Visualize joint and tendon limits by re-coloring.

PiperOrigin-RevId: 811590674
Change-Id: I5b65f2294a4bcda3c321fb51ebf29501bd0d416d

doc/XMLreference.rst

@@ -4886,7 +4886,9 @@ length X, as in the clip on the right of `this example model
 
 :at:`rgba`: :at-val:`real(4), "0.5 0.5 0.5 1"`
    Color and transparency of the tendon. When this value is different from the internal default, it overrides the
-   corresponding material properties.
+   corresponding material properties. If a :at:`material` is unspecified and :at:`rgba` has the default value, limited
+   tendons whose length exceeds the limit are recolored using the value of the :ref:`constraint impedance<soParameters>`
+   :math:`d` to mix the default color and :ref:`rgba/constraint<visual-rgba-constraint>`.
 
 .. _tendon-spatial-springlength:
 
@@ -8292,7 +8294,9 @@ disables the rendering of the corresponding object.
 .. _visual-rgba-joint:
 
 :at:`joint`: :at-val:`real(4), "0.2 0.6 0.8 1"`
-   Color of the arrows used to render joint axes.
+   Color of the arrows used to render joint axes. If a joint is limited and the joint value exceeds the limit, the
+   value of the :ref:`constraint impedance<soParameters>` :math:`d` is used to mix this color and
+   :ref:`rgba/constraint<visual-rgba-constraint>`.
 
 .. _visual-rgba-actuator:
 
@@ -8368,7 +8372,7 @@ disables the rendering of the corresponding object.
 .. _visual-rgba-constraint:
 
 :at:`constraint`: :at-val:`real(4), "0.9 0 0 1"`
-   Color of the capsules corresponding to spatial constraint violations.
+   Color corresponding to spatial constraint violations -- equality constraints, joint limits, and tendon limits.
 
 .. _visual-rgba-slidercrank:
 

doc/changelog.rst

@@ -19,6 +19,12 @@ General
 
      **Migration:** Replace ``meshdir`` and ``texturedir`` with ``compiler.meshdir`` and ``compiler.texturedir``.
 
+- Joint decorators and spatial tendons which have limits defined and whose current value (angle or length) exceeds the
+  limit, are recolored by using the :ref:`constraint impedance<soParameters>` :math:`d` to mix the existing color with
+  :ref:`visual/rgba/constraint<visual-rgba-constraint>`. For spatial tendons, this visualization aid is active only if
+  no :ref:`material<tendon-spatial-material>` is set and :ref:`rgba<tendon-spatial-rgba>` is default.
+
+
 Version 3.3.6 (September 15, 2025)
 ----------------------------------
 

src/engine/engine_vis_visualize.c

@@ -1462,7 +1462,24 @@ void mjv_addGeoms(const mjModel* m, mjData* d, const mjvOption* vopt,
           mjERROR("unknown joint type %d", m->jnt_type[i]);
         }
 
-        f2f(thisgeom->rgba, m->vis.rgba.joint, 4);
+        // loop over limit constraints, get impedance if this joint is limited
+        mjtNum imp = 0;
+        int efc_start = d->ne + d->nf;
+        int efc_end = efc_start + d->nl;
+        for (int k=efc_start; k < efc_end; k++) {
+          if (d->efc_type[k] == mjCNSTR_LIMIT_JOINT && d->efc_id[k] == i) {
+            imp = d->efc_KBIP[4*k + 2];
+          }
+        }
+
+        // use impedance to mix joint and constraint colors
+        float rgba[4];
+        rgba[0] = (1-imp) * m->vis.rgba.joint[0] + imp * m->vis.rgba.constraint[0];
+        rgba[1] = (1-imp) * m->vis.rgba.joint[1] + imp * m->vis.rgba.constraint[1];
+        rgba[2] = (1-imp) * m->vis.rgba.joint[2] + imp * m->vis.rgba.constraint[2];
+        rgba[3] = 1;
+
+        f2f(thisgeom->rgba, rgba, 4);
 
         // vopt->label
         if (vopt->label == mjLABEL_JOINT) {
@@ -2290,8 +2307,28 @@ void mjv_addGeoms(const mjModel* m, mjData* d, const mjvOption* vopt,
               mjv_connector(thisgeom, mjGEOM_CAPSULE, sz[0], d->wrap_xpos+3*j, d->wrap_xpos+3*j+3);
 
               // set material properties
-              float* rgba = m->tendon_rgba+4*i;
               int tendon_matid = m->tendon_matid[i];
+              float rgba[4];
+              f2f(rgba, m->tendon_rgba+4*i, 4);
+
+              // if tendon has no material and the color is the default gray, re-color it using limit impedance
+              if (tendon_matid == -1 && rgba[0] == 0.5 && rgba[1] == 0.5 && rgba[2] == 0.5 && rgba[3] == 1) {
+                // loop over limit constraints, get impedance if this tendon is limited
+                mjtNum imp = 0;
+                int efc_start = d->ne + d->nf;
+                int efc_end = efc_start + d->nl;
+                for (int k=efc_start; k < efc_end; k++) {
+                  if (d->efc_type[k] == mjCNSTR_LIMIT_TENDON && d->efc_id[k] == i) {
+                    imp = d->efc_KBIP[4*k + 2];
+                  }
+                }
+
+                // use impedance to mix tendon and constraint colors
+                rgba[0] = (1-imp) * rgba[0] + imp * m->vis.rgba.constraint[0];
+                rgba[1] = (1-imp) * rgba[1] + imp * m->vis.rgba.constraint[1];
+                rgba[2] = (1-imp) * rgba[2] + imp * m->vis.rgba.constraint[2];
+              }
+
               setMaterial(m, thisgeom, tendon_matid, rgba, vopt->flags);
 
               // override if visualizing islands

test/engine/testdata/catenary.xml

@@ -22,7 +22,7 @@
   </worldbody>
 
   <tendon>
-    <spatial limited="true" range="0 .5" rgba="0 .1 1 1" width=".005">
+    <spatial limited="true" range="0 .5" width=".005">
       <site site="hook"/>
       <site site="corner"/>
     </spatial>