Use mj_id2name in viewer code where appropriate.

PiperOrigin-RevId: 948945088
Change-Id: I5635c71fd285b128618592699ca64ce8c0f35e31

src/experimental/platform/ux/gui.cc

@@ -642,8 +642,8 @@ std::string GetCameraName(const mjModel* model, const mjvCamera& camera,
     return kCameraWasdName;
   } else if (index == kTrackingCameraIdx) {
     return "Tracking (" + std::to_string(camera.trackbodyid) + ")";
-  } else if (model->names[model->name_camadr[index]]) {
-    return std::string(model->names + model->name_camadr[index]);
+  } else if (const char* cam_name = mj_id2name(model, mjOBJ_CAMERA, index)) {
+    return cam_name;
   } else {
     return kCameraUnnamedName;
   }
@@ -1262,8 +1262,8 @@ void JointsGui(const mjModel* model, const mjData* data,
       continue;
     }
 
-    const char* jnt_name = model->names + model->name_jntadr[i];
-    if (*jnt_name) {
+    const char* jnt_name = mj_id2name(model, mjOBJ_JOINT, i);
+    if (jnt_name) {
       std::snprintf(name, sizeof(name), "%s", jnt_name);
     } else {
       std::snprintf(name, sizeof(name), "joint %d", i);
@@ -1315,8 +1315,8 @@ void ControlsGui(const mjModel* model, const mjData* data,
       continue;
     }
 
-    const char* ctrl_name = model->names + model->name_actuatoradr[i];
-    if (*ctrl_name) {
+    const char* ctrl_name = mj_id2name(model, mjOBJ_ACTUATOR, i);
+    if (ctrl_name) {
       std::snprintf(name, sizeof(name), "%s", ctrl_name);
     } else {
       std::snprintf(name, sizeof(name), "control %d", i);

src/experimental/platform/ux/picture_gui.cc

@@ -31,8 +31,8 @@ static bool PipGuiImpl(const mjModel* model, mjData* data, float aspect_ratio,
   bool result = true;
 
   auto get_camera_name = [model](int i) -> const char* {
-    if (model->names[model->name_camadr[i]]) {
-      return model->names + model->name_camadr[i];
+    if (const char* name = mj_id2name(model, mjOBJ_CAMERA, i)) {
+      return name;
     } else {
       return "Unnamed";
     }

src/experimental/studio/app.cc

@@ -1609,9 +1609,8 @@ void App::ModelOptionsGui() {
       ImGui::Spacing();
       {
         char key_fmt[128];
-        const char* key_name =
-            model()->names + model()->name_keyadr[ui_.key_idx];
-        if (key_name[0] != '\0') {
+        const char* key_name = mj_id2name(model(), mjOBJ_KEY, ui_.key_idx);
+        if (key_name) {
           std::snprintf(key_fmt, sizeof(key_fmt), "%s", key_name);
         } else {
           std::snprintf(key_fmt, sizeof(key_fmt), "Key %d", ui_.key_idx);