Remove grid from composite types.

Replaced it with flexcomp in the models.

PiperOrigin-RevId: 730070619
Change-Id: Icbc69e4c6784c6252743f689a1eafb6482d0c393

doc/XMLreference.rst

@@ -2982,7 +2982,7 @@ coordinates results in compiler error. See :ref:`CComposite` in the modeling gui
 
 .. _body-composite-type:
 
-:at:`type`: :at-val:`[particle, grid, cable, rope, loop, cloth, box, cylinder, ellipsoid], required`
+:at:`type`: :at-val:`[particle, cable], required`
    This attribute determines the type of composite object. The remaining attributes and sub-elements are then
    interpreted according to the type. Default settings are also adjusted depending on the type.
 
@@ -2993,48 +2993,13 @@ coordinates results in compiler error. See :ref:`CComposite` in the modeling gui
    and replace the default sphere with a custom geom. Note that the particle composite type is deprecated and might be
    removed in a future version. Instead of particle, it is recommended to use :ref:`replicate`.
 
-   The **grid** type creates a 1D or 2D grid of bodies, each having a sphere geom, a sphere site, and 3 orthogonal
-   sliding joints by default. The :el:`pin` sub-element can be used to specify that some bodies should not have joints,
-   and instead should be pinned to the parent body. Unlike the particle type, here each two neighboring bodies are
-   connected with a spatial tendon whose length is equality-constrained to its initial value (the sites are needed to
-   define the tendons). The "main" tendons are parallel to the axes of the grid. In addition one can create diagonal
-   "shear" tendons, using the :el:`tendon` sub-element. This type is suitable for simulating strings as well as cloth.
-
    The **cable** type creates a 1D chain of bodies connected with ball joints, each having a geom with user-defined type
    (cylinder, capsule or box). The geometry can either be defined with an array of 3D vertex coordinates :at:`vertex`
    or with prescribed functions with the option :at:`curve`. Currently, only linear and trigonometric functions are
    supported. For example, an helix can be obtained with curve="cos(s) sin(s) s". The size is set with the option
    :at:`size`, resulting in :math:`f(s)=\{\text{size}[1]\cdot\cos(2\pi\cdot\text{size}[2]),\;
    \text{size}[1]\cdot\sin(2\pi\cdot\text{size}[2]),\; \text{size}[0]\cdot s\}`.
 
-   The **cloth** type is a different way to model cloth, beyond type="grid". Here the elements are connected with
-   universal joints and form a kinematic spanning tree. The root of the tree is the parent body, and its coordinates in
-   the grid are inferred from its name - similar to rope but here the naming format is "CB2_0". Neighboring bodies that
-   are not connected with joints are then connected with equality-constrained spatial tendons. The resulting cloth is
-   non-homogeneous, because the kinematic constraints cannot be violated while the tendon equality constraints are soft.
-   One can make it more homogeneous by adding stretch and twist joints (similar to rope) and adjusting the strength of
-   their equality constraints. Shear tendons can also be added. In addition to the different physics, cloth can do
-   things that a 2D grid cannot do. This is because the elements of cloth have both position and orientation, while the
-   elements of grid can only translate. The geoms used in cloth can be ellipsoids and capsules in addition to spheres.
-   When elongated geoms are used, they are rotated and interleaved in a pattern that fills the holes, preventing objects
-   from penetrating the cloth. Furthermore the inertia of the cloth elements can be modified with the flatinertia
-   attribute, and can then be used with lift and drag forces to simulate ripple effects.
-
-   The **box** type creates a 3D arrangement of bodies forming the outer shell of a (soft) box. The parent body is at
-   the center of the box. Each element body has a geom (sphere, ellipsoid or capsule) and a single sliding joint
-   pointing away from the center of the box. The sliding joints are equality-constrained to their initial value.
-   Furthermore, to achieve smooth deformations of the sides of the box, each joint is equality-constrained to remain
-   equal to its neighbor joints. To preserve the volume of the soft box approximately, a fixed tendon is used to
-   constrain the sum of all joints to remain constant. When the user specifies elongated geoms (capsules or ellipsoids)
-   their long axis is aligned with the sliding joint axis. This makes the shell thicker for collision detection
-   purposes, preventing objects from penetrating the box. It is important to disable contacts between the elements of
-   the box. This is done by setting the default geom contype to 0. The user can change it of course, but if the geoms
-   comprising the soft box are allowed to contact each other the model will not work as intended.
-
-   The **cylinder** and **ellipsoid** types are the same as box, except the elements are projected on the surface of an
-   ellipsoid or a cylinder respectively. Thus the composite soft body shape is different, while everything else is the
-   same as in the box type.
-
 .. _body-composite-count:
 
 :at:`count`: :at-val:`int(3), required`

doc/changelog.rst

@@ -48,6 +48,7 @@ General
 
      Previously, if the volumetric inertia computation failed (for example due to a very flat mesh), the compiler
      would silently fall back to surface inertia computation. Now, the compiler will throw an informative error.
+   - Removed the composite type ``grid``. Users should instead use :ref:`flexcomp<body-flexcomp>`.
 
 MJX
 ^^^

doc/modeling.rst

@@ -1193,47 +1193,10 @@ a more complete treatment, see again the :ref:`deformable <CDeformable>` section
 flex, see the folder `elasticity/ <https://github.com/google-deepmind/mujoco/tree/main/model/plugin/elasticity>`__ for
 several examples.
 
-**1D grid**.
+**Grid**.
 
-|image6| |image7|
-
-.. code-block:: xml
-
-   <composite type="grid" count="20 1 1" spacing="0.045" offset="0 0 1">
-     <joint kind="main" damping="0.001"/>
-     <tendon kind="main" width="0.01"/>
-     <geom size=".02" rgba=".8 .2 .1 1"/>
-     <pin coord="1"/>
-     <pin coord="13"/>
-   </composite>
-
-The grid type can create 1D or 2D grids, depending on the :at:`count` attribute. Here we illustrate 1D grids. These
-are strings of spheres connected with tendons whose length is soft-equality-constrained. The softness can be adjusted.
-Similar to particles, the element bodies here have slider joints but no rotational joints. The plot on the right
-illustrates pinning. The :el:`pin` sub-element is used to specify the grid coordinates of the pinned bodies, and the
-model compiler does not generate joints for these bodies, thereby fixing them rigidly to the parent body (in this case
-the world). This makes the string in the right plot hang in space. The same mechanism can be used to model a whip for
-example; in that case the parent body would be moving, and the first element body would be pinned to the parent.
-
-**2D grid**.
-
-|image8| |image9|
-
-.. code-block:: xml
-
-   <composite type="grid" count="9 9 1" spacing="0.05" offset="0 0 1">
-     <skin material="matcarpet" inflate="0.001" subgrid="3" texcoord="true"/>
-     <geom size=".02"/>
-     <pin coord="0 0"/>
-     <pin coord="8 0"/>
-   </composite>
-
-A 2D grid can be used to simulate cloth. What it really simulates is a 2D grid of spheres connected with
-equality-constrained tendons (not shown). The model compiler can also generate skin, enabled with the :el:`skin`
-sub-element in the above XML. Some of the element bodies can also be pinned, similar to 1D grids but using two grid
-coordinates. The plot on the right shows a cloth pinned to the world body at the two corners, and draping over our
-capsule probe. The skin on the right is subdivided using bi-cubic interpolation, which increases visual quality in the
-absence of textures. When textures are present (left) the benefits of subdivision are less visible.
+The grid composite type has been removed. It is recommended to use 2D flex :ref:`deformable objects <CDeformable>` for
+simulating thin elastic structures.
 
 **Cable**.
 

model/hammock/hammock.xml

@@ -22,7 +22,7 @@
      Simple humanoid on a hammock, implemented as a 2D grid composite, pinned at the corners.
 -->
 
-  <option timestep="0.005" solver="CG" iterations="30" tolerance="1e-6"/>
+  <option timestep="0.001" solver="CG" iterations="30" tolerance="1e-6"/>
 
   <size memory="20M"/>
 
@@ -41,23 +41,22 @@
              width="512" height="512" mark="edge" markrgb=".8 .8 .8"/>
     <material name="plane" reflectance="0.3" texture="plane" texrepeat="1 1" texuniform="true"/>
     <material name="hammock" texture="hammock"/>
+    <model name="humanoid" file="../humanoid/humanoid.xml"/>
   </asset>
 
-  <include file="humanoid_body.xml"/>
-
   <worldbody>
     <geom name="floor" pos="0 0 -1" size="0 0 .25" type="plane" material="plane" condim="3"/>
     <light directional="true" diffuse=".2 .2 .2" specular="0 0 0" pos="0 0 5" dir="0 0 -1" castshadow="false"/>
     <light directional="false" diffuse=".8 .8 .8" specular="0.3 0.3 0.3" pos="0 0 4" dir="0 0 -1"/>
-    <composite type="grid" count="11 9 1" spacing="0.2" offset="0. 0. 0">
-      <skin texcoord="true" material="hammock" inflate="0.01" subgrid="3"/>
-      <pin coord="0 0"/>
-      <pin coord="10 0"/>
-      <pin coord="0 8"/>
-      <pin coord="10 8"/>
-      <geom size=".095"/>
-      <joint kind="main" damping="10"/>
-    </composite>
+    <flexcomp name="hammock" type="grid" count="11 9 1" spacing="0.2 0.2 0.2" material="hammock"
+              radius="0.04" dim="2">
+      <pin id="0 8 90 98"/>
+      <edge equality="true" solref="0.002 5"/>
+      <contact selfcollide="none" internal="false" solimp=".99 .999 .00001"/>
+    </flexcomp>
+    <frame euler="0 180 -15" pos="-.4 -.2 2">
+      <attach model="humanoid" body="torso" prefix="humanoid_"/>
+    </frame>
   </worldbody>
 
 </mujoco>