Add defaults to SDF plugin attributes.

Expose diameter and number of teeth for the gear SDF.

PiperOrigin-RevId: 577779297
Change-Id: Iec5890ab7f39080b395cf06a76f9c2216f4f39ed

doc/includes/references.h

@@ -1432,6 +1432,9 @@ struct mjpPlugin_ {
   // called during compilation for marching cubes
   mjtNum (*sdf_staticdistance)(const mjtNum point[3], const mjtNum* attributes);
 
+  // convert attributes and provide defaults if not present
+  void (*sdf_attribute)(mjtNum attribute[], const char* name[], const char* value[]);
+
   // bounding box of implicit surface
   void (*sdf_aabb)(mjtNum aabb[6], const mjtNum* attributes);
 };

doc/programming/extension.rst

@@ -250,20 +250,24 @@ A future version of this section will include:
 
 Currently, there are three directories of first-party plugins:
 
-* **elasticity:** The plugins in the `elasticity/ <https://github.com/google-deepmind/mujoco/tree/main/plugin/elasticity>`__
+* **elasticity:** The plugins in the `elasticity/
-  directory are passive forces based on continuum mechanics for 1-dimensional and 3-dimensional bodies. The
+  <https://github.com/google-deepmind/mujoco/tree/main/plugin/elasticity>`__ directory are passive forces based on
-  1D model is invariant under rotations and captures the large deformation of elastic cables, decoupling twisting and
+  continuum mechanics for 1-dimensional and 3-dimensional bodies. The 1D model is invariant under rotations and captures
-  bending strains. The 3D solid is a
+  the large deformation of elastic cables, decoupling twisting and bending strains. The 3D solid is a `Saint
-  `Saint Venant-Kirchhoff <https://en.wikipedia.org/wiki/Hyperelastic_material#Saint_Venant%E2%80%93Kirchhoff_model>`__
+  Venant-Kirchhoff <https://en.wikipedia.org/wiki/Hyperelastic_material#Saint_Venant%E2%80%93Kirchhoff_model>`__ model
-  model discretized with piecewise linear finite elements, which is suitable for large deformations with small strains.
+  discretized with piecewise linear finite elements, which is suitable for large deformations with small strains. See
-  See also :ref:`composite <CComposite>` and :ref:`deformable <CDeformable>` objects.
+  also :ref:`composite <CComposite>` and :ref:`deformable <CDeformable>` objects. For more information, please see the
+  `README <https://github.com/google-deepmind/mujoco/blob/main/plugin/elasticity/README.md>`__.
 * **sensor:** The plugins in the `sensor/ <https://github.com/google-deepmind/mujoco/tree/main/plugin/sensor>`__
   directory implement custom sensors. Currently the sole sensor plugin is the touch grid sensor, see the
   `README <https://github.com/google-deepmind/mujoco/blob/main/plugin/sensor/README.md>`__ for details.
-* **sdf:** The plugins in the `sdf/ <https://github.com/google-deepmind/mujoco/tree/main/plugin/sdf>`__
+* **sdf:** The plugins in the `sdf/ <https://github.com/google-deepmind/mujoco/tree/main/plugin/sdf>`__ directory
-  directory specify custom shapes in a mesh-free manner, by defining methods computing a signed distance field and its
+  specify custom shapes in a mesh-free manner, by defining methods computing a signed distance field and its gradient at
-  gradient at query points. This shape then acts as a new geom type in the collision table at the top of
+  query points. This shape then acts as a new geom type in the collision table at the top of `engine_collision_driver.c
-  `engine_collision_driver.c <https://github.com/google-deepmind/mujoco/blob/main/src/engine/engine_collision_driver.c>`__.
+  <https://github.com/google-deepmind/mujoco/blob/main/src/engine/engine_collision_driver.c>`__. For more information
+  concerning the available SDFs and how to write your own implicit geometry, please see the `README
+  <https://github.com/google-deepmind/mujoco/blob/main/plugin/sdf/README.md>`__. The rest of this section will give more
+  detail concerning the collision algorithm and the plugin engine interface.
 
   Collision points are found by minimizing the maximum of the two colliding SDFs via gradient descent.
   Because SDFs are non-convex, multiple starting points are required in order to converge to multiple local minima.

include/mujoco/mjplugin.h

@@ -17,6 +17,7 @@
 
 #include <mujoco/mjdata.h>
 #include <mujoco/mjmodel.h>
+#include <mujoco/mjtnum.h>
 #include <mujoco/mjvisualize.h>
 
 
@@ -117,6 +118,9 @@ struct mjpPlugin_ {
   // called during compilation for marching cubes
   mjtNum (*sdf_staticdistance)(const mjtNum point[3], const mjtNum* attributes);
 
+  // convert attributes and provide defaults if not present
+  void (*sdf_attribute)(mjtNum attribute[], const char* name[], const char* value[]);
+
   // bounding box of implicit surface
   void (*sdf_aabb)(mjtNum aabb[6], const mjtNum* attributes);
 };

plugin/elasticity/README.md

@@ -1,5 +1,5 @@
 <h1>
-  <a href="#"><img alt="MuJoCo" src="banner.png" width="100%"/></a>
+  <a href="#"><img alt="MuJoCo" src="../../banner.png" width="100%"/></a>
 </h1>
 
 ## Elasticity plugins

plugin/sdf/README.md

@@ -0,0 +1,100 @@
+<h1>
+  <a href="#"><img alt="MuJoCo" src="../../banner.png" width="100%"/></a>
+</h1>
+
+## Signed distance function (SDF) plugins
+
+These are first-party plugins that implement implicit geometries using SDFs. They can be applied to **geoms** and
+**meshes** (in the **asset** section). Sample models can be found in [this folder](../../model/plugin/sdf/).
+
+### Bolt
+
+Implemented in [bolt.cc](bolt.cc). Example usage in [nutbolt.xml](../../model/plugin/sdf/nutbolt.xml).
+
+This plugin implements a bolt with a hexagonal head, similar to https://www.shadertoy.com/view/XtffzX.
+
+Parameters:
+
+   - `radius` [m]: bolt radius (default `0.26`).
+
+### Bowl
+
+Implemented in [bowl.cc](bowl.cc). Example usage in [bowl.xml](../../model/plugin/sdf/bowl.xml).
+
+The plugin implements a cut hollow sphere from https://www.shadertoy.com/view/7tVXRt.
+
+Parameters:
+
+   - `height` [m]: location of the cut plane (default `0.4`).
+   - `radius` [m]: radius of the sphere (default `1`).
+   - `thickness` [m]: thickness of the bowl (default `0.02`).
+
+### Gear
+
+Implemented in [gear.cc](gear.cc). Example usage in [gear.xml](../../model/plugin/sdf/gear.xml).
+
+The plugin implements a 3D extrusion of the 2D gear geometry from https://www.shadertoy.com/view/3lG3WR.
+
+Parameters:
+
+   - `alpha` [m]: initial angle of rotation of the gear (default `0`).
+   - `diameter` [m]: gear diameter (default `2.8`).
+   - `teeth` []: number of teeth (default `25`).
+
+### Nut
+
+Implemented in [nut.cc](nut.cc). Example usage in [nutbolt.xml](../../model/plugin/sdf/nutbolt.xml).
+
+This plugin implements a hexagonal nut identical to the bolt head from https://www.shadertoy.com/view/XtffzX.
+
+Parameters:
+
+   - `radius` [m]: nut radius (default `0.26`).
+
+### Torus
+
+Implemented in [torus.cc](torus.cc). Example usage in [torus.xml](../../model/plugin/sdf/torus.xml).
+
+This plugin implements a torus.
+
+Parameters:
+
+  - `radius1` [m]: major radius (default `0.35`).
+  - `radius1` [m]: minor radius (default `0.15`).
+
+### How to make your own SDF
+
+Create your `MySDF.h` and `MySDF.cc` files in the SDF folder, where this README is located. Implement your SDF using the
+following interface:
+
+```
+struct MySDFAttribute {
+  static constexpr int nattribute =
+  /* insert the number of attributes */;
+  static constexpr char const* names[nattribute] =
+  /* an array of attributes with the same order as the attribute array in your SDF class */;
+  static constexpr mjtNum defaults[nattribute] =
+  /* an array of default values for your attributes */;
+};
+
+class MySDF {
+ public:
+  // creates a new MySDF instance or returns null on failure.
+  static std::optional<MySDF> Create(const mjModel* m, mjData* d, int instance);
+  MySDF(MySDF&&) = default;
+  ~MySDF() = default;
+
+  // functions that return the SDF and its gradient at a query point
+  mjtNum Distance(const mjtNum point[3]) const;
+  void Gradient(mjtNum grad[3], const mjtNum point[3]) const;
+
+  // a call to this needs to be added to register.cc
+  static void RegisterPlugin();
+
+  // an array of attributes with the same order as in the struct above
+  mjtNum attribute[MySDFAttribute::nattribute];
+
+ private:
+  Torus(const mjModel* m, mjData* d, int instance);
+};
+```

plugin/sdf/bolt.cc

@@ -13,8 +13,10 @@
 // limitations under the License.
 
 #include <cmath>
+#include <cstdint>
 #include <cstdlib>
 #include <optional>
+#include <utility>
 
 #include <mujoco/mjplugin.h>
 #include <mujoco/mjtnum.h>
@@ -75,7 +77,13 @@ std::optional<Bolt> Bolt::Create(
 
 // plugin constructor
 Bolt::Bolt(const mjModel* m, mjData* d, int instance) {
-  radius = strtod(mj_getPluginConfig(m, instance, "radius"), nullptr);
+  SdfDefault<BoltAttribute> defattribute;
+
+  for (int i=0; i < BoltAttribute::nattribute; i++) {
+    attribute[i] = defattribute.GetDefault(
+        BoltAttribute::names[i],
+        mj_getPluginConfig(m, instance, BoltAttribute::names[i]));
+  }
 }
 
 // add new element in the vector storing iteration counts
@@ -96,19 +104,19 @@ void Bolt::Visualize(const mjModel* m, mjData* d, const mjvOption* opt,
 
 // sdf
 mjtNum Bolt::Distance(const mjtNum point[3]) const {
-  return distance(point, &radius);
+  return distance(point, attribute);
 }
 
 // gradient of sdf
 void Bolt::Gradient(mjtNum grad[3], const mjtNum point[3]) const {
   mjtNum eps = 1e-8;
-  mjtNum dist0 = distance(point, &radius);
+  mjtNum dist0 = distance(point, attribute);
   mjtNum pointX[3] = {point[0]+eps, point[1], point[2]};
-  mjtNum distX = distance(pointX, &radius);
+  mjtNum distX = distance(pointX, attribute);
   mjtNum pointY[3] = {point[0], point[1]+eps, point[2]};
-  mjtNum distY = distance(pointY, &radius);
+  mjtNum distY = distance(pointY, attribute);
   mjtNum pointZ[3] = {point[0], point[1], point[2]+eps};
-  mjtNum distZ = distance(pointZ, &radius);
+  mjtNum distZ = distance(pointZ, attribute);
 
   grad[0] = (distX - dist0) / eps;
   grad[1] = (distY - dist0) / eps;
@@ -123,9 +131,8 @@ void Bolt::RegisterPlugin() {
   plugin.name = "mujoco.sdf.bolt";
   plugin.capabilityflags |= mjPLUGIN_SDF;
 
-  const char* attributes[] = {"radius"};
+  plugin.nattribute = BoltAttribute::nattribute;
-  plugin.nattribute = sizeof(attributes) / sizeof(attributes[0]);
+  plugin.attributes = BoltAttribute::names;
-  plugin.attributes = attributes;
   plugin.nstate = +[](const mjModel* m, int instance) { return 0; };
 
   plugin.init = +[](const mjModel* m, mjData* d, int instance) {
@@ -177,6 +184,11 @@ void Bolt::RegisterPlugin() {
         aabb[3] = aabb[4] = .6;
         aabb[5] = 1;
       };
+  plugin.sdf_attribute =
+      +[](mjtNum attribute[], const char* name[], const char* value[]) {
+        SdfDefault<BoltAttribute> defattribute;
+        defattribute.GetDefaults(attribute, name, value);
+      };
 
   mjp_registerPlugin(&plugin);
 }

plugin/sdf/bolt.h

@@ -28,6 +28,12 @@ namespace mujoco::plugin::sdf {
 // this plugin implements a modification of the signed distance function
 // from https://www.shadertoy.com/view/XtffzX  of a bolt with a hexagonal head
 
+struct BoltAttribute {
+  static constexpr int nattribute = 1;
+  static constexpr char const* names[nattribute] = {"radius"};
+  static constexpr mjtNum defaults[nattribute] = {0.26};
+};
+
 class Bolt {
  public:
   // Creates a new Bolt instance (allocated with `new`) or
@@ -45,7 +51,7 @@ class Bolt {
 
   static void RegisterPlugin();
 
-  mjtNum radius;
+  mjtNum attribute[BoltAttribute::nattribute];
 
  private:
   Bolt(const mjModel* m, mjData* d, int instance);

plugin/sdf/bowl.cc

@@ -12,12 +12,14 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include <sstream>
+#include <cstdint>
 #include <optional>
+#include <utility>
 
 #include <mujoco/mjplugin.h>
 #include <mujoco/mjtnum.h>
 #include <mujoco/mujoco.h>
+#include "sdf.h"
 #include "bowl.h"
 
 namespace mujoco::plugin::sdf {
@@ -51,9 +53,16 @@ std::optional<Bowl> Bowl::Create(
 
 // plugin constructor
 Bowl::Bowl(const mjModel* m, mjData* d, int instance) {
-  radius = strtod(mj_getPluginConfig(m, instance, "radius"), nullptr);
+  SdfDefault<BowlAttribute> defattribute;
-  height = strtod(mj_getPluginConfig(m, instance, "height"), nullptr);
+
-  thick = strtod(mj_getPluginConfig(m, instance, "thickness"), nullptr);
+  for (int i=0; i < BowlAttribute::nattribute; i++) {
+    attribute[i] = defattribute.GetDefault(
+        BowlAttribute::names[i],
+        mj_getPluginConfig(m, instance, BowlAttribute::names[i]));
+  }
+
+  mjtNum height = attribute[0];
+  mjtNum radius = attribute[1];
   width = mju_sqrt(radius*radius - height*height);
 }
 
@@ -75,8 +84,7 @@ void Bowl::Visualize(const mjModel* m, mjData* d, const mjvOption* opt, mjvScene
 
 // sdf
 mjtNum Bowl::Distance(const mjtNum point[3]) const {
-  mjtNum attributes[3]= {height, radius, thick};
+  return distance(point, attribute);
-  return distance(point, attributes);
 }
 
 // gradient of sdf
@@ -98,16 +106,15 @@ void Bowl::Gradient(mjtNum grad[3], const mjtNum point[3]) const {
   //   grad[1] = - grad_dist * point[1] / pnorm;
   //   grad[2] = - grad_dist * point[2] / pnorm;
   // }
-  mjtNum attributes[3]= {height, radius, thick};
   mjtNum eps = 1e-8;
-  mjtNum dist0 = distance(point, attributes);
+  mjtNum dist0 = distance(point, attribute);
 
   mjtNum pointX[3] = {point[0]+eps, point[1], point[2]};
-  mjtNum distX = distance(pointX, attributes);
+  mjtNum distX = distance(pointX, attribute);
   mjtNum pointY[3] = {point[0], point[1]+eps, point[2]};
-  mjtNum distY = distance(pointY, attributes);
+  mjtNum distY = distance(pointY, attribute);
   mjtNum pointZ[3] = {point[0], point[1], point[2]+eps};
-  mjtNum distZ = distance(pointZ, attributes);
+  mjtNum distZ = distance(pointZ, attribute);
 
   grad[0] = (distX - dist0) / eps;
   grad[1] = (distY - dist0) / eps;
@@ -122,9 +129,8 @@ void Bowl::RegisterPlugin() {
   plugin.name = "mujoco.sdf.bowl";
   plugin.capabilityflags |= mjPLUGIN_SDF;
 
-  const char* attributes[] = {"radius", "height", "thickness"};
+  plugin.nattribute = BowlAttribute::nattribute;
-  plugin.nattribute = sizeof(attributes) / sizeof(attributes[0]);
+  plugin.attributes = BowlAttribute::names;
-  plugin.attributes = attributes;
   plugin.nstate = +[](const mjModel* m, int instance) { return 0; };
 
   plugin.init = +[](const mjModel* m, mjData* d, int instance) {
@@ -177,6 +183,11 @@ void Bowl::RegisterPlugin() {
         aabb[0] = aabb[1] = aabb[2] = 0;
         aabb[3] = aabb[4] = aabb[5] = radius + thick;
       };
+  plugin.sdf_attribute =
+      +[](mjtNum attribute[], const char* name[], const char* value[]) {
+        SdfDefault<BowlAttribute> defattribute;
+        defattribute.GetDefaults(attribute, name, value);
+      };
 
   mjp_registerPlugin(&plugin);
 }

plugin/sdf/bowl.h

@@ -25,6 +25,13 @@
 
 namespace mujoco::plugin::sdf {
 
+struct BowlAttribute {
+  static constexpr int nattribute = 3;
+  static constexpr char const* names[nattribute] = {"height", "radius",
+                                                    "thickness"};
+  static constexpr mjtNum defaults[nattribute] = {.4, 1., .02};
+};
+
 class Bowl {
  public:
   // Creates a new Bowl instance (allocated with `new`) or
@@ -42,9 +49,7 @@ class Bowl {
 
   static void RegisterPlugin();
 
-  mjtNum radius;
+  mjtNum attribute[BowlAttribute::nattribute];
-  mjtNum height;
-  mjtNum thick;
   mjtNum width;
 
  private:

plugin/sdf/gear.cc

@@ -12,14 +12,16 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include <cstdio>
+#include <cstdint>
-#include <sstream>
+#include <cstdlib>
 #include <optional>
+#include <utility>
 
 #include <mujoco/mjplugin.h>
 #include <mujoco/mjtnum.h>
 #include <mujoco/mjvisualize.h>
 #include <mujoco/mujoco.h>
+#include "sdf.h"
 #include "gear.h"
 
 namespace mujoco::plugin::sdf {
@@ -50,10 +52,11 @@ static mjtNum mod(mjtNum x, mjtNum y) {
   return x - y * floor(x/y);
 }
 
-static mjtNum distance2D(const mjtNum p[3], const mjtNum attributes[3]) {
+static mjtNum distance2D(const mjtNum p[3],
+                         const mjtNum attributes[GearAttribute::nattribute]) {
   // see https://www.shadertoy.com/view/3lG3WR
-  mjtNum D = 2.8;  // should be an attribute
+  mjtNum D = attributes[1];
-  mjtNum N = 25;  // should be an attribute
+  mjtNum N = attributes[2];
   mjtNum psi = 3.096e-5 * N * N -6.557e-3 * N + 0.551;  // pressure angle
   mjtNum alpha = attributes[0];
 
@@ -78,7 +81,7 @@ static mjtNum distance2D(const mjtNum p[3], const mjtNum attributes[3]) {
 
   mjtNum h = 2.2 / Pd;
 
-  mjtNum innerR = Ro - h - 0.4;
+  mjtNum innerR = Ro - h - 0.14*D;
 
   // Early exit
   if (innerR - rho > 0.0)
@@ -126,15 +129,16 @@ static mjtNum distance2D(const mjtNum p[3], const mjtNum attributes[3]) {
                                    fib - (alphaStride - shift));
 
   cogs = Intersection(baseWalls, cogs);
-  cogs = smoothIntersection(gearOuter, cogs, 0.01);
+  cogs = smoothIntersection(gearOuter, cogs, 0.0035*D);
   cogs = smoothUnion(gearLowBase, cogs, Rb - Ro + h);
   cogs = Subtraction(cogs, crownBase);
 
-  return extrusion(p, cogs, .1);
+  return cogs;
 }
 
-static mjtNum distance(const mjtNum p[3], const mjtNum attributes[3]) {
+static mjtNum distance(const mjtNum p[3],
-  return extrusion(p, distance2D(p, attributes), .1) - .005;
+                       const mjtNum attributes[GearAttribute::nattribute]) {
+  return extrusion(p, distance2D(p, attributes), attributes[3]/2.);
 }
 
 }  // namespace
@@ -142,17 +146,24 @@ static mjtNum distance(const mjtNum p[3], const mjtNum attributes[3]) {
 // factory function
 std::optional<Gear> Gear::Create(
   const mjModel* m, mjData* d, int instance) {
-  if (CheckAttr("alpha", m, instance)) {
+  if (CheckAttr("alpha", m, instance) && CheckAttr("diameter", m, instance) &&
-    return Gear(m, d, instance);
+      CheckAttr("teeth", m, instance)) {
+      return Gear(m, d, instance);
   } else {
-    mju_warning("Invalid parameter specification in Gear plugin");
+      mju_warning("Invalid parameter specification in Gear plugin");
-    return std::nullopt;
+      return std::nullopt;
   }
 }
 
 // plugin constructor
 Gear::Gear(const mjModel* m, mjData* d, int instance) {
-  alpha = strtod(mj_getPluginConfig(m, instance, "alpha"), nullptr);
+  SdfDefault<GearAttribute> defattribute;
+
+  for (int i=0; i < GearAttribute::nattribute; i++) {
+      attribute[i] = defattribute.GetDefault(
+          GearAttribute::names[i],
+          mj_getPluginConfig(m, instance, GearAttribute::names[i]));
+  }
 }
 
 // plugin computation
@@ -173,21 +184,20 @@ void Gear::Visualize(const mjModel* m, mjData* d, const mjvOption* opt,
 
 // sdf
 mjtNum Gear::Distance(const mjtNum point[3]) const {
-  return distance(point, &alpha);
+  return distance(point, attribute);
 }
 
 // gradient of sdf
 void Gear::Gradient(mjtNum grad[3], const mjtNum point[3]) const {
-  mjtNum attributes[1]= {alpha};
   mjtNum eps = 1e-8;
-  mjtNum dist0 = distance(point, attributes);
+  mjtNum dist0 = distance(point, attribute);
 
   mjtNum pointX[3] = {point[0]+eps, point[1], point[2]};
-  mjtNum distX = distance(pointX, attributes);
+  mjtNum distX = distance(pointX, attribute);
   mjtNum pointY[3] = {point[0], point[1]+eps, point[2]};
-  mjtNum distY = distance(pointY, attributes);
+  mjtNum distY = distance(pointY, attribute);
   mjtNum pointZ[3] = {point[0], point[1], point[2]+eps};
-  mjtNum distZ = distance(pointZ, attributes);
+  mjtNum distZ = distance(pointZ, attribute);
 
   grad[0] = (distX - dist0) / eps;
   grad[1] = (distY - dist0) / eps;
@@ -202,9 +212,8 @@ void Gear::RegisterPlugin() {
   plugin.name = "mujoco.sdf.gear";
   plugin.capabilityflags |= mjPLUGIN_SDF;
 
-  const char* attributes[] = {"alpha"};
+  plugin.nattribute = GearAttribute::nattribute;
-  plugin.nattribute = sizeof(attributes) / sizeof(attributes[0]);
+  plugin.attributes = GearAttribute::names;
-  plugin.attributes = attributes;
   plugin.nstate = +[](const mjModel* m, int instance) { return 0; };
 
   plugin.init = +[](const mjModel* m, mjData* d, int instance) {
@@ -253,8 +262,13 @@ void Gear::RegisterPlugin() {
   plugin.sdf_aabb =
       +[](mjtNum aabb[6], const mjtNum* attributes) {
         aabb[0] = aabb[1] = aabb[2] = 0;
-        aabb[3] = aabb[4] = 1.7;
+        aabb[3] = aabb[4] = attributes[1] / 2. * 1.25;
-        aabb[5] = .11;
+        aabb[5] = attributes[3] / 2. * 1.1;
+  };
+  plugin.sdf_attribute =
+      +[](mjtNum attribute[], const char* name[], const char* value[]) {
+        SdfDefault<GearAttribute> defattribute;
+        defattribute.GetDefaults(attribute, name, value);
       };
 
   mjp_registerPlugin(&plugin);

plugin/sdf/gear.h

@@ -26,6 +26,13 @@
 
 namespace mujoco::plugin::sdf {
 
+struct GearAttribute {
+  static constexpr int nattribute = 4;
+  static constexpr char const* names[nattribute] = {"alpha", "diameter",
+                                                    "teeth", "thickness"};
+  static constexpr mjtNum defaults[nattribute] = { 0, 2.8, 25, .2 };
+};
+
 class Gear {
  public:
   // Creates a new Gear instance (allocated with `new`) or
@@ -43,7 +50,7 @@ class Gear {
 
   static void RegisterPlugin();
 
-  mjtNum alpha;
+  mjtNum attribute[GearAttribute::nattribute];
 
  private:
   Gear(const mjModel* m, mjData* d, int instance);

plugin/sdf/nut.cc

@@ -13,12 +13,15 @@
 // limitations under the License.
 
 #include <cmath>
-#include <sstream>
+#include <cstdint>
+#include <cstdlib>
 #include <optional>
+#include <utility>
 
 #include <mujoco/mjplugin.h>
 #include <mujoco/mjtnum.h>
 #include <mujoco/mujoco.h>
+#include "sdf.h"
 #include "nut.h"
 
 namespace mujoco::plugin::sdf {
@@ -74,7 +77,13 @@ std::optional<Nut> Nut::Create(
 
 // plugin constructor
 Nut::Nut(const mjModel* m, mjData* d, int instance) {
-  radius = strtod(mj_getPluginConfig(m, instance, "radius"), nullptr);
+  SdfDefault<NutAttribute> defattribute;
+
+  for (int i=0; i < NutAttribute::nattribute; i++) {
+    attribute[i] = defattribute.GetDefault(
+        NutAttribute::names[i],
+        mj_getPluginConfig(m, instance, NutAttribute::names[i]));
+  }
 }
 
 // plugin computation
@@ -95,19 +104,19 @@ void Nut::Visualize(const mjModel* m, mjData* d, const mjvOption* opt,
 
 // sdf
 mjtNum Nut::Distance(const mjtNum point[3]) const {
-  return distance(point, &radius);
+  return distance(point, attribute);
 }
 
 // gradient of sdf
 void Nut::Gradient(mjtNum grad[3], const mjtNum point[3]) const {
   mjtNum eps = 1e-8;
-  mjtNum dist0 = distance(point, &radius);
+  mjtNum dist0 = distance(point, attribute);
   mjtNum pointX[3] = {point[0]+eps, point[1], point[2]};
-  mjtNum distX = distance(pointX, &radius);
+  mjtNum distX = distance(pointX, attribute);
   mjtNum pointY[3] = {point[0], point[1]+eps, point[2]};
-  mjtNum distY = distance(pointY, &radius);
+  mjtNum distY = distance(pointY, attribute);
   mjtNum pointZ[3] = {point[0], point[1], point[2]+eps};
-  mjtNum distZ = distance(pointZ, &radius);
+  mjtNum distZ = distance(pointZ, attribute);
 
   grad[0] = (distX - dist0) / eps;
   grad[1] = (distY - dist0) / eps;
@@ -122,9 +131,8 @@ void Nut::RegisterPlugin() {
   plugin.name = "mujoco.sdf.nut";
   plugin.capabilityflags |= mjPLUGIN_SDF;
 
-  const char* attributes[] = {"radius"};
+  plugin.nattribute = NutAttribute::nattribute;
-  plugin.nattribute = sizeof(attributes) / sizeof(attributes[0]);
+  plugin.attributes = NutAttribute::names;
-  plugin.attributes = attributes;
   plugin.nstate = +[](const mjModel* m, int instance) { return 0; };
 
   plugin.init = +[](const mjModel* m, mjData* d, int instance) {
@@ -176,6 +184,11 @@ void Nut::RegisterPlugin() {
         aabb[3] = aabb[4] = .6;
         aabb[5] = 1;
       };
+  plugin.sdf_attribute =
+      +[](mjtNum attribute[], const char* name[], const char* value[]) {
+        SdfDefault<NutAttribute> defattribute;
+        defattribute.GetDefaults(attribute, name, value);
+      };
 
   mjp_registerPlugin(&plugin);
 }

plugin/sdf/nut.h

@@ -28,6 +28,12 @@ namespace mujoco::plugin::sdf {
 // this plugin implements a modification of the signed distance function
 // from https://www.shadertoy.com/view/XtffzX  of hexagonal nut
 
+struct NutAttribute {
+  static constexpr int nattribute = 1;
+  static constexpr char const* names[nattribute] = {"radius"};
+  static constexpr mjtNum defaults[nattribute] = {0.26};
+};
+
 class Nut {
  public:
   // Creates a new Nut instance (allocated with `new`) or
@@ -45,7 +51,7 @@ class Nut {
 
   static void RegisterPlugin();
 
-  mjtNum radius;
+  mjtNum attribute[NutAttribute::nattribute];
 
  private:
   Nut(const mjModel* m, mjData* d, int instance);

plugin/sdf/sdf.cc

@@ -13,10 +13,12 @@
 // limitations under the License.
 
 #include <algorithm>
+#include <cctype>
 #include <cstddef>
+#include <cstdio>
+#include <cstdlib>
 #include <string>
 
-#include <mujoco/mjplugin.h>
 #include <mujoco/mujoco.h>
 #include "sdf.h"
 

plugin/sdf/sdf.h

@@ -15,6 +15,9 @@
 #ifndef MUJOCO_PLUGIN_SDF_SDF_H_
 #define MUJOCO_PLUGIN_SDF_SDF_H_
 
+#include <map>
+#include <stdexcept>
+#include <string>
 #include <vector>
 
 #include <mujoco/mujoco.h>
@@ -40,6 +43,43 @@ inline mjtNum Fract(mjtNum x) {
 // reads numeric attributes
 bool CheckAttr(const char* name, const mjModel* m, int instance);
 
+// converts attributes to numeric or returns default if not present
+template <typename T>
+class SdfDefault {
+ public:
+  SdfDefault() {
+    for (int i = 0; i < T::nattribute; i++) {
+      default_[T::names[i]] = T::defaults[i];
+    }
+  }
+
+  // get a single default value
+  mjtNum GetDefault(const char* name, const char* value) {
+    if (std::string(value).empty()) {
+      return default_[name];
+    }
+    try {
+      mjtNum num = std::stod(value);
+      return num;
+    } catch (const std::invalid_argument& e) {
+      mju_error("invalid attribute value for '%s'", name);
+      return 0;
+    }
+  }
+
+  // populate attribute array
+  void GetDefaults(mjtNum* attribute, const char* names[],
+                   const char* values[]) {
+    for (int i = 0; i < default_.size(); i++) {
+      attribute[i] = GetDefault(names[i], values[i]);
+    }
+  }
+
+ private:
+  std::map<std::string, mjtNum> default_;
+};
+
+// stores the history of gradient descent iterations
 class SdfVisualizer {
  public:
   SdfVisualizer();

plugin/sdf/torus.cc

@@ -12,12 +12,14 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include <sstream>
+#include <cstdint>
 #include <optional>
+#include <utility>
 
 #include <mujoco/mjplugin.h>
 #include <mujoco/mjtnum.h>
 #include <mujoco/mujoco.h>
+#include "sdf.h"
 #include "torus.h"
 
 namespace mujoco::plugin::sdf {
@@ -43,19 +45,24 @@ std::optional<Torus> Torus::Create(
 
 // plugin constructor
 Torus::Torus(const mjModel* m, mjData* d, int instance) {
-  radius[0] = strtod(mj_getPluginConfig(m, instance, "radius1"), nullptr);
+  SdfDefault<TorusAttribute> defattribute;
-  radius[1] = strtod(mj_getPluginConfig(m, instance, "radius2"), nullptr);
+
+  for (int i=0; i < TorusAttribute::nattribute; i++) {
+    attribute[i] = defattribute.GetDefault(
+        TorusAttribute::names[i],
+        mj_getPluginConfig(m, instance, TorusAttribute::names[i]));
+  }
 }
 
 // sdf
 mjtNum Torus::Distance(const mjtNum point[3]) const {
-  return distance(point, radius);
+  return distance(point, attribute);
 }
 
 // gradient of sdf
 void Torus::Gradient(mjtNum grad[3], const mjtNum p[3]) const {
   mjtNum len_xy = mju_sqrt(p[0]*p[0] + p[1]*p[1]);
-  mjtNum q = len_xy - radius[0];
+  mjtNum q = len_xy - attribute[0];
   mjtNum grad_q[2] = { p[0] / len_xy, p[1] / len_xy };
   mjtNum len_qz = mju_sqrt(q*q + p[2]*p[2]);
   grad[0] = q*grad_q[0] / mjMAX(len_qz, mjMINVAL);
@@ -71,9 +78,8 @@ void Torus::RegisterPlugin() {
   plugin.name = "mujoco.sdf.torus";
   plugin.capabilityflags |= mjPLUGIN_SDF;
 
-  const char* attributes[] = {"radius1", "radius2", "axis"};
+  plugin.nattribute = TorusAttribute::nattribute;
-  plugin.nattribute = sizeof(attributes) / sizeof(attributes[0]);
+  plugin.attributes = TorusAttribute::names;
-  plugin.attributes = attributes;
   plugin.nstate = +[](const mjModel* m, int instance) { return 0; };
 
   plugin.init = +[](const mjModel* m, mjData* d, int instance) {
@@ -117,8 +123,13 @@ void Torus::RegisterPlugin() {
         aabb[3] = aabb[4] = attributes[0] + attributes[1];
         aabb[5] = attributes[1];
       };
+  plugin.sdf_attribute =
+      +[](mjtNum attribute[], const char* name[], const char* value[]) {
+        SdfDefault<TorusAttribute> defattribute;
+        defattribute.GetDefaults(attribute, name, value);
+      };
 
-      mjp_registerPlugin(&plugin);
+  mjp_registerPlugin(&plugin);
 }
 
 }  // namespace mujoco::plugin::sdf

plugin/sdf/torus.h

@@ -24,6 +24,12 @@
 
 namespace mujoco::plugin::sdf {
 
+struct TorusAttribute {
+  static constexpr int nattribute = 2;
+  static constexpr char const* names[nattribute] = {"radius1", "radius2"};
+  static constexpr mjtNum defaults[nattribute] = { .35, .15 };
+};
+
 class Torus {
  public:
   // Creates a new Torus instance or returns null on failure.
@@ -36,7 +42,7 @@ class Torus {
 
   static void RegisterPlugin();
 
-  mjtNum radius[2];
+  mjtNum attribute[TorusAttribute::nattribute];
 
  private:
   Torus(const mjModel* m, mjData* d, int instance);

src/user/user_mesh.cc

@@ -21,7 +21,6 @@
 #include <functional>
 #include <memory>
 #include <optional>
-#include <stdexcept>
 #include <string>
 #include <unordered_map>
 #include <utility>
@@ -303,15 +302,16 @@ void mjCMesh::LoadSDF() {
     throw mjCError(this, "plugin '%s' does not support signed distance fields", plugin->name);
   }
 
-  int i=0;
   std::vector<mjtNum> attributes(plugin->nattribute, 0);
-  for (auto const& pair : plugin_instance->config_attribs) {
+  std::vector<const char*> names(plugin->nattribute, 0);
-    try {
+  std::vector<const char*> values(plugin->nattribute, 0);
-      attributes[i++] = std::stod(pair.second);
+  for (int i=0; i < plugin->nattribute; i++) {
-    } catch (const std::invalid_argument& e) {
+    names[i] = plugin->attributes[i];
-      throw mjCError(this, "invalid attribute value for '%s'",
+    values[i] = plugin_instance->config_attribs[names[i]].c_str();
-                     pair.first.c_str());
+  }
-    }
+
+  if (plugin->sdf_attribute) {
+    plugin->sdf_attribute(attributes.data(), names.data(), values.data());
   }
 
   mjtNum aabb[6] = {0};