t5de-renderer/docs/interface.md

# IMVU Classic's renderer interface

Findings from client 554.8, from IMVU's exported symbols and Ghidra (headless). Ghidra projects hold IMVU's code:
keep them out of this repository (`%LOCALAPPDATA%\t5de-dev\ghidra`; Ghidra also refuses paths with a dot-folder).
Our declarations are in `include/t5de/imvu/`.

## How the client creates a renderer

Settings > Advanced > Graphics offers three renderers. The Python setting `graphicsRenderer` maps to a
`RendererType`, passed to `SceneWindow.dll`'s `createRenderer(boost::shared_ptr<INativeWindow>, RendererType)`:

| Setting | Type | Renderer |
|---|---|---|
| Direct3D | 1 | `RenderEngineDx`, Direct3D 9, built into `SceneWindow.dll` |
| OpenGL | 2 | a plugin, `glRenderEngine.dll` (`RenderEngineGl`), via `createDLLRenderer` |
| Software ("Standard graphics") | other | `RenderEngineDx` on SwiftShader (`swiftshader.dll`, Direct3D 9 on the CPU) |

`D3D11` and `OpenGLSM2` exist only as Python enum values.

`createDLLRenderer(window, const wchar_t* dll)` (exported) loads a renderer plugin: `LoadLibraryW(dll)`, then the
DLL's `isRenderEngineSupported()` (nonzero to accept), then `createRenderEngine(window)`. For type 2,
`createRenderer` passes `L"glRenderEngine.dll"` with a single `push` of the string's address, so a T5DE patch can
point that operand at our DLL's name in memory (after checking the instruction) to load this plugin instead.

The plugin's exports:

- `int isRenderEngineSupported()`: IMVU's OpenGL plugin returns 1.
- `boost::shared_ptr<PiRenderEngine> createRenderEngine(boost::shared_ptr<INativeWindow> window)`: `__cdecl`; the
  result through a hidden pointer; the callee destroys the by-value argument (MSVC), releasing its reference.

## `PiRenderEngine`

A 2006-era fixed-function interface: 35 virtual methods (`__thiscall`). Names come from `RenderEngineDx`'s exported
methods; signatures from their decorated names. `include/t5de/imvu/render_engine.hpp` declares them in vtable order.

- Frame: `beginRendering(PiRenderTarget*)`, `endRendering()`, `clear(flags, color)`, `resizeFrameBuffer(Vec2<int>)`,
  `enable2dFrameBuffer()`, render targets (`newPixmapRenderTarget`, `newTextureRenderTarget`, returning
  `boost::shared_ptr`s).
- Transforms: `setProjection(Vec2<int>, const Mat4f&)`, `setViewSpace` and `setModelSpace(const Transform&)`,
  texture transforms per stage.
- State: depth test and write, blend mode and blending, alpha test and value, back-face culling, normal inverting,
  lighting, `setLightProperties(index, const LightProperties&)`, fog, environment ambient color.
- Resources: `newRenderTexture(Pixmap*, flags)`, index and vertex buffers.
- Drawing: `drawTriangles` (an index buffer and four vertex buffers), `drawLine`, `drawPointSprites`,
  `drawTrianglesFromMemory` (vertex attribute arrays).
- Two textures per draw (`setTextures(texture0, texture1)`).

`SceneWindow.dll` defines the base class: its constructor and destructor, defaults for the buffer methods and
`drawTriangles` (IMVU's OpenGL plugin uses them), and an asserting `enable2dFrameBuffer`. Its fields, after the vtable
pointer: a magic number (`0x3b913159`, checked by the destructor) and a size counter (12 bytes in all). Outside the
virtual methods, IMVU's code only uses an engine through helpers that call its virtual methods (render states'
`apply`, resources' `allocateInternalResource`).

## ABI

- 32-bit; virtual methods are `__thiscall`. Modern MSVC and clang-cl lay out single-inheritance vtables and pass
  arguments the same way as Visual C++ 2010, which built the client.
- `boost::shared_ptr` (Boost 1.61): the object pointer, then the control block: a vtable (destructor, `dispose`,
  `destroy`, `get_deleter`, `get_untyped_deleter`), the use count and the weak count, changed atomically.
  `include/t5de/imvu/shared_ptr.hpp` matches it. With non-trivial copy and destruction, it's returned through a
  hidden pointer and passed on the stack, as IMVU's code expects.
- Still to work out, for a renderer of our own: `Mat4f`, `Transform`, `LightProperties`, `TextureTransform`,
  `Pixmap` (`pixmap.dll`), the buffers' contents, `VertexAttributeArray<T>`, `std::vector<PointSprite>` as
  Visual C++ 2010 lays it out, and `RenderBlendMode`'s values.
- Memory crosses the boundary only through virtual methods (destructors, control blocks), so each side can use its
  own C runtime. Exceptions must not cross it.

## What a renderer can do

- Every draw passes through the interface between `beginRendering` and `endRendering`, so a renderer can record a
  frame and draw it however it likes: shaders emulating the fixed-function state, per-pixel lighting,
  anti-aliasing, anisotropic filtering, correct gamma and tone mapping, shadows and screen-space effects from the
  recorded frame, post-processing.
- It receives only two textures, a few lights, fog and blend state per draw. Newer material features (normal,
  roughness, emissive maps) aren't passed: they'd need hooks where `SceneWindow.dll` loads products, and the maps
  must exist in the product data the servers send Classic (unverified).
- Frame rate is not the renderer's: the scene advances in 33 ms ticks, and animations one frame per tick
  (`ActionInstance::step` calls `incrementActionFrame` per tick; `FrameIntervalMaker::getFrameInterval` counts the
  ticks; `runApp` waits out `zero.dll`'s `millisecondsPerFrame`). Shorter ticks speed animations up. Smooth high
  frame rates need in-between poses: hooks in `SceneWindow.dll`'s animation update.

## Plan

1. Pass-through plugin (this repository's first version): wraps IMVU's OpenGL engine and logs what each frame
   calls, proving our declarations.
2. Work out the layouts above.
3. A renderer reproducing the current look on Direct3D 11 or OpenGL 4, then the improvements.