Rephrase documentation in places where "parent body" creates ambiguity between "the body which contains the element" and "the parent body of the body which contains the element".

Fixes #1127

PiperOrigin-RevId: 576486971
Change-Id: I54d0c351e95101c5f7907a3c5bb6be18904c03ad

doc/XMLreference.rst

@@ -2161,10 +2161,10 @@ This element creates a joint. As explained in :ref:`Kinematic tree <CTree>`, a j
 between the body where it is defined and the body's parent. If multiple joints are defined in the same body, the
 corresponding spatial transformations (of the body frame relative to the parent frame) are applied in order. If no
 joints are defined, the body is welded to its parent. Joints cannot be defined in the world body. At runtime the
-positions and orientations of all joints defined in the model are stored in the vector mjData.qpos, in the order in
+positions and orientations of all joints defined in the model are stored in the vector ``mjData.qpos``, in the order in
-which the appear in the kinematic tree. The linear and angular velocities are stored in the vector mjData.qvel. These
+which the appear in the kinematic tree. The linear and angular velocities are stored in the vector ``mjData.qvel``.
-two vectors have different dimensionality when free or ball joints are used, because such joints represent rotations as
+These two vectors have different dimensionality when free or ball joints are used, because such joints represent
-unit quaternions.
+rotations as unit quaternions.
 
 .. _body-joint-name:
 
@@ -2211,7 +2211,8 @@ unit quaternions.
 .. _body-joint-pos:
 
 :at:`pos`: :at-val:`real(3), "0 0 0"`
-   Position of the joint, specified in the frame of the parent body. For free joints this attribute is ignored.
+   Position of the joint, specified in the frame of the body where the joint is defined.
+   For free joints this attribute is ignored.
 
 .. _body-joint-axis:
 
@@ -2654,7 +2655,7 @@ helps clarify the role of bodies and geoms in MuJoCo.
 .. _body-geom-pos:
 
 :at:`pos`: :at-val:`real(3), "0 0 0"`
-   Position of the geom, specified in the frame of the parent body.
+   Position of the geom, specified in the frame of the body where the geom is defined.
 
 .. _body-geom-quat:
 
@@ -2698,8 +2699,8 @@ helps clarify the role of bodies and geoms in MuJoCo.
 
 :at:`fluidshape`: :at-val:`[none, ellipsoid], "none"`
    "ellipsoid" activates the geom-level fluid interaction model based on an ellipsoidal approximation of the geom
-   shape. When active, the model based on :ref:`body inertia sizes <flInertia>` is disabled for the parent body.
+   shape. When active, the model based on :ref:`body inertia sizes <flInertia>` is disabled for the body in which the
-   See section on :ref:`ellipsoid-based<flEllipsoid>` fluid interaction model for details.
+   geom is defined. See section on :ref:`ellipsoid-based<flEllipsoid>` fluid interaction model for details.
 
 .. _body-geom-fluidcoef:
 
@@ -2879,15 +2880,15 @@ and the +Y axis points up. Thus the frame position and orientation are the key a
 :at:`mode`: :at-val:`[fixed, track, trackcom, targetbody, targetbodycom], "fixed"`
    This attribute specifies how the camera position and orientation in world coordinates are computed in forward
    kinematics (which in turn determine what the camera sees). "fixed" means that the position and orientation specified
-   below are fixed relative to the parent (i.e., the body where the camera is defined). "track" means that the camera
+   below are fixed relative to the the body where the camera is defined. "track" means that the camera position is at a
-   position is at a constant offset from the parent in world coordinates, while the camera orientation is constant in
+   constant offset from the body in world coordinates, while the camera orientation is constant in world coordinates.
-   world coordinates. These constants are determined by applying forward kinematics in qpos0 and treating the camera as
+   These constants are determined by applying forward kinematics in qpos0 and treating the camera as fixed. Tracking can
-   fixed. Tracking can be used for example to position a camera above a body, point it down so it sees the body, and
+   be used for example to position a camera above a body, point it down so it sees the body, and have it always remain
-   have it always remain above the body no matter how the body translates and rotates. "trackcom" is similar to "track"
+   above the body no matter how the body translates and rotates. "trackcom" is similar to "track" but the constant
-   but the constant spatial offset is defined relative to the center of mass of the kinematic subtree starting at the
+   spatial offset is defined relative to the center of mass of the kinematic subtree starting at the body in which the
-   parent body. This can be used to keep an entire mechanism in view. Note that the subtree center of mass for the world
+   camera is defined. This can be used to keep an entire mechanism in view. Note that the subtree center of mass for the
-   body is the center of mass of the entire model. So if a camera is defined in the world body in mode "trackcom", it
+   world body is the center of mass of the entire model. So if a camera is defined in the world body in mode "trackcom",
-   will track the entire model. "targetbody" means that the camera position is fixed in the parent body, while the
+   it will track the entire model. "targetbody" means that the camera position is fixed in the body frame, while the
    camera orientation is adjusted so that it always points towards the targeted body (which is specified with the target
    attribute below). This can be used for example to model an eye that fixates a moving object; the object will be the
    target, and the camera/eye will be defined in the body corresponding to the head. "targetbodycom" is the same as

doc/modeling.rst

@@ -198,8 +198,8 @@ Coordinate frames
 ~~~~~~~~~~~~~~~~~
 
 The positions and orientations of all elements defined in the kinematic tree are expressed in local coordinates,
-relative to the parent body for bodies, and relative to the body that owns the element for geoms, joints, sites, cameras
+relative to the parent body for bodies, and relative to the body that contains the element for geoms, joints, sites,
-and lights.
+cameras and lights.
 
 A related attribute is :ref:`compiler/angle<compiler-angle>`. It specifies whether angles in the MJCF file are expressed
 in degrees or radians (after compilation, angles are always expressed in radians).
@@ -981,7 +981,7 @@ Cameras
 Besides the default, user-controllable, free camera, "fixed" cameras can be attached to the kinematic tree.
 
 Extrinsics
-   By default, camera frames are attached to the parent body. The optional :ref:`mode<body-camera-mode>` and
+   By default, camera frames are attached to the containing body. The optional :ref:`mode<body-camera-mode>` and
    :ref:`target<body-camera-target>` attributes can be used to specify camera that track (move with) or target (look at)
    a body or subtree. Cameras look towards the negative Z axis of the camera frame, while positive X and Y correspond to
    *right* and *up* in the image plane, respectively.