Added support for GMSH22 surface loading

PiperOrigin-RevId: 629734063
Change-Id: I6a250b81de4e7c8f7520795bfe74a81d719dfb50

doc/XMLreference.rst

@@ -3452,9 +3452,10 @@ saving the XML:
 .. _body-flexcomp-file:
 
 :at:`file`: :at-val:`string, optional`
-   The name of the file from which a **mesh** or a **gmsh** is loaded. For mesh, the file extentsion is used to
+   The name of the file from which a **surface** (triangular) or **volumetric** (tetrahedral) mesh is loaded. For
-   determine the file format. Supported formats are the same as in :ref:`mesh assets<asset-mesh>`. For gmsh, the file is
+   surface meshes, the file extension is used to determine the file format. Supported formats are the same as in
-   expected to be in GMSH format 4.1 or 2.2, ascii or binary, see :ref:`here<gmsh-file-docs>`.
+   :ref:`mesh assets<asset-mesh>` and also including GMSH. Volumetric meshes are supported only in GMSH format.
+   See :ref:`here<gmsh-file-docs>` for more information on GMSH files.
 
 .. _body-flexcomp-rigid:
 

doc/changelog.rst

@@ -24,39 +24,40 @@ General
      field.
 4. The type of the ``size`` argument of :ref:`mj_stackAllocNum` and :ref:`mj_stackAllocInt` was changed from ``int``
    to ``size_t``.
+5. Added support for gmsh format version 2.2 surface meshes in :ref:`flexcomp<body-flexcomp-file>`.
 
 MJX
 ^^^
 .. admonition:: Breaking API changes
    :class: attention
 
-   5. Removed deprecated ``mjx.device_get_into`` and ``mjx.device_put`` functions as they lack critical new
+   6. Removed deprecated ``mjx.device_get_into`` and ``mjx.device_put`` functions as they lack critical new
       functionality.
 
       **Migration:** Use ``mjx.get_data_into`` instead of ``mjx.device_get_into``, and ``mjx.put_data`` instead of
       ``mjx.device_put``.
 
-6. Added cylinder plane collisions.
+7. Added cylinder plane collisions.
-7. Added ``efc_type`` to ``mjx.Data`` and ``dim``, ``efc_address`` to ``mjx.Contact``.
+8. Added ``efc_type`` to ``mjx.Data`` and ``dim``, ``efc_address`` to ``mjx.Contact``.
-8. Added ``geom`` to ``mjx.Contact`` and marked ``geom1``, ``geom2`` deprecated.
+9. Added ``geom`` to ``mjx.Contact`` and marked ``geom1``, ``geom2`` deprecated.
-9. Added ``ne``, ``nf``, ``nl``, ``nefc``, and ``ncon`` to ``mjx.Data`` to match ``mujoco.MjData``.
+10. Added ``ne``, ``nf``, ``nl``, ``nefc``, and ``ncon`` to ``mjx.Data`` to match ``mujoco.MjData``.
-10. Given the above added fields, removed ``mjx.get_params``, ``mjx.ncon``, and ``mjx.count_constraints``.
+11. Given the above added fields, removed ``mjx.get_params``, ``mjx.ncon``, and ``mjx.count_constraints``.
-11. Changed the way meshes are organized on device to speed up collision detection when a mesh is replicated for many
+12. Changed the way meshes are organized on device to speed up collision detection when a mesh is replicated for many
     geoms.
-12. Fixed a bug where capsules might be ignored in broadphase colliision checking.
+13. Fixed a bug where capsules might be ignored in broadphase colliision checking.
-13. Added cylinder collisions using SDFs.
+14. Added cylinder collisions using SDFs.
-14. Added support for all :ref:`condim <coContact>`: 1, 3, 4, 6.
+15. Added support for all :ref:`condim <coContact>`: 1, 3, 4, 6.
-15. Add support functions for ``id2name`` and ``name2id``, MJX versions of :ref:`mj_id2name` and :ref:`mj_name2id`.
+16. Add support functions for ``id2name`` and ``name2id``, MJX versions of :ref:`mj_id2name` and :ref:`mj_name2id`.
-16. Added support for :ref:`gravcomp<body-gravcomp>` and :ref:`actuatorgravcomp<body-joint-actuatorgravcomp>`.
+17. Added support for :ref:`gravcomp<body-gravcomp>` and :ref:`actuatorgravcomp<body-joint-actuatorgravcomp>`.
-17. Fixed a bug in ``mjx.ray`` for sometimes allowed negative distances for ray-mesh tests.
+18. Fixed a bug in ``mjx.ray`` for sometimes allowed negative distances for ray-mesh tests.
 
 Bug fixes
 ^^^^^^^^^
-18. Defaults of lights were not being saved, now fixed.
+19. Defaults of lights were not being saved, now fixed.
-19. Prevent overwriting of frame names by body names when saving an XML. Bug introduced in 3.1.4.
+20. Prevent overwriting of frame names by body names when saving an XML. Bug introduced in 3.1.4.
-20. Fixed bug in Python binding of :ref:`mj_saveModel`: ``buffer`` argument was documented as optional but was actually
+21. Fixed bug in Python binding of :ref:`mj_saveModel`: ``buffer`` argument was documented as optional but was actually
     not optional.
-21. Fixed bug that prevented memory allocations larger than 2.15 GB.
+22. Fixed bug that prevented memory allocations larger than 2.15 GB.
 
 
 Version 3.1.4 (April 10th, 2024)

src/user/user_flexcomp.cc

@@ -62,7 +62,7 @@ static void ReadStrFromBuffer(char* dest, const char* src, int maxlen) {
   std::strncpy(dest, src, maxlen);
 }
 
-bool IsValidElementHeader22(const std::string& line) {
+bool IsValidElementOrNodeHeader22(const std::string& line) {
   // making sure characters are numbers
   for (char c : line) {
     if (!std::isdigit(c)) {
@@ -1296,10 +1296,21 @@ void mjCFlexcomp::LoadGMSH41(char* buffer, int binary, int nodeend,
 // load GMSH format 2.2
 void mjCFlexcomp::LoadGMSH22(char* buffer, int binary, int nodeend,
                              int nodebegin, int elemend, int elembegin) {
+  // number of nodes
+  size_t numNodes = 0;
+
   // ascii nodes
   if (binary == 0) {
     // convert node char buffer to stringstream
     stringstream ss(std::string(buffer + nodebegin, nodeend - nodebegin));
+    std::string line;
+
+    // checking header template
+    std::getline(ss, line);
+    if (!IsValidElementOrNodeHeader22(line)) {
+      throw mjCError(NULL, "Invalid node header");
+    }
+    ss.seekg(-(line.size()+1), std::ios::cur);
 
     // read header
     size_t maxNodeTag = 0;
@@ -1307,7 +1318,7 @@ void mjCFlexcomp::LoadGMSH22(char* buffer, int binary, int nodeend,
     if (!ss.good()) {
       throw mjCError(NULL, "Error reading Nodes header");
     }
-    size_t numNodes = maxNodeTag;
+    numNodes = maxNodeTag;
 
     if (numNodes < 0 || numNodes >= INT_MAX / 3) {
       throw mjCError(NULL, "Invalid number of nodes.");
@@ -1349,7 +1360,7 @@ void mjCFlexcomp::LoadGMSH22(char* buffer, int binary, int nodeend,
     ReadStrFromBuffer(maxNodeTagChar, buffer + nodebegin, std::min(10, nodeend - nodebegin));
     size_t measuredHeaderSize = strnlen(maxNodeTagChar, 10) - 1;
     size_t maxNodeTag = std::stoi(maxNodeTagChar);
-    size_t numNodes = maxNodeTag;
+    numNodes = maxNodeTag;
 
     // check number of nodes is a positive number
     if (numNodes < 0) {
@@ -1396,7 +1407,7 @@ void mjCFlexcomp::LoadGMSH22(char* buffer, int binary, int nodeend,
 
     // checking header template
     std::getline(ss, line);
-    if (!IsValidElementHeader22(line)) {
+    if (!IsValidElementOrNodeHeader22(line)) {
       throw mjCError(NULL, "Invalid elements header");
     }
     ss.seekg(-(line.size()+1), std::ios::cur);
@@ -1404,7 +1415,7 @@ void mjCFlexcomp::LoadGMSH22(char* buffer, int binary, int nodeend,
     size_t maxElementTag = 0;
     ss >> maxElementTag;
     if (!ss.good()) {
-      throw mjCError(NULL, "GetMaxElementTag::Error reading Elements header");
+      throw mjCError(NULL, "Error reading Elements header");
     }
     size_t numElements = maxElementTag;
 
@@ -1419,18 +1430,30 @@ void mjCFlexcomp::LoadGMSH22(char* buffer, int binary, int nodeend,
     }
 
     // reading first element's type
-    int tag = 0, type = 0, numTags = 0;
+    int tag = 0, elementType = 0, numTags = 0;
-    ss >> tag >> type >> numTags;
+    ss >> tag >> elementType >> numTags;
     if (!ss.good()) {
       throw mjCError(NULL, "Error reading Elements");
     }
-    int numNodeTags = type;
+
+    size_t entityDim = 0;
+    int numNodeTags = 0;
+    // surface
+    if (elementType == 2) {
+      entityDim = 2;
+      numNodeTags = 3;
+    }
+    // tetrahedral
+    else if (elementType == 4) {
+      entityDim = 3;
+      numNodeTags = 4;
+    }
+
     if (numNodeTags < 1 || numNodeTags >4) {
       throw mjCError(NULL, "Invalid number of node tags");
     }
 
     // setting entityDim
-    size_t entityDim = numNodeTags - 1;
     def.spec.flex->dim = entityDim;
 
     // read elements, discard all tags
@@ -1438,7 +1461,7 @@ void mjCFlexcomp::LoadGMSH22(char* buffer, int binary, int nodeend,
     for (size_t i=0; i<numElements; i++) {
       int nodeTag = 0, physicalEntityTag = 0, elmentModelEntityTag = 0;
       if (i != 0) {
-        ss >> tag >> type >> numTags;
+        ss >> tag >> elementType >> numTags;
         if (!ss.good()) {
           throw mjCError(NULL, "Error reading Elements");
         }
@@ -1454,7 +1477,7 @@ void mjCFlexcomp::LoadGMSH22(char* buffer, int binary, int nodeend,
         if (!ss.good()) {
           throw mjCError(NULL, "Error reading Elements");
         }
-        if (nodeTag > numElements || nodeTag < 1) {
+        if (nodeTag > numNodes || nodeTag < 1) {
           throw mjCError(NULL, "Invalid node tag");
         }
         element.push_back((int)(nodeTag-1));
@@ -1464,7 +1487,7 @@ void mjCFlexcomp::LoadGMSH22(char* buffer, int binary, int nodeend,
   // binary elements
   else {
     // header size for gmshApp
-    constexpr int elementHeaderSizeGmshApp = 6;
+    constexpr int elementHeaderSizeGmshApp = 4;
     // header size for Ftetwild
     constexpr int elementHeaderSizeFtetwild = 17;
     // check header size
@@ -1480,7 +1503,7 @@ void mjCFlexcomp::LoadGMSH22(char* buffer, int binary, int nodeend,
     int numElements = maxElementTag;
     int tag, numTags;
     int nodeTag;
-    int numNodeTags;
+    int elementType;
 
     // check number of elements is a positive number
     if (numElements < 0) {
@@ -1491,14 +1514,28 @@ void mjCFlexcomp::LoadGMSH22(char* buffer, int binary, int nodeend,
     int componentSize = sizeof(int);
     // element buffer
     const char* elementsBuffer = buffer + elembegin + measuredHeaderSize;
-    ReadFromBuffer(&numNodeTags, elementsBuffer);
+    ReadFromBuffer(&elementType, elementsBuffer);
     ReadFromBuffer(&numTags, elementsBuffer + componentSize*2);
     ReadFromBuffer(&tag, elementsBuffer + componentSize*3);
 
+    // tetrahedral has 4 node tags and surface has 3
+    int numNodeTags = 0;
+    size_t entityDim = 0;
+    // surface
+    if (elementType == 2) {
+      entityDim = 2;
+      numNodeTags = 3;
+    }
+    // tetrahedral
+    else if (elementType == 4) {
+      entityDim = 3;
+      numNodeTags = 4;
+    }
+
     if (numNodeTags < 1 || numNodeTags >4) {
       throw mjCError(NULL, "Invalid number of node tags");
     }
-    size_t entityDim = numNodeTags - 1;
+
     def.spec.flex->dim = entityDim;
 
     // element data(Ftetwild): tag and 4 nodeTag
@@ -1534,7 +1571,7 @@ void mjCFlexcomp::LoadGMSH22(char* buffer, int binary, int nodeend,
       // read first element
       for (int k =0; k<numNodeTags; k++) {
         ReadFromBuffer(&nodeTag, elementsBuffer + componentSize*(6+k));
-        if (nodeTag > numElements || nodeTag < 1) {
+        if (nodeTag > numNodes || nodeTag < 1) {
           throw mjCError(NULL, "Invalid node tag");
         }
         element.push_back(nodeTag-1);
@@ -1564,7 +1601,7 @@ void mjCFlexcomp::LoadGMSH22(char* buffer, int binary, int nodeend,
       for (int k = 0; k < numNodeTags; k++) {
         const char* nodeTagBuffer = elementsBuffer + componentSize*(4+k);
         ReadFromBuffer(&nodeTag, nodeTagBuffer);
-        if (nodeTag > numElements || nodeTag < 1) {
+        if (nodeTag > numNodes || nodeTag < 1) {
           throw mjCError(NULL, "Invalid node tag");
         }
         element.push_back(nodeTag-1);

test/user/testdata/cube_22_ascii_surf_gmshApp.msh

@@ -0,0 +1,47 @@
+$MeshFormat
+2.2 0 8
+$EndMeshFormat
+$Nodes
+14
+1 -0.5 -0.5 0
+2 0.5 -0.5 0
+3 0.5 0.5 0
+4 -0.5 0.5 0
+5 -0.5 -0.5 1
+6 0.5 -0.5 1
+7 0.5 0.5 1
+8 -0.5 0.5 1
+9 0 0 0
+10 0 -0.5 0.5
+11 0.5 0 0.5
+12 0 0.5 0.5
+13 -0.5 0 0.5
+14 0 0 1
+$EndNodes
+$Elements
+24
+1 2 2 0 1 9 4 3
+2 2 2 0 1 9 3 2
+3 2 2 0 1 9 1 4
+4 2 2 0 1 9 2 1
+5 2 2 0 1 2 10 1
+6 2 2 0 1 1 10 5
+7 2 2 0 1 6 10 2
+8 2 2 0 1 5 10 6
+9 2 2 0 1 3 11 2
+10 2 2 0 1 2 11 6
+11 2 2 0 1 7 11 3
+12 2 2 0 1 6 11 7
+13 2 2 0 1 4 12 3
+14 2 2 0 1 3 12 7
+15 2 2 0 1 8 12 4
+16 2 2 0 1 7 12 8
+17 2 2 0 1 1 13 4
+18 2 2 0 1 5 13 1
+19 2 2 0 1 4 13 8
+20 2 2 0 1 8 13 5
+21 2 2 0 1 5 6 14
+22 2 2 0 1 8 5 14
+23 2 2 0 1 6 7 14
+24 2 2 0 1 7 8 14
+$EndElements

test/user/testdata/cube_22_ascii_surf_gmshApp.xml

@@ -0,0 +1,26 @@
+<mujoco model="gmsh">
+  <option timestep="0.01" integrator="implicitfast" solver="CG"/>
+
+  <asset>
+    <texture type="skybox" builtin="gradient" rgb1=".3 .5 .7" rgb2="0 0 0" width="512" height="512"/>
+    <texture name="grid" type="2d" builtin="checker" width="512" height="512" rgb1=".1 .2 .3" rgb2=".2 .3 .4"/>
+    <material name="grid" texture="grid" texrepeat="1 1" texuniform="true" reflectance=".2"/>
+  </asset>
+
+  <visual>
+    <headlight diffuse="0.9 0.9 0.9" ambient="0.3 0.3 0.3" specular="0 0 0"/>
+    <map force="0.1" zfar="30"/>
+    <rgba haze="0.15 0.25 0.35 1"/>
+    <quality shadowsize="4096"/>
+    <global offwidth="800" offheight="800"/>
+  </visual>
+
+  <worldbody>
+    <geom name="floor" size="1 1 .05" type="plane" material="grid" condim="3"/>
+    <light directional="false" diffuse=".2 .2 .2" specular="0 0 0" pos="0 0 5" dir="0 0 -1"/>
+    <flexcomp name="cube" type="gmsh" dim="2" rgba=".8 .2 .2 1" radius="0.001" pos="0 0 0"
+              mass="10" scale="1 1 1" file="cube_22_ascii_surf_gmshApp.msh">
+      <edge equality="true"/>
+    </flexcomp>
+  </worldbody>
+</mujoco>

test/user/testdata/cube_22_binary_surf_gmshApp.xml

@@ -0,0 +1,26 @@
+<mujoco model="gmsh">
+  <option timestep="0.01" integrator="implicitfast" solver="CG"/>
+
+  <asset>
+    <texture type="skybox" builtin="gradient" rgb1=".3 .5 .7" rgb2="0 0 0" width="512" height="512"/>
+    <texture name="grid" type="2d" builtin="checker" width="512" height="512" rgb1=".1 .2 .3" rgb2=".2 .3 .4"/>
+    <material name="grid" texture="grid" texrepeat="1 1" texuniform="true" reflectance=".2"/>
+  </asset>
+
+  <visual>
+    <headlight diffuse="0.9 0.9 0.9" ambient="0.3 0.3 0.3" specular="0 0 0"/>
+    <map force="0.1" zfar="30"/>
+    <rgba haze="0.15 0.25 0.35 1"/>
+    <quality shadowsize="4096"/>
+    <global offwidth="800" offheight="800"/>
+  </visual>
+
+  <worldbody>
+    <geom name="floor" size="1 1 .05" type="plane" material="grid" condim="3"/>
+    <light directional="false" diffuse=".2 .2 .2" specular="0 0 0" pos="0 0 5" dir="0 0 -1"/>
+    <flexcomp name="cube" type="gmsh" dim="2" rgba=".8 .2 .2 1" radius="0.001" pos="0 0 0"
+              mass="10" scale="1 1 1" file="cube_22_binary_surf_gmshApp.msh">
+      <edge equality="true"/>
+    </flexcomp>
+  </worldbody>
+</mujoco>

test/user/testdata/cube_41_ascii_surf_gmshApp.msh

@@ -0,0 +1,67 @@
+$MeshFormat
+4.1 0 8
+$EndMeshFormat
+$Entities
+0 0 1 0
+1 -0.5 -0.5 0 0.5 0.5 1 0 0 
+$EndEntities
+$Nodes
+1 14 1 14
+2 1 0 14
+1
+2
+3
+4
+5
+6
+7
+8
+9
+10
+11
+12
+13
+14
+-0.5 -0.5 0
+0.5 -0.5 0
+0.5 0.5 0
+-0.5 0.5 0
+-0.5 -0.5 1
+0.5 -0.5 1
+0.5 0.5 1
+-0.5 0.5 1
+0 0 0
+0 -0.5 0.5
+0.5 0 0.5
+0 0.5 0.5
+-0.5 0 0.5
+0 0 1
+$EndNodes
+$Elements
+1 24 1 24
+2 1 2 24
+1 9 4 3 
+2 9 3 2 
+3 9 1 4 
+4 9 2 1 
+5 2 10 1 
+6 1 10 5 
+7 6 10 2 
+8 5 10 6 
+9 3 11 2 
+10 2 11 6 
+11 7 11 3 
+12 6 11 7 
+13 4 12 3 
+14 3 12 7 
+15 8 12 4 
+16 7 12 8 
+17 1 13 4 
+18 5 13 1 
+19 4 13 8 
+20 8 13 5 
+21 5 6 14 
+22 8 5 14 
+23 6 7 14 
+24 7 8 14 
+$EndElements

test/user/testdata/cube_41_ascii_surf_gmshApp.xml

@@ -0,0 +1,26 @@
+<mujoco model="gmsh">
+  <option timestep="0.01" integrator="implicitfast" solver="CG"/>
+
+  <asset>
+    <texture type="skybox" builtin="gradient" rgb1=".3 .5 .7" rgb2="0 0 0" width="512" height="512"/>
+    <texture name="grid" type="2d" builtin="checker" width="512" height="512" rgb1=".1 .2 .3" rgb2=".2 .3 .4"/>
+    <material name="grid" texture="grid" texrepeat="1 1" texuniform="true" reflectance=".2"/>
+  </asset>
+
+  <visual>
+    <headlight diffuse="0.9 0.9 0.9" ambient="0.3 0.3 0.3" specular="0 0 0"/>
+    <map force="0.1" zfar="30"/>
+    <rgba haze="0.15 0.25 0.35 1"/>
+    <quality shadowsize="4096"/>
+    <global offwidth="800" offheight="800"/>
+  </visual>
+
+  <worldbody>
+    <geom name="floor" size="1 1 .05" type="plane" material="grid" condim="3"/>
+    <light directional="false" diffuse=".2 .2 .2" specular="0 0 0" pos="0 0 5" dir="0 0 -1"/>
+    <flexcomp name="cube" type="gmsh" dim="2" rgba=".8 .2 .2 1" radius="0.001" pos="0 0 0"
+              mass="10" scale="1 1 1" file="cube_41_ascii_surf_gmshApp.msh">
+      <edge equality="true"/>
+    </flexcomp>
+  </worldbody>
+</mujoco>

test/user/testdata/cube_41_binary_surf_gmshApp.xml

@@ -0,0 +1,26 @@
+<mujoco model="gmsh">
+  <option timestep="0.01" integrator="implicitfast" solver="CG"/>
+
+  <asset>
+    <texture type="skybox" builtin="gradient" rgb1=".3 .5 .7" rgb2="0 0 0" width="512" height="512"/>
+    <texture name="grid" type="2d" builtin="checker" width="512" height="512" rgb1=".1 .2 .3" rgb2=".2 .3 .4"/>
+    <material name="grid" texture="grid" texrepeat="1 1" texuniform="true" reflectance=".2"/>
+  </asset>
+
+  <visual>
+    <headlight diffuse="0.9 0.9 0.9" ambient="0.3 0.3 0.3" specular="0 0 0"/>
+    <map force="0.1" zfar="30"/>
+    <rgba haze="0.15 0.25 0.35 1"/>
+    <quality shadowsize="4096"/>
+    <global offwidth="800" offheight="800"/>
+  </visual>
+
+  <worldbody>
+    <geom name="floor" size="1 1 .05" type="plane" material="grid" condim="3"/>
+    <light directional="false" diffuse=".2 .2 .2" specular="0 0 0" pos="0 0 5" dir="0 0 -1"/>
+    <flexcomp name="cube" type="gmsh" dim="2" rgba=".8 .2 .2 1" radius="0.001" pos="0 0 0"
+              mass="10" scale="1 1 1" file="cube_41_binary_surf_gmshApp.msh">
+      <edge equality="true"/>
+    </flexcomp>
+  </worldbody>
+</mujoco>

test/user/testdata/malformed_shark_41_ascii_missing_node_index.xml

@@ -19,7 +19,7 @@
   <worldbody>
     <geom name="floor" size="20 20 .05" type="plane" material="grid" condim="3" />
     <light directional="false" diffuse=".2 .2 .2" specular="0 0 0" pos="0 0 5" dir="0 0 -1" />
-    <flexcomp name="shark binary 2.2" type="gmsh" rgba=".8 .2 .2 1" radius="0.001" pos="0 0 4"
+    <flexcomp name="shark binary 2.2" type="gmsh" dim="3" rgba=".8 .2 .2 1" radius="0.001" pos="0 0 4"
       mass="10" scale="1 1 1" file="shark_41_ascii_missing_node_index.msh">
       <edge equality="true" />
     </flexcomp>

test/user/testdata/shark_41_ascii_missing_node_index.msh

@@ -8,7 +8,6 @@ $EndEntities
 $Nodes
 1 652 1 652
 3 0 0 652
-1
 2
 3
 4

test/user/user_flex_test.cc

@@ -23,6 +23,8 @@
 #include <mujoco/mujoco.h>
 #include "test/fixture.h"
 
+
+
 namespace mujoco {
 namespace {
 
@@ -32,6 +34,8 @@ using ::testing::HasSubstr;
 using UserFlexTest = MujocoTest;
 
 
+
+
 TEST_F(UserFlexTest, ParentMustHaveName) {
   static constexpr char xml[] = R"(
   <mujoco>
@@ -255,6 +259,56 @@ TEST_F(UserFlexTest, LoadMSHBinaryGMSH_22_Success) {
   mj_deleteData(d);
 }
 
+TEST_F(UserFlexTest, LoadMSHSurfaceBinaryGMSH_41_Success) {
+  const std::string xml_path =
+      GetTestDataFilePath("user/testdata/cube_41_binary_surf_gmshApp.xml");
+  std::array<char, 1024> error;
+  mjModel* m = mj_loadXML(xml_path.c_str(), 0, error.data(), error.size());
+  ASSERT_THAT(m, NotNull()) << error.data();
+  mjData* d = mj_makeData(m);
+  EXPECT_EQ(m->nflexvert, 14);
+  EXPECT_EQ(m->nflexelem, 24);
+
+  // first node x y z
+  EXPECT_EQ(m->flex_xvert0[0], -0.5 );
+  EXPECT_EQ(m->flex_xvert0[1], -0.5 );
+  EXPECT_EQ(m->flex_xvert0[2], 0 );
+
+  // first element
+  EXPECT_EQ(m->flex_elem[0], 9-1 );
+  EXPECT_EQ(m->flex_elem[1], 4-1 );
+  EXPECT_EQ(m->flex_elem[2], 3-1 );
+
+  mj_step(m, d);
+  mj_deleteModel(m);
+  mj_deleteData(d);
+}
+
+TEST_F(UserFlexTest, LoadMSHSurfaceBinaryGMSH_22_Success) {
+  const std::string xml_path =
+      GetTestDataFilePath("user/testdata/cube_22_binary_surf_gmshApp.xml");
+  std::array<char, 1024> error;
+  mjModel* m = mj_loadXML(xml_path.c_str(), 0, error.data(), error.size());
+  ASSERT_THAT(m, NotNull()) << error.data();
+  mjData* d = mj_makeData(m);
+  EXPECT_EQ(m->nflexvert, 14);
+  EXPECT_EQ(m->nflexelem, 24);
+
+  // first node x y z
+  EXPECT_EQ(m->flex_xvert0[0], -0.5 );
+  EXPECT_EQ(m->flex_xvert0[1], -0.5 );
+  EXPECT_EQ(m->flex_xvert0[2], 0 );
+
+  // first element
+  EXPECT_EQ(m->flex_elem[0], 9-1 );
+  EXPECT_EQ(m->flex_elem[1], 4-1 );
+  EXPECT_EQ(m->flex_elem[2], 3-1 );
+
+  mj_step(m, d);
+  mj_deleteModel(m);
+  mj_deleteData(d);
+}
+
 TEST_F(UserFlexTest, LoadMSHBinaryFTETWILD_22_Success) {
   const std::string xml_path =
       GetTestDataFilePath("user/testdata/shark_22_binary_fTetWild.xml");
@@ -297,6 +351,56 @@ TEST_F(UserFlexTest, LoadMSHASCIIGMSH_22_Success) {
   mj_deleteData(d);
 }
 
+TEST_F(UserFlexTest, LoadMSHSurfaceASCIIGMSH_41_Success) {
+  const std::string xml_path =
+      GetTestDataFilePath("user/testdata/cube_41_ascii_surf_gmshApp.xml");
+  std::array<char, 1024> error;
+  mjModel* m = mj_loadXML(xml_path.c_str(), 0, error.data(), error.size());
+  ASSERT_THAT(m, NotNull()) << error.data();
+  mjData* d = mj_makeData(m);
+  EXPECT_EQ(m->nflexvert, 14);
+  EXPECT_EQ(m->nflexelem, 24);
+
+  // first node x y z
+  EXPECT_EQ(m->flex_xvert0[0], -0.5 );
+  EXPECT_EQ(m->flex_xvert0[1], -0.5 );
+  EXPECT_EQ(m->flex_xvert0[2], 0 );
+
+  // first element
+  EXPECT_EQ(m->flex_elem[0], 9-1 );
+  EXPECT_EQ(m->flex_elem[1], 4-1 );
+  EXPECT_EQ(m->flex_elem[2], 3-1 );
+
+  mj_step(m, d);
+  mj_deleteModel(m);
+  mj_deleteData(d);
+}
+
+TEST_F(UserFlexTest, LoadMSHSurfaceASCIIGMSH_22_Success) {
+  const std::string xml_path =
+      GetTestDataFilePath("user/testdata/cube_22_ascii_surf_gmshApp.xml");
+  std::array<char, 1024> error;
+  mjModel* m = mj_loadXML(xml_path.c_str(), 0, error.data(), error.size());
+  ASSERT_THAT(m, NotNull()) << error.data();
+  mjData* d = mj_makeData(m);
+  EXPECT_EQ(m->nflexvert, 14);
+  EXPECT_EQ(m->nflexelem, 24);
+
+  // first node x y z
+  EXPECT_EQ(m->flex_xvert0[0], -0.5 );
+  EXPECT_EQ(m->flex_xvert0[1], -0.5 );
+  EXPECT_EQ(m->flex_xvert0[2], 0 );
+
+  // first element
+  EXPECT_EQ(m->flex_elem[0], 9-1 );
+  EXPECT_EQ(m->flex_elem[1], 4-1 );
+  EXPECT_EQ(m->flex_elem[2], 3-1 );
+
+  mj_step(m, d);
+  mj_deleteModel(m);
+  mj_deleteData(d);
+}
+
 TEST_F(UserFlexTest, LoadMSHASCIIFTETWILD_22_Success) {
   const std::string xml_path =
       GetTestDataFilePath("user/testdata/shark_22_ascii_fTetWild.xml");
@@ -329,7 +433,7 @@ TEST_F(UserFlexTest, LoadMSHASCII_41_MissingNodeIndex_Fail) {
   std::array<char, 1024> error;
   mjModel* m = mj_loadXML(xml_path.c_str(), 0, error.data(), error.size());
   EXPECT_THAT(error.data(), HasSubstr(
-        "XML Error: Element size must be a multiple of dim+1"));
+        "XML Error: Error: Node tags must be sequential"));
   mj_deleteModel(m);
 }
 
@@ -361,8 +465,9 @@ TEST_F(UserFlexTest, LoadMSHASCII_22_MissingNumNodes_Fail) {
           "user/testdata/malformed_shark_22_ascii_missing_num_nodes.xml");
   std::array<char, 1024> error;
   mjModel* m = mj_loadXML(xml_path.c_str(), 0, error.data(), error.size());
-  EXPECT_THAT(error.data(), HasSubstr(
+  // TODO(mohammadhamid): Replace with an assertion about the error message. For
-        "XML Error: Element size must be a multiple of dim+1"));
+  // some reason, on Windows the error message is different on GH Actions
+  EXPECT_THAT(m, IsNull());
   mj_deleteModel(m);
 }