First-pass of a plugin system for GUI elements.

Refactor filament's debug drawing to be one such plugin.

PiperOrigin-RevId: 859002002
Change-Id: Ibe9000dec19fce648eef6d57170ce435b9a8c6ff

src/experimental/platform/CMakeLists.txt

@@ -47,6 +47,8 @@ target_sources(${MUJOCO_PLATFORM_TARGET_NAME}
         interaction.h
         picture_gui.h
         picture_gui.cc
+        plugin.cc
+        plugin.h
         renderer.cc
         renderer.h
         sim_history.cc

src/experimental/platform/plugin.cc

@@ -0,0 +1,63 @@
+// Copyright 2026 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <functional>
+#include <string_view>
+
+#include <mujoco/mujoco.h>
+#include "experimental/platform/plugin.h"
+#include "engine/engine_global_table.h"
+
+namespace mujoco::platform {
+
+void RegisterGuiPlugin(const GuiPlugin* plugin) {
+  if (plugin->name == nullptr || plugin->name[0] == '\0') {
+    mju_error("Plugin name must not be empty or null.");
+  }
+  GlobalTable<GuiPlugin>::GetSingleton().AppendIfUnique(*plugin);
+}
+
+void ForEachGuiPlugin(const std::function<void(GuiPlugin*)>& fn) {
+  auto& table = GlobalTable<GuiPlugin>::GetSingleton();
+  for (int i = 0; i < table.count(); ++i) {
+    const GuiPlugin* plugin = table.GetAtSlot(i);
+    fn(const_cast<GuiPlugin*>(plugin));
+  }
+}
+
+}  // namespace mujoco::platform
+
+using mujoco::GlobalTable;
+using GuiPlugin = mujoco::platform::GuiPlugin;
+
+template <>
+const char* GlobalTable<GuiPlugin>::HumanReadableTypeName() {
+  return "gui plugin";
+}
+
+template <>
+std::string_view GlobalTable<GuiPlugin>::ObjectKey(const GuiPlugin& plugin) {
+  return std::string_view(plugin.name);
+}
+
+template <>
+bool GlobalTable<GuiPlugin>::ObjectEqual(const GuiPlugin& p1, const GuiPlugin& p2) {
+  return CaseInsensitiveEqual(p1.name, p2.name);
+}
+
+template <>
+bool GlobalTable<GuiPlugin>::CopyObject(GuiPlugin& dst, const GuiPlugin& src, ErrorMessage& err) {
+  dst = src;
+  return true;
+}

src/experimental/platform/plugin.h

@@ -0,0 +1,58 @@
+// Copyright 2026 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef MUJOCO_SRC_EXPERIMENTAL_PLATFORM_PLUGIN_H_
+#define MUJOCO_SRC_EXPERIMENTAL_PLATFORM_PLUGIN_H_
+
+#include <functional>
+
+namespace mujoco::platform {
+
+// Important: Do not inherit from these plugin structs. They are copied by value
+// and therefore any derived classes will be sliced. We assume plugins are
+// effectively globals and so any pointers will be valid for the lifetime
+// of the process.
+
+// Plugin for processing custom UI windows. The plugin will be listed in the
+// "Plugins" main menu and, when selected, an ImGui window will be opened with
+// the name of the plugin as the title. The `update` function can then be used
+// to process the GUI. All functions will be called by passing `this` as the
+// first argument.
+struct GuiPlugin final {
+  using UpdateFn = void (*)(GuiPlugin* self);
+
+  // Whether or not to display the plugin window.
+  bool active = false;
+
+  // The name of the plugin; must be unique.
+  const char* name = "";
+
+  // The function that will update the plugin's window. Plugin GUI updates
+  // happen when the window is `active` and after all other Studio GUI updates.
+  UpdateFn update = nullptr;
+
+  // Optional data pointer.
+  void* data = nullptr;
+};
+
+// Registers a plugin with a global registry. The plugin must have a
+// case-insensitive unique name.
+void RegisterGuiPlugin(const GuiPlugin* plugin);
+
+// Executes the given function for each registered plugin.
+void ForEachGuiPlugin(const std::function<void(GuiPlugin*)>& fn);
+
+}  // namespace mujoco::platform
+
+#endif  // MUJOCO_SRC_EXPERIMENTAL_PLATFORM_PLUGIN_H_

src/experimental/platform/renderer.cc

@@ -20,6 +20,7 @@
 
 #if defined(USE_FILAMENT_OPENGL) || defined(USE_FILAMENT_VULKAN)
 #include "experimental/filament/render_context_filament.h"
+#include "experimental/platform/plugin.h"
 #elif defined(USE_CLASSIC_OPENGL)
 #include <imgui.h>
 #include <backends/imgui_impl_opengl3.h>
@@ -136,3 +137,14 @@ int Renderer::UploadImage(int texture_id, const std::byte* pixels, int width,
 double Renderer::GetFps() { return fps_; }
 
 }  // namespace mujoco::platform
+
+#if !defined(USE_CLASSIC_OPENGL)
+mjPLUGIN_LIB_INIT {
+  mujoco::platform::GuiPlugin plugin;
+  plugin.name = "Filament";
+  plugin.update = [](mujoco::platform::GuiPlugin* self) {
+    mjr_updateGui(nullptr);
+  };
+  mujoco::platform::RegisterGuiPlugin(&plugin);
+}
+#endif