From 38c33880467024d484927248dcd82df4c2db06d4 Mon Sep 17 00:00:00 2001
From: Yuval Tassa <tassa@google.com>
Date: Wed, 17 Jun 2026 04:23:16 -0700
Subject: [PATCH] Fix rotation Jacobian offset in mj_jacSum sparse path.

PiperOrigin-RevId: 933640368
Change-Id: Ib30acd651b6022b0835e07b3f3b01e648791a511
---
 src/engine/engine_core_util.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/engine/engine_core_util.c b/src/engine/engine_core_util.c
index 7cedb477..008b279a 100644
--- a/src/engine/engine_core_util.c
+++ b/src/engine/engine_core_util.c
@@ -520,12 +520,10 @@ int mj_jacSum(const mjModel* m, mjData* d, int* chain,
               const mjtNum point[3], mjtNum* jac, int flg_rot) {
   int nv = m->nv, NV;
   mjtNum* jacp = jac;
-  mjtNum* jacr = flg_rot ? jac + 3*nv : NULL;
 
   mj_markStack(d);
   mjtNum* jtmp = mjSTACKALLOC(d, flg_rot ? 6*nv : 3*nv, mjtNum);
   mjtNum* jp = jtmp;
-  mjtNum* jr = flg_rot ? jtmp + 3*nv : NULL;
 
   // sparse
   if (mj_isSparse(m)) {
@@ -537,6 +535,7 @@ int mj_jacSum(const mjModel* m, mjData* d, int* chain,
     NV = mj_bodyChain(m, body[0], chain);
     if (NV) {
       // get Jacobian
+      mjtNum* jacr = flg_rot ? jac + 3*NV : NULL;
       if (m->body_simple[body[0]]) {
         mj_jacSparseSimple(m, d, jacp, jacr, point, body[0], 1, NV, 0);
       } else {
@@ -554,6 +553,7 @@ int mj_jacSum(const mjModel* m, mjData* d, int* chain,
       if (!bodyNV) {
         continue;
       }
+      mjtNum* jr = flg_rot ? jtmp + 3*bodyNV : NULL;
       if (m->body_simple[body[i]]) {
         mj_jacSparseSimple(m, d, jp, jr, point, body[i], 1, bodyNV, 0);
       } else {
@@ -568,6 +568,9 @@ int mj_jacSum(const mjModel* m, mjData* d, int* chain,
 
   // dense
   else {
+    mjtNum* jacr = flg_rot ? jac + 3*nv : NULL;
+    mjtNum* jr = flg_rot ? jtmp + 3*nv : NULL;
+
     // set first
     mj_jac(m, d, jacp, jacr, point, body[0]);
     mju_scl(jac, jac, weight[0], flg_rot ? 6*nv : 3*nv);