# 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. Its swap chain is a flip model one, which the
compositor shows above the client's own drawing. The client's window must not clip its children (`WS_CLIPCHILDREN`):
the client draws a frame when its window gets `WM_PAINT` (`SceneWindow.dll` invalidates it, and draws on the paint),
and a window entirely covered by clipped children never gets one, so the client fell to about one frame a second.
The process exits when the engine is destroyed or the client exits.

## Status

The renderer process draws the whole 3D view in place of the client's renderer (the full view, the default). The
adapter still forwards everything to the client's Direct3D renderer (state and resources: it draws render targets and
reads textures back, which the renderer process doesn't yet), but while the renderer process covers the whole view
(`Channel::covering`, set after each frame it presents) the client's renderer skips its clears and draws, decided per
frame; it draws again as soon as the renderer process stops covering the view (the view changes, or the process
fails). Checked in rooms with screenshots of both renderers side by side (the compare view): they match, particles
and the 2D overlay (name tags) included.

For comparing while the client runs (development):

- Views: `T5DE_RENDERER_VIEW` (off, split, full (the default) or compare) when the client starts, or
  `%TEMP%\t5de-renderer-view.txt` holding one of those at any time. Compare draws the left half's scene over the
  right half, so both renderers show the same thing side by side.
- `%TEMP%\t5de-renderer-shaders\fixed_function.hlsl`, if present, replaces the built-in shaders and is reloaded
  when it changes.
- The renderer process runs from a copy (`%TEMP%\t5de-renderer`), so `t5de-renderer.exe` can be replaced while the
  client runs; engines started afterwards run the new one (the room's view keeps its engine until the client
  restarts).
- The adapter's log (`%TEMP%\t5de-renderer.log`) sums up what the client draws every 300 frames (the first few
  times), and logs textures and render targets the client renderer made itself.

- The command stream (`include/t5de/protocol/commands.hpp`) carries state, transforms, lights, fog, textures,
  buffers (copied when created or updated) and draws, in chunks through the channel; the adapter waits for each to
  be taken, and gives up on a renderer process that stops taking them (the client's renderer carries on).
- The renderer process reproduces IMVU's Direct3D 9 setup (`renderer/shaders/fixed_function.hlsl`), particles
  included (point sprites, drawn as instanced squares), with a pipeline per combination of states.
- Speed: on a PC with a graphics card, a room frame takes about 2 ms to draw (some 430 draws), and the time from
  the client finishing a frame to ours being presented is about 3 ms: the halves can't be told apart, even while
  the camera moves. On the development VM (software rendering, over Remote Desktop) drawing takes 8 to 60 ms, and
  the right half visibly trails, mostly from compositing another process's window over Remote Desktop. The
  renderer process draws only the newest frame when it falls behind, and keeps at most one frame queued.

Next: render targets (offscreen frames are skipped, and drawn by the client's renderer), and one renderer process for
all of a client's views.

## 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.