Add per-body gravity compensation (buoyancy) passive force.

PiperOrigin-RevId: 485342531
Change-Id: Icb3e6bb5080b6ef17a3d3b1cc67e0503cfeaeffb

src/xml/xml_native_reader.cc

@@ -227,8 +227,8 @@ static const char* MJCF[nMJCF][mjXATTRNUM] = {
             "emission", "specular", "shininess", "reflectance", "rgba"},
     {">"},
 
-    {"body", "R", "10", "name", "childclass", "pos", "quat", "mocap",
-        "axisangle", "xyaxes", "zaxis", "euler", "user"},
+    {"body", "R", "11", "name", "childclass", "pos", "quat", "mocap",
+        "axisangle", "xyaxes", "zaxis", "euler", "gravcomp", "user"},
     {"<"},
         {"plugin", "*", "3", "name", "plugin", "instance"},
         {"<"},
@@ -2714,6 +2714,9 @@ void mjXReader::Body(XMLElement* section, mjCBody* pbody) {
       }
       ReadAlternative(elem, pchild->alt);
 
+      // read gravcomp
+      ReadAttr(elem, "gravcomp", 1, &pchild->gravcomp, text);
+
       // read userdata
       ReadVector(elem, "user", pchild->userdata, text);
 

src/xml/xml_native_writer.cc

@@ -1338,6 +1338,9 @@ void mjXWriter::Body(XMLElement* elem, mjCBody* body) {
       WriteAttrKey(elem, "mocap", bool_map, 2, 1);
     }
 
+    // gravity compensation
+    WriteAttr(elem, "gravcomp", 1, &body->gravcomp);
+
     // userdata
     WriteVector(elem, "user", body->userdata);