Model Directives mechanism for scene assembly.

 * Toward #13282
 * A domain-specific language for assembling MultibodyPlant scenes
   from multiple SDF files.
 * Helpful for assembling large scenes without huge unwieldy sdf/xacro
   files.
 * A temporary accomodation until sdformat adds similar functionality.

This code is copied and adapted from TRI's Project Anzu.

Co-authored-by: Eric Cousineau <[email protected]>
Co-authored-by: Siyuan Feng <[email protected]>
Co-authored-by: Grant Gould <[email protected]>
Co-authored-by: Jeremy Nimmer <[email protected]>
Co-authored-by: Calder Phillips-Grafflin <[email protected]>

common/schema/dev/BUILD.bazel

@@ -39,6 +39,7 @@ drake_cc_library(
     name = "transform",
     srcs = ["transform.cc"],
     hdrs = ["transform.h"],
+    visibility = ["//multibody/parsing/dev:__pkg__"],
     deps = [
         ":rotation",
         ":stochastic",

common/schema/dev/rotation.h

@@ -34,7 +34,7 @@ struct Rotation {
     Identity() = default;
 
     template <typename Archive>
-    void Serialize(Archive* a) {}
+    void Serialize(Archive*) {}
   };
 
   /// A roll-pitch-yaw rotation, using the angle conventions of Drake's
@@ -72,7 +72,7 @@ struct Rotation {
     Uniform() = default;
 
     template <typename Archive>
-    void Serialize(Archive* a) {}
+    void Serialize(Archive*) {}
   };
 
   /// Returns true iff this is fully deterministic.

multibody/parsing/dev/BUILD.bazel

@@ -0,0 +1,64 @@
+# -*- python -*-
+
+load(
+    "@drake//tools/skylark:drake_cc.bzl",
+    "drake_cc_googletest",
+    "drake_cc_library",
+)
+load("//tools/lint:lint.bzl", "add_lint_tests")
+
+drake_cc_library(
+    name = "process_model_directives",
+    srcs = ["process_model_directives.cc"],
+    hdrs = ["process_model_directives.h"],
+    data = [
+        "//manipulation/models/jaco_description:models",
+    ],
+    deps = [
+        ":model_directives",
+        "//common:filesystem",
+        "//common:find_resource",
+        "//common/yaml:yaml_read_archive",
+        "//multibody/parsing",
+        "//multibody/plant",
+    ],
+)
+
+filegroup(
+    name = "process_model_directives_test_models",
+    testonly = True,
+    data = glob(["test/process_model_directives_test/**"]),
+)
+
+drake_cc_googletest(
+    name = "process_model_directives_test",
+    data = [
+        ":process_model_directives_test_models",
+    ],
+    deps = [
+        ":process_model_directives",
+    ],
+)
+
+drake_cc_library(
+    name = "model_directives",
+    hdrs = [
+        "model_directives.h",
+    ],
+    deps = [
+        "//common:essential",
+        "//common:name_value",
+        "//common/schema/dev:transform",
+        "//math:geometric_transform",
+    ],
+)
+
+drake_cc_googletest(
+    name = "model_directives_test",
+    deps = [
+        ":model_directives",
+        "//common/yaml:yaml_read_archive",
+    ],
+)
+
+add_lint_tests()

multibody/parsing/dev/README.md

@@ -0,0 +1,89 @@
+The Model Directives mechanism
+==============================
+
+Model Directives is a small yaml-based language for building a complex
+MultibodyPlant-based scene out of numerous SDFs.  For instance in the TRI
+dish-loading demo we have individual SDF files for the counter, sink, cameras,
+pedestal, arm, gripper, and each manipuland.  A single SDF for this would be
+unwieldy and difficult to maintain and collaborate on, but SDF's file
+inclusion mechanisms have not yet proven adequate to this task.
+
+We expect that this mechanism will be temporary and will be removed when
+sdformat adds similar functionality.  Users should be aware that this library
+will be deprecated if/when sdformat reaches feature parity with it.
+
+
+## Syntax
+
+The easiest syntax reference is the unit test files in `test/models/*.yaml` of
+this directory.
+
+A model directives file is a yaml file with a top level `directives:` group.
+Within this group are a series of directives:
+
+ * `AddModel` takes a `file` and `name` and loads the SDF/URDF file indicated
+   as a new model instance with the given name.
+ * `AddModelInstance` Creates a new, empty model instance in the plant with
+   the indicated `name`.
+ * `AddPackagePath` takes `name` and `path` and makes `package://name` URIs
+   be resolved to `path`.  This directive is due for deprecation soon and
+   should generally be avoided.
+ * `AddFrame` takes a `name` and a `X_PF` transform and adds a new frame to
+   the model.  Note that the transform is as specified in the `Transform`
+   scenario schema and can reference an optional base frame, in which case
+   the frame will be added to the base frame's model instance.
+ * `AddDirectives` takes a `file` naming a model directives file and an
+   optional `model_namespace`; it loads the model directives from that file
+   with the namespace prefixed to them (see Scoping, below).
+ * `AddWeld` takes a `parent` and `child` frame and welds them together.
+
+
+## Use
+
+The easiest use reference is the unit test `process_model_directives_test.cc`.
+
+A simple example of a use would be:
+
+```cpp
+  ModelDirectives station_directives = LoadModelDirectives(
+      FindResourceOrThrow("my_exciting_project/models/my_scene.yaml"));
+  MultibodyPlant<double> plant;
+  ProcessModelDirectives(station_directives, &plant);
+  plant.Finalize();
+```
+
+This loads the model directives from that filename, constructs a plant, and
+uses the model directives to populate the plant.
+
+
+## Scoping
+
+Elements (frames, bodies, etc.) in `MultibodyPlant` belong to model instances.
+Model instances can have any name specifiers, and can contain the "namespace"
+delimiter `::`. Element names should not contain `::`.
+
+Examples:
+
+- `my_frame` implies no explicit model instance.
+- `my_model::my_frame` implies the model instance `my_model`, the frame
+`my_frame`.
+- `top_level::my_model::my_frame` implies the model instance
+`top_level::my_model`, the frame `my_frame`.
+
+
+## Conditions for deprecation
+
+We expect and hope to deprecate this mechanism when either:
+
+SDF format properly specifies, and Drake supports, the following:
+
+ * What `<include/>` statements should *really* do (e.g. namespacing models,
+   joints, etc.) without kludging Drake's parsing
+ * How to weld models together with joints external to the models
+
+OR if we find a mechanism whereby xacro could accomplish the same thing:
+
+ * Drake's `package://` / `model://` mechanism were mature and correct, and
+   `sdformat` didn't use singletons for search paths.
+ * It was easier for one xacro to locate other xacros (possibly via a
+   workaround using a wrapper script to inject `DRAKE_PATH`)