Initial implementation of the filament rendering backend for mujoco.

This is intended as a drop-in replacement for mjr_render to enable high
quality, physically-based rendering (PBR) using the filament render engine.

This is a work-in-progress and, as such, some features may be missing. This
is also a code-only drop; build files (including instructions on how to
generate the filament materials) will be provided at a later date.

This library is intended to be run with the experimental studio app.

PiperOrigin-RevId: 817593684
Change-Id: Ib9cd14c1881e4897c1111275bd749aa38ebb5317

src/experimental/filament/render_context_filament.cc

@@ -0,0 +1,149 @@
+// Copyright 2025 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 "experimental/filament/render_context_filament.h"
+
+#include <cstdint>
+#include <cstdlib>
+#include <cstring>
+#include <filesystem>
+#include <fstream>
+#include <ios>
+
+#include <mujoco/mjmodel.h>
+#include <mujoco/mjrender.h>
+#include <mujoco/mjvisualize.h>
+#include <mujoco/mujoco.h>
+#include "experimental/filament/filament/filament_context.h"
+
+#if defined(TLS_FILAMENT_CONTEXT)
+static thread_local mujoco::FilamentContext* g_filament_context = nullptr;
+#else
+static mujoco::FilamentContext* g_filament_context = nullptr;
+#endif
+
+// Default asset loader to use when calling mjr_makeContext. This is only
+// intended for basic backwards compatibility with the existing mjr_makeContext
+// API. If this doesn't work as expected, you should be calling
+// mjr_makeFilamentContext instead and provide your own asset loading callbacks.
+// Returns 0 on success and non-zero to indicate an error.
+static int DefaultLoadAsset(const char* asset_filename, void* user_data,
+                             unsigned char** contents, uint64_t* out_size) {
+  std::filesystem::path full_path;
+  full_path.append("filament/assets/data");
+  full_path.append(asset_filename);
+
+  std::ifstream file(full_path, std::ios::binary);
+  if (!file) {
+    mju_error("File does not exist: %s", full_path.c_str());
+    return 1;
+  }
+
+  file.seekg(0, std::ios::end);
+  *out_size = static_cast<uint64_t>(file.tellg());
+  if (*out_size == 0) {
+    mju_error("File is empty: %s", full_path.c_str());
+    return 1;
+  }
+  file.seekg(0, std::ios::beg);
+
+  *contents = (unsigned char*)malloc(*out_size);
+  file.read((char*)*contents, *out_size);
+  file.close();
+  return 0;
+}
+
+static void CheckFilamentContext() {
+  if (g_filament_context == nullptr) {
+    mju_error("Missing context; did you call mjr_makeFilamentContext?");
+  }
+}
+
+
+extern "C" {
+
+void mjr_defaultFilamentConfig(mjrFilamentConfig* config) {
+  memset(config, 0, sizeof(mjrFilamentConfig));
+}
+
+void mjr_makeFilamentContext(const mjModel* m, mjrContext* con,
+                             const mjrFilamentConfig* config) {
+  // TODO: Support multiple contexts and multiple threads. For now, we'll just
+  // assume a single, global context.
+  if (g_filament_context != nullptr) {
+    mju_error("Context already exists!");
+  }
+  g_filament_context = new mujoco::FilamentContext(config, m, con);
+}
+
+void mjr_defaultContext(mjrContext* con) { memset(con, 0, sizeof(mjrContext)); }
+
+void mjr_makeContext(const mjModel* m, mjrContext* con, int fontscale) {
+  mjr_freeContext(con);
+  mjrFilamentConfig cfg;
+  mjr_defaultFilamentConfig(&cfg);
+  cfg.load_asset = DefaultLoadAsset;
+  mjr_makeFilamentContext(m, con, &cfg);
+}
+
+void mjr_freeContext(mjrContext* con) {
+  // mjr_freeContext may be called multiple times.
+  if (g_filament_context) {
+    delete g_filament_context;
+    g_filament_context = nullptr;
+  }
+  mjr_defaultContext(con);
+}
+
+void mjr_render(mjrRect viewport, mjvScene* scn, const mjrContext* con) {
+  CheckFilamentContext();
+  g_filament_context->Render(viewport, scn, con);
+}
+
+void mjr_uploadMesh(const mjModel* m, const mjrContext* con, int meshid) {
+  CheckFilamentContext();
+  g_filament_context->UploadMesh(m, meshid);
+}
+
+void mjr_uploadTexture(const mjModel* m, const mjrContext* con, int texid) {
+  CheckFilamentContext();
+  g_filament_context->UploadTexture(m, texid);
+}
+
+void mjr_uploadHField(const mjModel* m, const mjrContext* con, int hfieldid) {
+  CheckFilamentContext();
+  g_filament_context->UploadHeightField(m, hfieldid);
+}
+
+void mjr_setBuffer(int framebuffer, mjrContext* con) {
+  CheckFilamentContext();
+  g_filament_context->SetFrameBuffer(framebuffer);
+}
+
+void mjr_readPixels(unsigned char* rgb, float* depth, mjrRect viewport,
+                    const mjrContext* con) {
+  CheckFilamentContext();
+  g_filament_context->ReadPixels(viewport, rgb, depth);
+}
+
+void mjr_uploadFont(unsigned char* pixels, int width, int height, int bpp,
+                    int id, const mjrContext* con) {
+  CheckFilamentContext();
+  if (bpp != 4) {
+    mju_error("Only 4bpp fonts are supported, got %d", bpp);
+  }
+  g_filament_context->UploadFont(pixels, width, height, id);
+}
+
+}  // extern "C"