Add touch sensor for estimating the surface contact stresses from the SDF.

PiperOrigin-RevId: 758556339
Change-Id: Ib56d76fab084b530fd68d61a29294cb6f1d05f3f

plugin/sensor/README.md

@@ -9,6 +9,9 @@ plugins](https://mujoco.readthedocs.io/en/latest/programming/extension.html#engi
   - [Illustration of foveal deformation](#illustration-of-foveal-deformation)
   - [Illustration combining resolution, fields-of-view and foveal deformation](#illustration-combining-resolution-fields-of-view-and-foveal-deformation)
 
+- [Touch Stress](#touch-stress)
+  - [Example model with analytical SDF](#example-model-with-analytical-sdf)
+
 ## [Touch Grid](touch_grid.h)
 
 This sensor aggregates contact forces into "taxels": a rectangular array of pixel-like elements.
@@ -23,7 +26,7 @@ The output of the sensor is a stack of 1 to 6 "touch images" corresponding to fo
 and torques in the frame of the sensor. Forces and torques are in the in [z, x,
 y] order, corresponding to the ordering in contact frames: [normal, tangent,
 tangent] and [torsional, rolling, rolling]. Each "taxel" corresponds to an angular bin
-in spherical coordinates, and aggregates all the forces occuring inside this bin, which occur
+in spherical coordinates, and aggregates all the forces occurring inside this bin, which occur
 between the body in which the sensor's site is defined and any other body.
 
 The sensor is parametrized by 6 numbers:
@@ -84,3 +87,64 @@ See [touch_grid.xml](../../model/plugin/sensor/touch_grid.xml) to play with the
 ### Illustration combining resolution, fields-of-view and foveal deformation
 
 [![touch grid illustration](https://img.youtube.com/vi/YScjmR8LwQI/0.jpg)](https://www.youtube.com/watch?v=YScjmR8LwQI)
+
+## [Touch Stress](touch_stress.h)
+
+This sensor is based on similar concepts and parametrization as the `touch_grid`,
+while overcoming some of its limitations. In particular, the `touch_grid` can
+only provide sparse information, depending on the number of contact points
+generated. The `touch_stress` sensor can instead generate a high-resolution
+touch image. In order to do this, it requires a signed distance function (SDF)
+of the object that is in contact with the sensor. This is handled internally for
+primitives or it must be declared explicitly in the model using SDF plugins.
+
+There is one important difference with respect to the `touch_grid`: in this case,
+the force is computed in the local taxel frame and not in the frame of the sensor.
+This allows for a more intuitive interpretation of normal and tangential stresses,
+as shown in the images below.
+
+Note that in this case, the absolute values of the stresses reported by the
+sensor are unrelated to the contact forces. They are purely based on geometric
+and kinematic considerations, i.e. the SDF for the normal stress and the sliding
+velocity for the tangential contributions.
+
+### Example model with analytical SDF
+
+```xml
+<extension>
+  <plugin plugin="mujoco.sdf.gear">
+    <instance name="gear">
+      <config key="alpha" value="0"/>
+    </instance>
+  </plugin>
+  <plugin plugin="mujoco.sensor.touch_stress">
+    <instance name="touch_stress">
+      <config key="size" value="37 37"/>
+      <config key="fov" value="45 45"/>
+      <config key="gamma" value="0"/>
+      <config key="nchannel" value="3"/>
+    </instance>
+  </plugin>
+  ...
+  <asset>
+    <mesh name="gear">
+      <plugin instance="gear"/>
+    </mesh>
+  </asset>
+  ...
+  <worldbody>
+    <geom type="sdf" name="gear" mesh="gear">
+      <plugin instance="gear"/>
+    </geom>
+  ...
+  <sensor>
+    <plugin instance="touch_stress" objtype="site" objname="touch_site"/>
+  </sensor>
+</extension>
+```
+The images below show a static sphere over a gear described by an analytic SDF
+and the same sphere dragged along the x and y axes.
+
+<img src="images/normal.png" style="width: 300px;"/>
+<img src="images/tangential1.png" style="width: 300px;"/>
+<img src="images/tangential2.png" style="width: 300px;"/>