Merge pull request #3451 from smallquail:flex-geometric-stiffness

PiperOrigin-RevId: 958995742 Change-Id: Ibb60c3112b6dbc2373a5cd98df0477d639a5a001
2026-08-04 06:56:37 -07:00
parent 3d1d133a48 5497994763
commit 10793e539e
3 changed files with 138 additions and 14 deletions
@@ -65,6 +65,11 @@ Bug fixes
  large penetration depth between nearly touching thin boxes with positive margin, causing resting stacks to explode.
 - Fixed a bug in the box-box collider where penetrations deeper than a box's smallest half-size could produce no
  contacts, letting boxes tunnel through thin boxes. Fixes :issue:`1800`.
+- Fixed the flex stretch stiffness operator, which was the Gauss-Newton Hessian of the stretch force rather than its
+  Jacobian: the geometric (stress-proportional) term was missing. Only the tensile part of that term is added, since
+  it is positive semi-definite exactly when the edge is in tension, and its consumers require an SPD operator; the
+  stretch force itself is unchanged. This affects the implicit integrators and the implicit effective metric, so
+  flexes using ``elastic2d="stretch"`` integrate slightly differently. Bending-only flexes are unaffected.

 Version 3.11.0 (July 27, 2026)
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