From a897805683fef38173020e544d4ff2f98b282b2b Mon Sep 17 00:00:00 2001
From: Yuval Tassa <tassa@google.com>
Date: Sun, 17 May 2026 12:02:52 -0700
Subject: [PATCH] Improve test comparing forward and inverse dynamics mismatch

PiperOrigin-RevId: 916864072
Change-Id: I9952c7dd1bde2033dfe1380552f2c4b6b7344b26
---
 test/engine/engine_inverse_test.cc | 41 +++++++++++++++++++++---------
 1 file changed, 29 insertions(+), 12 deletions(-)

diff --git a/test/engine/engine_inverse_test.cc b/test/engine/engine_inverse_test.cc
index 988e73e6..97a48e2d 100644
--- a/test/engine/engine_inverse_test.cc
+++ b/test/engine/engine_inverse_test.cc
@@ -41,19 +41,36 @@ TEST_F(InverseTest, ForwardInverseMatch) {
   ASSERT_THAT(model, NotNull()) << error;
   mjData* data = mj_makeData(model);
 
-  // simulate, call mj_forward
-  for (int i = 0; i < kSteps; ++i) {
-    mj_step(model, data);
+  // set small tolerance and enough iterations for all solvers to converge
+  model->opt.iterations = 500;
+  model->opt.tolerance = 0;
+
+  // solver names for diagnostics
+  const char* solver_name[] = {"PGS", "CG", "Newton"};
+
+  for (mjtSolver solver : {mjSOL_PGS, mjSOL_CG, mjSOL_NEWTON}) {
+    model->opt.solver = solver;
+    mj_resetData(model, data);
+
+    // simulate, call mj_forward
+    for (int i = 0; i < kSteps; ++i) {
+      mj_step(model, data);
+    }
+    mj_forward(model, data);
+
+    // call built-in testing function
+    mj_compareFwdInv(model, data);
+
+    // per-solver tolerances
+    mjtNum epsilon;
+    switch (solver) {
+    case mjSOL_PGS:    epsilon = MjTol(1e-6, 1e-2);   break;
+    case mjSOL_CG:     epsilon = MjTol(1e-3, 1e-1);   break;
+    case mjSOL_NEWTON: epsilon = MjTol(1e-10, 5e-3);  break;
+    }
+    EXPECT_LT(data->solver_fwdinv[0], epsilon) << solver_name[solver];
+    EXPECT_LT(data->solver_fwdinv[1], epsilon) << solver_name[solver];
   }
-  mj_forward(model, data);
-
-  // call built-in testing function
-  mj_compareFwdInv(model, data);
-
-  // expect mismatch to be small
-  mjtNum epsilon = MjTol(1e-10, 0.05);
-  EXPECT_LT(data->solver_fwdinv[0], epsilon);
-  EXPECT_LT(data->solver_fwdinv[1], epsilon);
 
   mj_deleteData(data);
   mj_deleteModel(model);