Disable tendon catenary visualization when tendon is actuated. Add no…

…te about catenary visualization.

PiperOrigin-RevId: 706161179
Change-Id: Ifaf571e039306309869078d1a0bcf9a69f1fe9de

doc/XMLreference.rst

@@ -4661,6 +4661,18 @@ A second form of wrapping is where the tendon is constrained to pass *through* a
 wrap around it. This is enabled automatically when a sidesite is specified and its position is inside the volume of
 the obstacle geom.
 
+.. youtube:: I2q7D0Vda-A
+   :width: 300px
+   :align: right
+
+**Visualization:** Tendon paths are visualized as in the image above, respecting the :ref:`width<tendon-spatial-width>`,
+:ref:`material<tendon-spatial-material>` and :ref:`rgba<tendon-spatial-rgba>` attributes below. A special kind of
+visualization is used for unactuated 2-point tendons with :ref:`range<tendon-spatial-range>` or
+:ref:`springlength<tendon-spatial-springlength>` of the form :at-val:`[0 X]`, with positive X. Such tendons act like a
+cable, applying force only when stretched. Therefore when not stretched, they are drawn as a catenary of
+length X, as in the clip on the right of `this example model
+<https://github.com/google-deepmind/mujoco/blob/main/test/engine/testdata/catenary.xml>`__.
+
 .. _tendon-spatial-name:
 
 :at:`name`: :at-val:`string, optional`

src/engine/engine_vis_visualize.c

@@ -1729,6 +1729,16 @@ void mjv_addGeoms(const mjModel* m, mjData* d, const mjvOption* vopt,
   objtype = mjOBJ_TENDON;
   category = mjCAT_DYNAMIC;
   if (vopt->flags[mjVIS_TENDON] && (category & catmask)) {
+    // mark actuated tendons
+    int* tendon_actuated = mjSTACKALLOC(d, m->ntendon, int);
+    mju_zeroInt(tendon_actuated, m->ntendon);
+    for (int i=0; i < m->nu; i++) {
+      if (m->actuator_trntype[i] == mjTRN_TENDON) {
+        tendon_actuated[m->actuator_trnid[2*i]] = 1;
+      }
+    }
+
+    // draw tendons
     for (int i=0; i < m->ntendon; i++) {
       if (vopt->tendongroup[mjMAX(0, mjMIN(mjNGROUP-1, m->tendon_group[i]))]) {
         // tendon has a deadband spring
@@ -1752,9 +1762,10 @@ void mjv_addGeoms(const mjModel* m, mjData* d, const mjvOption* vopt,
           !mjDISABLED(mjDSBL_GRAVITY)           &&    // gravity enabled
           mju_norm3(m->opt.gravity) > mjMINVAL  &&    // gravity strictly nonzero
           m->tendon_num[i] == 2                 &&    // only two sites on the tendon
-          (limitedspring || limitedconstraint)  &&    // either spring or constraint length limits
+          (limitedspring != limitedconstraint)  &&    // either spring or constraint length limits
           m->tendon_damping[i] == 0             &&    // no damping
-          m->tendon_frictionloss[i] == 0;             // no frictionloss
+          m->tendon_frictionloss[i] == 0        &&    // no frictionloss
+          tendon_actuated[i] == 0;                    // no actuator
 
         // conditions not met: draw straight lines
         if (!draw_catenary) {