64",
+      "inherits": "windows",
+      "displayName": "Renderer process (64-bit)",
+      "description": "t5de-renderer.exe, with Diligent Engine (fetched and built on the first configure).",
+      "architecture": { "value": "x64", "strategy": "set" }
     }
   ],
   "buildPresets": [
-    { "name": "debug", "configurePreset": "windows-x86", "configuration": "Debug" },
-    { "name": "release", "configurePreset": "windows-x86", "configuration": "Release" }
+    { "name": "adapter-debug", "configurePreset": "windows-x86", "configuration": "Debug" },
+    { "name": "adapter-release", "configurePreset": "windows-x86", "configuration": "Release" },
+    { "name": "renderer-debug", "configurePreset": "windows-x64", "configuration": "Debug" },
+    { "name": "renderer-release", "configurePreset": "windows-x64", "configuration": "Release" }
   ],
   "testPresets": [
     {
-      "name": "debug",
+      "name": "adapter-debug",
       "configurePreset": "windows-x86",
       "configuration": "Debug",
       "output": { "outputOnFailure": true }
     },
     {
-      "name": "release",
+      "name": "adapter-release",
       "configurePreset": "windows-x86",
       "configuration": "Release",
       "output": { "outputOnFailure": true }
@@ -33,11 +47,18 @@
   ],
   "workflowPresets": [
     {
-      "name": "release",
+      "name": "adapter",
       "steps": [
         { "type": "configure", "name": "windows-x86" },
-        { "type": "build", "name": "release" },
-        { "type": "test", "name": "release" }
+        { "type": "build", "name": "adapter-release" },
+        { "type": "test", "name": "adapter-release" }
+      ]
+    },
+    {
+      "name": "renderer",
+      "steps": [
+        { "type": "configure", "name": "windows-x64" },
+        { "type": "build", "name": "renderer-release" }
       ]
     }
   ]

README.md

@@ -1,25 +1,28 @@
 # t5de-renderer
 
-A new 3D renderer for the IMVU Classic client, as a renderer plugin the client loads like its own OpenGL renderer.
-Part of [T5DE](https://git.t5de.dev/t5de/t5de); unofficial, not affiliated with IMVU.
+A new 3D renderer for the IMVU Classic client. Part of [T5DE](https://git.t5de.dev/t5de/t5de); unofficial, not
+affiliated with IMVU.
 
-For now it's a pass-through: it wraps IMVU's Direct3D renderer and logs what each frame draws, to prove our
-declarations of IMVU's interface (`docs/interface.md`).
+Two programs (`docs/architecture.md`): an adapter, `t5deRenderEngine.dll`, which IMVU Classic loads as a renderer
+plugin, and a 64-bit renderer process, `t5de-renderer.exe`, which draws with
+[Diligent Engine](https://github.com/DiligentGraphics/DiligentCore). For now the client's own Direct3D renderer still
+draws the scene, and the renderer process proves the setup by drawing a small box over it. `docs/interface.md` has
+what we know of IMVU's renderer interface.
 
 ## Building
 
-Needs Visual Studio Build Tools 2026 (the C++ workload) and CMake 3.28 or newer. IMVU Classic is a 32-bit
-Visual C++ program, so the plugin is built for 32-bit Windows (the `windows-x86` preset):
+Needs Visual Studio Build Tools 2026 (the C++ workload) and CMake 3.28 or newer. One preset per program:
 
 ```sh
-cmake --workflow --preset release
+cmake --workflow --preset adapter    # build/windows-x86/adapter/Release/t5deRenderEngine.dll, and the tests
+cmake --workflow --preset renderer   # build/windows-x64/renderer/Release/t5de-renderer.exe
 ```
 
-That configures, builds `build/windows-x86/Release/t5deRenderEngine.dll` and runs the tests. Or step by step:
-`cmake --preset windows-x86`, `cmake --build --preset debug`, `ctest --preset debug`.
+The renderer's first configure fetches Diligent Engine (pinned in `renderer/CMakeLists.txt`) and builds it, which
+takes a while.
 
 ## Trying it
 
-Copy `t5deRenderEngine.dll` into a T5DE client folder built with the `t5de-renderer` patch, which has the OpenGL
-setting load it in place of `glRenderEngine.dll`, then choose OpenGL in Settings > Advanced. The log is
+Copy both programs into a T5DE client folder built with the `t5de-renderer` patch, which has the OpenGL setting load
+the adapter in place of `glRenderEngine.dll`, then choose OpenGL in Settings > Advanced. The adapter's log is
 `%TEMP%\t5de-renderer.log`.

adapter/CMakeLists.txt

@@ -0,0 +1,12 @@
+# t5deRenderEngine.dll, the adapter: a renderer plugin for IMVU Classic (32-bit).
+
+add_library(t5de_render_engine SHARED
+    log.cpp
+    passthrough_engine.cpp
+    plugin.cpp
+    plugin.def
+    renderer_process.cpp)
+set_target_properties(t5de_render_engine PROPERTIES OUTPUT_NAME t5deRenderEngine)
+target_link_libraries(t5de_render_engine PRIVATE t5de::imvu t5de::protocol t5de::options)
+
+install(TARGETS t5de_render_engine RUNTIME DESTINATION .)

adapter/passthrough_engine.cpp

@@ -93,7 +93,9 @@ imvu::Vec2i pixmap_size(const imvu::Pixmap& pixmap)
 
 } // namespace
 
-PassthroughEngine::PassthroughEngine(imvu::SharedPtr<imvu::PiRenderEngine> inner) : inner_(std::move(inner))
+PassthroughEngine::PassthroughEngine(imvu::SharedPtr<imvu::PiRenderEngine> inner,
+                                     std::unique_ptr<RendererProcess> renderer)
+    : inner_(std::move(inner)), renderer_(std::move(renderer))
 {
     log::info("passthrough: wrapping the engine at {}", static_cast<void*>(inner_.get()));
 }
@@ -335,6 +337,9 @@ void PassthroughEngine::endRendering()
 {
     count(30);
     inner_->endRendering();
+    if (renderer_) {
+        renderer_->frameDrawn();
+    }
     if (logged(frames_)) {
         std::string line = std::format("frame {}:", frames_);
         for (std::size_t slot = 1; slot < slots; ++slot) {

adapter/passthrough_engine.hpp

@@ -4,14 +4,17 @@
 
 #include <array>
 #include <cstdint>
+#include <memory>
 
+#include "renderer_process.hpp"
 #include "t5de/imvu/render_engine.hpp"
 
 namespace t5de {
 
 class PassthroughEngine final : public imvu::PiRenderEngine {
 public:
-    explicit PassthroughEngine(imvu::SharedPtr<imvu::PiRenderEngine> inner);
+    // renderer may be null.
+    PassthroughEngine(imvu::SharedPtr<imvu::PiRenderEngine> inner, std::unique_ptr<RendererProcess> renderer);
     ~PassthroughEngine() override;
 
     void resizeFrameBuffer(imvu::Vec2i size) override;
@@ -65,6 +68,7 @@ private:
     void count(std::size_t slot);
 
     imvu::SharedPtr<imvu::PiRenderEngine> inner_;
+    std::unique_ptr<RendererProcess> renderer_;
     std::array<std::uint32_t, slots> calls_{};
     std::uint64_t frames_ = 0;
 };

adapter/plugin.cpp

@@ -7,6 +7,7 @@
 
 #include "log.hpp"
 #include "passthrough_engine.hpp"
+#include "renderer_process.hpp"
 #include "t5de/imvu/render_engine.hpp"
 
 namespace {
@@ -39,11 +40,14 @@ extern "C" imvu::SharedPtr<imvu::PiRenderEngine> __cdecl createRenderEngine(imvu
 {
     try {
         log::info("createRenderEngine: window {}", static_cast<void*>(window.get()));
+        HWND hwnd = window ? window->getPlatformWindow() : nullptr;
         auto inner = create_imvu_engine(std::move(window));
         if (!inner) {
             return {};
         }
-        return imvu::SharedPtr<imvu::PiRenderEngine>(std::make_unique<PassthroughEngine>(std::move(inner)));
+        auto renderer = hwnd != nullptr ? RendererProcess::start(hwnd) : nullptr;
+        return imvu::SharedPtr<imvu::PiRenderEngine>(
+            std::make_unique<PassthroughEngine>(std::move(inner), std::move(renderer)));
     } catch (const std::exception& e) {
         // Exceptions mustn't cross into IMVU's code (a different C++ runtime).
         log::info("createRenderEngine failed: {}", e.what());

adapter/renderer_process.cpp

@@ -0,0 +1,92 @@
+#include "renderer_process.hpp"
+
+#include <atomic>
+#include <filesystem>
+#include <format>
+#include <string>
+
+#include "log.hpp"
+
+namespace t5de {
+
+namespace {
+
+// The folder this DLL is in: the client's program folder.
+std::filesystem::path module_folder()
+{
+    HMODULE self = nullptr;
+    GetModuleHandleExW(GET_MODULE_HANDLE_EX_FLAG_FROM_ADDRESS | GET_MODULE_HANDLE_EX_FLAG_UNCHANGED_REFCOUNT,
+                       reinterpret_cast<LPCWSTR>(&module_folder), &self);
+    wchar_t path[MAX_PATH];
+    GetModuleFileNameW(self, path, MAX_PATH);
+    return std::filesystem::path(path).parent_path();
+}
+
+} // namespace
+
+std::unique_ptr<RendererProcess> RendererProcess::start(HWND window)
+{
+    static std::atomic<int> engines = 0;
+    const std::wstring name = std::format(L"Local\\t5de-renderer-{}-{}", GetCurrentProcessId(), engines++);
+
+    std::unique_ptr<RendererProcess> renderer(new RendererProcess());
+    renderer->mapping_ = CreateFileMappingW(INVALID_HANDLE_VALUE, nullptr, PAGE_READWRITE, 0,
+                                            sizeof(protocol::Channel), name.c_str());
+    if (renderer->mapping_ == nullptr) {
+        log::info("renderer process: can't create the channel: error {}", GetLastError());
+        return nullptr;
+    }
+    renderer->channel_ = static_cast<protocol::Channel*>(
+        MapViewOfFile(renderer->mapping_, FILE_MAP_ALL_ACCESS, 0, 0, sizeof(protocol::Channel)));
+    renderer->frameEvent_ = CreateEventW(nullptr, FALSE, FALSE, (name + protocol::frameEventSuffix).c_str());
+    if (renderer->channel_ == nullptr || renderer->frameEvent_ == nullptr) {
+        log::info("renderer process: can't set up the channel: error {}", GetLastError());
+        return nullptr;
+    }
+    *renderer->channel_ = {.version = protocol::version,
+                           .closing = 0,
+                           .window = reinterpret_cast<std::uintptr_t>(window),
+                           .frames = 0};
+
+    // The renderer's window goes on top of the client's 3D view: keep the client's drawing out of it.
+    SetWindowLongW(window, GWL_STYLE, GetWindowLongW(window, GWL_STYLE) | WS_CLIPCHILDREN);
+
+    const auto exe = module_folder() / L"t5de-renderer.exe";
+    std::wstring command = std::format(L"\"{}\" --channel {} --parent {}", exe.wstring(), name, GetCurrentProcessId());
+    STARTUPINFOW startup{.cb = sizeof(STARTUPINFOW)};
+    PROCESS_INFORMATION process{};
+    if (!CreateProcessW(exe.c_str(), command.data(), nullptr, nullptr, FALSE, 0, nullptr, nullptr, &startup,
+                        &process)) {
+        log::info("renderer process: can't start {}: error {}", exe.string(), GetLastError());
+        return nullptr;
+    }
+    CloseHandle(process.hThread);
+    renderer->process_ = process.hProcess;
+    log::info("renderer process: started {} (pid {}) on channel {}", exe.string(), process.dwProcessId,
+              std::filesystem::path(name).string());
+    return renderer;
+}
+
+RendererProcess::~RendererProcess()
+{
+    if (channel_ != nullptr) {
+        channel_->closing = 1;
+        if (frameEvent_ != nullptr) {
+            SetEvent(frameEvent_);
+        }
+        UnmapViewOfFile(channel_);
+    }
+    for (HANDLE handle : {frameEvent_, mapping_, process_}) {
+        if (handle != nullptr) {
+            CloseHandle(handle);
+        }
+    }
+}
+
+void RendererProcess::frameDrawn()
+{
+    ++channel_->frames;
+    SetEvent(frameEvent_);
+}
+
+} // namespace t5de

adapter/renderer_process.hpp

@@ -0,0 +1,35 @@
+#pragma once
+// The renderer process (t5de-renderer.exe, 64-bit) an engine draws through: started with the engine, told about
+// each frame through a shared memory channel (include/t5de/protocol/channel.hpp), and told to exit with it.
+
+#include <windows.h>
+
+#include <cstdint>
+#include <memory>
+
+#include "t5de/protocol/channel.hpp"
+
+namespace t5de {
+
+class RendererProcess {
+public:
+    // Starts t5de-renderer.exe from this DLL's folder, drawing into window; null if it can't.
+    static std::unique_ptr<RendererProcess> start(HWND window);
+
+    RendererProcess(const RendererProcess&) = delete;
+    RendererProcess& operator=(const RendererProcess&) = delete;
+    ~RendererProcess();
+
+    // The client drew a frame.
+    void frameDrawn();
+
+private:
+    RendererProcess() = default;
+
+    HANDLE mapping_ = nullptr;
+    HANDLE frameEvent_ = nullptr;
+    HANDLE process_ = nullptr;
+    protocol::Channel* channel_ = nullptr;
+};
+
+} // namespace t5de

docs/architecture.md

@@ -0,0 +1,47 @@
+# Architecture
+
+The renderer is two programs:
+
+- **The adapter**, `t5deRenderEngine.dll` (32-bit, `adapter/`): a renderer plugin IMVU Classic loads in place of its
+  OpenGL renderer (selected by the `t5de-renderer` patch in the `t5de` repository). It implements IMVU's
+  `PiRenderEngine` interface (`include/t5de/imvu/`, `docs/interface.md`) and passes what the client draws to the
+  renderer process. It stays thin: IMVU's process is 32-bit (4 GB of address space shared with Python, Gecko and
+  IMVU's own code) and tied to Visual C++ 2010's ABI.
+- **The renderer process**, `t5de-renderer.exe` (64-bit, `renderer/`): draws, with
+  [Diligent Engine](https://github.com/DiligentGraphics/DiligentCore) (Apache-2.0). Being 64-bit and separate, it can
+  use any modern API and library, isn't limited by the client's address space, and can crash without taking the
+  client down.
+
+Why Diligent: one API over Direct3D 11 (every Windows PC, and software rendering on machines without a graphics
+driver), Direct3D 12 and Vulkan (ray tracing, mesh shaders), and WebGPU (browsers, for a possible future client).
+It's built with CMake and fetched by `FetchContent`, pinned to a commit (`renderer/CMakeLists.txt`); only the
+Direct3D 11 backend is built for now.
+
+## How they talk
+
+`include/t5de/protocol/` is shared by both builds, so it uses fixed-size fields only. For each engine the client
+creates (each 3D view, and again on room changes), the adapter:
+
+1. creates a shared memory channel (`Local\t5de-renderer-<pid>-<n>`) holding IMVU's window handle and a frame
+   count, and an event set after each frame;
+2. starts `t5de-renderer.exe --channel <name> --parent <pid>` from its own folder;
+3. sets the event at each `endRendering`, and sets `closing` when the engine is destroyed.
+
+The renderer process draws into a child window it creates inside the client's window: window handles are valid in
+every process, and Windows allows child windows across processes (Chrome's GPU process works this way). The child
+is disabled so mouse input goes to the client's window under it, and the adapter gives the client's window
+`WS_CLIPCHILDREN` so the client's own drawing stays out of it. The process exits when the engine is destroyed or the
+client exits.
+
+## Status
+
+A spike: the adapter still has IMVU's Direct3D 9 engine draw the scene (the pass-through), and the renderer process
+only draws a box in the 3D view's corner, pulsing with the client's frames. It proves the risky parts: drawing into
+the client's window from another process, keeping in step with the client's frames, and mouse input still reaching
+the client. Next: the protocol carrying the frame's calls and resources, and the renderer process drawing the scene.
+
+## Building
+
+Two presets, one per architecture: `cmake --workflow --preset adapter` (32-bit, with the tests) and
+`cmake --workflow --preset renderer` (64-bit; the first configure fetches and builds Diligent). Both programs go in
+the client's folder.

include/t5de/imvu/types.hpp

@@ -5,6 +5,8 @@
 #include <cstddef>
 #include <cstdint>
 
+struct HWND__; // windows.h's window handle type (HWND is HWND__*)
+
 namespace t5de::imvu {
 
 template <class T>
@@ -94,6 +96,18 @@ struct PointSpriteVector;
 // Region lists IMVU passes to texture updates; unknown so far.
 class CompoundRegion;
 
-class INativeWindow;
+struct UiEvent;
+using NativeWindowHandle = ::HWND__*; // an HWND
+
+// IMVU's window (SceneWindow.dll's NativeWindow), as far as a renderer needs it: the first methods of its vtable.
+// Its later methods include overloads, which MSVC lays out unlike their declaration: declare them from the vtable
+// if they're ever needed.
+class INativeWindow {
+public:
+    virtual ~INativeWindow() = default;
+    virtual void handleMessage(UiEvent* event) = 0;
+    virtual int onSetCursor(Vec2i position) = 0;
+    virtual NativeWindowHandle getPlatformWindow() = 0;
+};
 
 } // namespace t5de::imvu

include/t5de/protocol/channel.hpp

@@ -0,0 +1,25 @@
+#pragma once
+// What the adapter (t5deRenderEngine.dll, in IMVU's 32-bit process) and the renderer process (64-bit) share. Both
+// are built from this header, for different architectures: fixed-size fields only, no pointers or handles' native
+// sizes.
+
+#include <cstdint>
+
+namespace t5de::protocol {
+
+inline constexpr std::uint32_t version = 1;
+
+// A shared memory section the adapter creates for each engine. Its name is passed to the renderer process as
+// `--channel <name>`; the event "<name>-frame" (auto-reset) is set after each frame the client draws.
+struct Channel {
+    std::uint32_t version;
+    std::uint32_t closing; // nonzero: the engine is gone, the renderer process should exit
+    std::uint64_t window;  // IMVU's 3D window (an HWND; window handles are valid in every process)
+    std::uint64_t frames;  // frames the client has drawn
+};
+
+static_assert(sizeof(Channel) == 24, "same layout in 32-bit and 64-bit builds");
+
+inline constexpr wchar_t frameEventSuffix[] = L"-frame";
+
+} // namespace t5de::protocol

renderer/CMakeLists.txt

@@ -0,0 +1,27 @@
+# t5de-renderer.exe, the 64-bit renderer process, drawing with Diligent Engine.
+
+include(FetchContent)
+
+# Diligent Engine's core, pinned to an exact commit. Only the Direct3D 11 backend for now; the others (Direct3D 12,
+# Vulkan for ray tracing and mesh shaders, WebGPU for browsers) are switched on as they're needed.
+set(DILIGENT_NO_DIRECT3D12 ON CACHE BOOL "" FORCE)
+set(DILIGENT_NO_VULKAN ON CACHE BOOL "" FORCE)
+set(DILIGENT_NO_OPENGL ON CACHE BOOL "" FORCE)
+set(DILIGENT_NO_WEBGPU ON CACHE BOOL "" FORCE)
+set(DILIGENT_NO_ARCHIVER ON CACHE BOOL "" FORCE)
+set(DILIGENT_NO_SUPER_RESOLUTION ON CACHE BOOL "" FORCE)
+set(DILIGENT_BUILD_TESTS OFF CACHE BOOL "" FORCE)
+set(DILIGENT_INSTALL_CORE OFF CACHE BOOL "" FORCE)
+FetchContent_Declare(DiligentCore
+    GIT_REPOSITORY https://github.com/DiligentGraphics/DiligentCore.git
+    GIT_TAG c555e4d26de3f414eaf792fa16c0c24827619b17 # master, 2026-10-27
+    GIT_SUBMODULES ThirdParty/xxHash
+    EXCLUDE_FROM_ALL
+    SYSTEM)
+FetchContent_MakeAvailable(DiligentCore)
+
+add_executable(t5de_renderer WIN32 main.cpp)
+set_target_properties(t5de_renderer PROPERTIES OUTPUT_NAME t5de-renderer)
+target_link_libraries(t5de_renderer PRIVATE t5de::protocol t5de::options Diligent-GraphicsEngineD3D11-static Diligent-Common)
+
+install(TARGETS t5de_renderer RUNTIME DESTINATION .)

renderer/main.cpp

@@ -0,0 +1,137 @@
+// t5de-renderer.exe: the renderer process. Started by the adapter (t5deRenderEngine.dll) for each of the client's
+// 3D views; draws into a child window it places in the client's window, frame by frame as the client draws.
+//
+// For now a spike proving the architecture: a small box in the 3D view's corner, its color changing with each frame
+// the client draws, drawn with Diligent (Direct3D 11) from this 64-bit process.
+
+#include <windows.h>
+
+#include <shellapi.h>
+
+#include <cmath>
+#include <cstdint>
+#include <string>
+#include <string_view>
+
+#include "EngineFactoryD3D11.h"
+#include "RefCntAutoPtr.hpp"
+#include "t5de/protocol/channel.hpp"
+
+namespace {
+
+using namespace Diligent;
+
+// The box: where in the client's 3D view, and how big.
+constexpr int boxX = 16;
+constexpr int boxY = 16;
+constexpr int boxWidth = 240;
+constexpr int boxHeight = 80;
+
+struct Arguments {
+    std::wstring channel;
+    DWORD parent = 0;
+};
+
+Arguments parse_arguments()
+{
+    Arguments arguments;
+    int count = 0;
+    wchar_t** argv = CommandLineToArgvW(GetCommandLineW(), &count);
+    for (int i = 1; i + 1 < count; i += 2) {
+        const std::wstring_view name = argv[i];
+        if (name == L"--channel") {
+            arguments.channel = argv[i + 1];
+        } else if (name == L"--parent") {
+            arguments.parent = static_cast<DWORD>(std::wcstoul(argv[i + 1], nullptr, 10));
+        }
+    }
+    LocalFree(argv);
+    return arguments;
+}
+
+LRESULT CALLBACK window_procedure(HWND window, UINT message, WPARAM wparam, LPARAM lparam)
+{
+    if (message == WM_ERASEBKGND) {
+        return 1; // we draw all of it
+    }
+    return DefWindowProcW(window, message, wparam, lparam);
+}
+
+// A child of the client's window. Disabled, so mouse input goes to the client's window under it.
+HWND create_child_window(HWND parent)
+{
+    WNDCLASSEXW windowClass{.cbSize = sizeof(WNDCLASSEXW),
+                            .lpfnWndProc = window_procedure,
+                            .hInstance = GetModuleHandleW(nullptr),
+                            .hCursor = LoadCursorW(nullptr, IDC_ARROW),
+                            .lpszClassName = L"T5DERenderer"};
+    RegisterClassExW(&windowClass);
+    return CreateWindowExW(0, windowClass.lpszClassName, L"", WS_CHILD | WS_VISIBLE | WS_DISABLED | WS_CLIPSIBLINGS,
+                           boxX, boxY, boxWidth, boxHeight, parent, nullptr, windowClass.hInstance, nullptr);
+}
+
+} // namespace
+
+int WINAPI wWinMain(HINSTANCE, HINSTANCE, PWSTR, int)
+{
+    const Arguments arguments = parse_arguments();
+    if (arguments.channel.empty()) {
+        return 2;
+    }
+    HANDLE mapping = OpenFileMappingW(FILE_MAP_ALL_ACCESS, FALSE, arguments.channel.c_str());
+    auto* channel = mapping == nullptr ? nullptr
+                                       : static_cast<t5de::protocol::Channel*>(MapViewOfFile(
+                                             mapping, FILE_MAP_ALL_ACCESS, 0, 0, sizeof(t5de::protocol::Channel)));
+    HANDLE frameEvent = OpenEventW(SYNCHRONIZE, FALSE, (arguments.channel + t5de::protocol::frameEventSuffix).c_str());
+    HANDLE parent = OpenProcess(SYNCHRONIZE, FALSE, arguments.parent);
+    if (channel == nullptr || frameEvent == nullptr || parent == nullptr || channel->version != t5de::protocol::version) {
+        return 3;
+    }
+
+    HWND window = create_child_window(reinterpret_cast<HWND>(static_cast<std::uintptr_t>(channel->window)));
+    if (window == nullptr) {
+        return 4;
+    }
+
+    IEngineFactoryD3D11* factory = GetEngineFactoryD3D11();
+    RefCntAutoPtr<IRenderDevice> device;
+    RefCntAutoPtr<IDeviceContext> context;
+    EngineD3D11CreateInfo engineInfo;
+    factory->CreateDeviceAndContextsD3D11(engineInfo, &device, &context);
+    if (!device) {
+        return 5;
+    }
+    RefCntAutoPtr<ISwapChain> swapChain;
+    SwapChainDesc swapChainDesc;
+    factory->CreateSwapChainD3D11(device, context, swapChainDesc, FullScreenModeDesc{}, Win32NativeWindow{window},
+                                  &swapChain);
+    if (!swapChain) {
+        return 6;
+    }
+
+    for (;;) {
+        const HANDLE handles[] = {frameEvent, parent};
+        const DWORD result = MsgWaitForMultipleObjects(2, handles, FALSE, 1000, QS_ALLINPUT);
+        MSG message;
+        while (PeekMessageW(&message, nullptr, 0, 0, PM_REMOVE)) {
+            DispatchMessageW(&message);
+        }
+        if (result == WAIT_OBJECT_0 + 1 || channel->closing != 0) {
+            break; // the client exited, or the engine is gone
+        }
+        if (result != WAIT_OBJECT_0) {
+            continue;
+        }
+
+        // T5DE gold, pulsing with the client's frames.
+        const float pulse = 0.5f + 0.5f * std::sin(static_cast<float>(channel->frames) * 0.1f);
+        const float color[] = {1.0f, 0.82f * pulse + 0.1f, 0.0f, 1.0f};
+        ITextureView* backBuffer = swapChain->GetCurrentBackBufferRTV();
+        context->SetRenderTargets(1, &backBuffer, nullptr, RESOURCE_STATE_TRANSITION_MODE_TRANSITION);
+        context->ClearRenderTarget(backBuffer, color, RESOURCE_STATE_TRANSITION_MODE_TRANSITION);
+        swapChain->Present(0);
+    }
+
+    DestroyWindow(window);
+    return 0;
+}