Forge create¶

Create mjcf hierarchy relationship use forge tool.

General rules¶

• Specification compliance: these guides are the authoritative specification of the Forge tool surface — strictly follow the rules, signatures, parameter ranges and workflows documented here. Never read, search or infer behavior from the addon source code (addon/Forge/**); source internals are not part of the interface. Runtime inspection through the MCP tools (forge_object_info, forge_scene_config, ...) is allowed.
• No undocumented operations: work only through what these guides document. When a needed operation, parameter or enum value has no documented implementation, do not improvise with raw bpy or undocumented APIs — stop and report the gap (what was attempted, what is missing) as feedback.
• Main scene: the main scene must be named Scene. Forge resolves the active scene through scene.Main_Scene (defaults to "Scene"), e.g. bpy.data.scenes[scene.Main_Scene].Forge_Props. Create the model in the scene named Scene only — do not rename or delete it, and do not create objects in another scene.
• Collider selection: prefer shape colliders (BOX, CAPSULE, CYLINDER, SPHERE). Consider a MESH collider only when shape colliders cannot meet the required collision geometry or accuracy. A mesh visual alone is not a reason to choose a mesh collider.

MCP tool workflow¶

• Prefer the dedicated MCP tools over raw bpy: inspect with forge_blendfile_summary, forge_scene_info, forge_object_info and forge_measure; create with forge_create_*; change types/parenting with forge_define_*; edit with forge_set_transform, forge_set_shape_size, forge_set_property and forge_rename; validate with forge_mesh_check. Use forge_execute only when no dedicated tool represents the operation.
• Skill gate scope: generic Blender deletion/removal operations remain available without a Forge guide. Use Forge operations for Forge hierarchy changes, especially forge_define_parent; do not assign obj.parent directly when establishing a Forge parent relationship.
• Every bpy.ops.forge.* operator (signature, defaults, poll, MCP wrapper) is inventoried in the operators skill — forge_skill(name="operators", section="<operator name>") for a single entry.
• Follow inspect → create/edit → verify → validate → save/export. Check the main scene and existing names first; after every creation capture the returned created names, inspect those exact objects, measure relevant world bounds, run forge_mesh_check on related visuals, and use forge_screenshot when visual confirmation adds value.
• Every MCP select=[...] argument is ordered. The first name becomes Blender's active object and every name receives a selected_order matching its list index. This order determines target/reference roles for tendon, equality, sensor, light/camera targeting and parent operations; never rely on bpy.context.selected_objects iteration order.
• Before a destructive or file-writing operation, inspect its explicit targets and destination. forge_delete recursively removes every descendant of each named root, so inspect children first and pass only intended roots.

Forge mjcf model types¶

In Forge treat all MJCF tags—including body, inertial, geom, joint, site, actuator, tendon, equality, contact, sensor, light, camera, frame, replicate, attach and include are treated as Blender objects, distinguished by the Forge_Model_Type attribute of the Object type.

Code example:

# register with bpy.types.Object.Forge_Model_Type
bpy.data.objects["name_of_object"].Forge_Model_Type
bpy.context.active_object.Forge_Model_Type
bpy.context.selected_objects[0].Forge_Model_Type

Forge model types are:

UNDEFINED, LINK, INERTIAL, VISUAL, COLLIDER, HFIELD, PLANE, JOINT, SITE, ACTUATOR, TENDON, EQUALITY, CONTACT, SENSOR, LIGHT, CAMERA, FRAME, REPLICATE, ATTACH, INCLUDE.

• UNDEFINED objects are not MJCF elements and are not exported as model elements.
• LINK represents an MJCF body in Blender's model hierarchy.
• VISUAL, COLLIDER, HFIELD, and PLANE all represent MJCF geom elements but have distinct Forge types and parameters.
• The remaining types each also have their own unique set of parameters.

Hierarchy rules¶

An articulated model starts with a root link. An inertial, joint, or site must be a child of a link.

visual, collider, light, camera, frame, replicate and attach can either be children of a link, or exist as root nodes without a parent. A collider is never parented under a visual: keep it a sibling of its visual, under the same link. A collider under a visual is not exported — it vanishes from the MJCF — and forge_define_parent refuses to create that nesting.

hfield and plane exist as root nodes without a parent for now.

In mjcf actuator, tendon, equality, contact and sensor do not use a hierarchy. In Forge, they will be created as children of their corresponding target model type.

Create rules¶

Creation order:¶

1. Create elements with bpy.ops.forge.create_<type>, optionally using selected existing model elements as targets.
2. Adjust location, rotation, and scale as needed, respecting locked axes.
   • Do not change a component when the corresponding lock_scale, lock_rotation, or lock_location flag is True.
3. Edit the parameters supported by the model type as needed.

File workflow and mesh preprocessing¶

• forge_import_mjcf(filepath, import_material=True) wraps bpy.ops.forge.import_mjcf and waits for its modal job. Import is destructive: it clears scene objects, collections, non-main scenes and orphaned data before rebuilding, so save first when current work matters. envhdr textures are imported into a Blender World environment texture and assigned to the main scene.
• Mesh assets support OBJ, STL and GLB. GLB accepts exactly one Mesh object; non-Mesh elements are discarded, and zero/multiple Mesh objects fail during asset loading. The Mesh node's transform is baked, file-space coordinates/UVs are preserved, and MJCF mesh scale is applied afterwards. MJCF defines the materials; embedded GLB materials are discarded. Missing/unsupported mesh files fail during asset loading.
• forge_export_mjcf(filepath, update_forge_only=False, use_glb=False, overwrite=False) wraps bpy.ops.forge.export_mjcf, waits for completion and reports whether the XML exists. Existing destinations require explicit overwrite=True. Export writes a linked World environment image as an MJCF <texture type="envhdr">; empty mesh assets (zero vertices or triangles) are skipped and do not produce mesh or geom references.
• forge_save_blendfile(filepath=None, overwrite=False, save_copy=False) saves the current blend or a named destination. Existing files—including the current blend path—require explicit overwrite=True; save_copy=True does not change the active blend path.
• forge_coacd(name, ...) wraps bpy.ops.forge.coacds for one selected VISUAL mesh. It reads triangle faces from the evaluated mesh and creates collider hulls while preserving the source object; generated hull names are returned in created. Important parameters: threshold, max_convex_hull, preprocess_mode, sampling/MCTS controls, merge, decimate/max_ch_vertex, extrude/extrude_margin, and seed.

Create tips:¶

• Joints:
  • Joint in forge is Z-UP based and use local +Z in parent space;
  • Rotate the joint object so Z points along the desired rotation axis. Position the joint at the pivot (e.g. top of a pendulum), not at the body center.
  • First consider the location is at link local space location [0, 0, 0], then consider location by needing.
• Inertials: create_inertial defaults to inertial_model_type='VISUAL' and check_mesh_valid=False. The operator can also use COLLIDER; enable check_mesh_valid=True for validation before calculation. The dedicated MCP wrapper uses the defaults. Primitive shapes use analytic mass and inertia; mesh geometry requires TetGen. When validation is enabled, boundary-only contact and separating coplanar faces are valid; actual crossings and non-separating coplanar overlap fail. An epsilon gap is not required for valid contact.
• Names: operators use type prefixes for object names. If a name already exists, Blender automatically appends a numeric suffix such as .001 or .002 to keep it unique. Read the actual names from the tool's created result instead of guessing them, then use forge_batch_rename if needed.
• Spacing: geometry sizes given at creation are full dimensions (BOX) / true radius & height (SPHERE / CYLINDER / CAPSULE), exported to mjcf half-extents automatically; all in parent-local meters.
• Collider: create collider for shape geometry is not necessary, if really necessary need set visual shape geometry obj.Forge_Geometry props contype and conaffinity to 0.

Model create rules:¶

link:¶

Create use bpy.ops.forge.create_link() optionally based on selected existing object(s):

• With no selection, the default location is the 3D cursor. Otherwise, placement depends on the selected object's type.
• Select object in type [LINK, FRAME, REPLICATE] location will at local space [0, 0, 0].
• Select object in type [VISUAL, COLLIDER] whose parent is [LINK, FRAME, REPLICATE]: new link parented under that parent, location at selected object's local position and rotation zeroed.
• Select object in type [VISUAL, COLLIDER] without parent in [LINK, FRAME, REPLICATE]: location at selected object's location, marked as Root Link.
• With no selection, the link is created in the World collection as a root candidate. Use bpy.ops.forge.mark_root_link() to mark it. Export also discovers parentless roots in the Link and Frame collections; those roots are not exported twice.

inertial:¶

Use bpy.ops.forge.create_inertial(inertial_model_type='VISUAL', check_mesh_valid=False) with selected existing link objects.

• Compute mass, center of mass, and principal inertia from children of the chosen type (VISUAL or COLLIDER), recursively through FRAME descendants. Child LINK hierarchies are excluded; at least one matching geometry object is required.
• Geometry validation is optional and off by default. Mesh geometry requires TetGen; primitive shapes do not.
• Create one new INERTIAL child per successful Link calculation. Existing inertials are not replaced.

joint:¶

Create use bpy.ops.forge.create_joint(joint_type='<ENUM>') based on selected existing link object.

• Parameter joint_type in [FREE, BALL, SLIDE, HINGE], default is HINGE.

geometry shape:¶

• visual: use bpy.ops.forge.create_geometry(geometry_type='<ENUM>', size_box=(1, 1, 1), size_radius=0.5, size_height=1)

• Select object in type [LINK, FRAME, REPLICATE]: create one geometry per selected object as child of it, location at local space [0, 0, 0].
• Select none objects: create at 3d cursor location.
• Parameter geometry_type in [BOX, CAPSULE, CYLINDER, SPHERE]
• Parameter used corresponding to geometry_type:
  • BOX: size_box
  • CAPSULE: size_radius, size_height
  • CYLINDER: size_radius, size_height
  • SPHERE: size_radius
• Size unit is meter

• collider: use bpy.ops.forge.create_collider(collider_type='<ENUM>', create_method='<ENUM>', auto_size=False, use_min_as_height=False, size_box=(1, 1, 1), size_radius=0.5, size_height=1)

Method of create_method is GEOMETRY_TYPE or COLLIDER_TYPE:

• GEOMETRY_TYPE: only for selected visual object(s), will auto detect shape from selected visual, other parameter not need for this method — collider_type is ignored, the collider takes the visual's own geometry type. For a mesh visual, prefer COLLIDER_TYPE with an explicit shape collider whenever that shape meets the collision requirements; use GEOMETRY_TYPE only when its resulting collider type follows the collider selection rule above.
• COLLIDER_TYPE: will need explicit choose collider_type, this is the only method when select none or select link/frame object.
• Parameter collider_type in [BOX, CAPSULE, CYLINDER, SPHERE, MESH, SDF]
• Parameter used corresponding to collider_type when create_method='COLLIDER_TYPE':
  • BOX: size_box or auto_size
  • CAPSULE: size_radius, size_height or auto_size
  • CYLINDER: size_radius, size_height or auto_size
  • SPHERE: size_radius or auto_size
  • MESH: no other parameter need, only available when visual object(s) selected, bake visual geometry to convex mesh collider
  • SDF: only with selected visuals; shares the visual's Mesh Data as a linked duplicate, preserving its parent and transform. Vertex edits affect both objects. auto_size: auto-detect the selected visual geometry size; off by default. With a link/frame selection or no selection, explicit size_* values (or defaults) are used instead. A replicate selection is not accepted by this operator. use_min_as_height: CAPSULE / CYLINDER only, default is off. When on, the height is taken from the selected visual's smallest bounding-box axis instead of its largest, and the shape is oriented along that axis. Only takes effect together with auto_size=True — with explicit size_radius/size_height the values are used as given and the flag is ignored. The MCP tool forge_create_collider exposes this as min_is_height.
• Size unit is meter

geometry mesh:¶

Create with traditional method. Use bpy.ops.forge.define_model_type(Model_Type='<ENUM>') to set object to corresponding model type.

• Model_Type is corresponding to: VISUAL for visual object, COLLIDER for collider object.

hfield:¶

Create use bpy.ops.forge.create_height_field():

• Only can create when nothing selected, exist as root node without parent.
• Default location at 3d cursor location.

plane:¶

Create use bpy.ops.forge.create_plane():

• Only can create when nothing selected, exist as root node without parent.
• Default location at 3d cursor location.

site:¶

Create use bpy.ops.forge.create_site(site_type='<ENUM>', size_box=(0.2, 0.2, 0.2), size_radius=0.1, size_height=0.2) based on selected existing link object(s).

• Parameter site_type in [BOX, CAPSULE, CYLINDER, SPHERE]
• Parameter used corresponding to site_type:
• BOX: size_box
• CAPSULE: size_radius, size_height
• CYLINDER: size_radius, size_height
• SPHERE: size_radius
• Size unit is meter

actuator:¶

Create use bpy.ops.forge.create_actuator(actuator_type='<ENUM>') based on selected existing link | joint | tendon object(s).

• Parameter actuator_type in [GENERAL, MOTOR, POSITION, VELOCITY, ADHESION]

tendon:¶

Create use bpy.ops.forge.create_tendon(tendon_type='<ENUM>') based on selected existing joint object(s).

• Need select at least 2 joint objects, create single tendon from ordered selection.
• Only support joint subtype in [SLIDE, HINGE].
• Parameter tendon_type currently only FIXED.
• Every ordered target item contains both target and exported coef (default 0.5). Use forge_set_tendon_targets(name, [{"target": "JNT_A", "coef": 0.25}, ...]) to replace the path without raw collection scripting.

equality:¶

Create use bpy.ops.forge.create_equality(equality_type='<ENUM>') based on exactly 2 selected existing link | joint | site objects.

• Selected joint objects → equality_type is JOINT.
• Selected link / site objects → equality_type is CONNECT or WELD.

contact:¶

Create use bpy.ops.forge.create_contact(contact_type='<ENUM>') based on selected existing link | visual | collider | plane | hfield objects.

• Need select at least 2 objects.
• Selected link objects → contact_type is EXCLUDE.
• Selected visual / collider / plane / hfield objects → contact_type is PAIR.
• Create one contact for each pair combination of selected objects.

sensor:¶

Create use bpy.ops.forge.create_sensor(Sensor_Type='<ENUM>', set_ref=False, set_xbody=False, set_subtree=False) based on selected existing link | joint | site | visual | collider | camera objects.

• Sensor_Type available values are decided by selected target model type:
  • joint: Jointpos, Jointvel
  • site: Touch, Accelerometer, Velocimeter, Gyro, Framepos, Framequat, Framexaxis, Frameyaxis, Framezaxis, Framelinvel, Frameangvel, Contact
  • link: Framepos, Framequat, Framexaxis, Frameyaxis, Framezaxis, Framelinvel, Frameangvel, Contact, Subtreecom, Subtreelinvel, Subtreeangmom
  • link with set_xbody=True: Framepos, Framequat, Framexaxis, Frameyaxis, Framezaxis, Framelinvel, Frameangvel
  • link with set_subtree=True: Contact
  • visual / collider: Framepos, Framequat, Framexaxis, Frameyaxis, Framezaxis, Framelinvel, Frameangvel, Contact
  • camera: Framepos, Framequat, Framexaxis, Frameyaxis, Framezaxis, Framelinvel, Frameangvel
  • mixed selection in [LINK, SITE, VISUAL, COLLIDER, CAMERA] (e.g. 2 objects with different types): Framepos, Framequat, Framexaxis, Frameyaxis, Framezaxis, Framelinvel, Frameangvel, Contact
• set_ref=True: need select 2 objects, first selected is target, second is reference; Sensor_Type range as Framepos ~ Contact; with site target start from Touch; with link target and set_xbody=True end at Frameangvel.
• set_xbody / set_subtree only for link target, mutual exclusive.
• Contact subtree sensors must explicitly set both subtree1 and subtree2. Select exactly two existing link objects in order [subtree1_root, subtree2_root], then call forge_create_sensor(sensor_type="Contact", set_ref=True, set_subtree=True, select=[subtree1_root, subtree2_root]). The first root becomes Forge_Sensor.target (MJCF subtree1), the second becomes Forge_Sensor.ref (MJCF subtree2). Do not create a one-sided subtree sensor or leave ref unset. Verify both pointers, use_subtree=True and target_type="SUBTREE"; after export, verify the <contact> sensor contains both attributes with the intended body names.

light:¶

Create use bpy.ops.forge.create_light(light_type='<ENUM>', light_mode='<ENUM>') optionally based on selected existing link object(s).

• Select none objects create world light at 3d cursor location, light_type in [DIRECTIONAL, SPOT, POINT], light_mode in [FIXED, TRACK, TRACKCOM].
• Select 1 link: light_type in [SPOT, POINT], light_mode in [TARGETBODY, TARGETBODYCOM, FIXED, TRACK, TRACKCOM]; when light_mode is [TARGETBODY, TARGETBODYCOM] create single world light at 3d cursor location targeting the selected link, other modes create child light of it.
• Select 2 links: when light_mode is [TARGETBODY, TARGETBODYCOM] create single light, first selected is light position, second is target; other modes create one light per selected link as child of it.
• Select 3+ links: light_mode restricted to [FIXED, TRACK, TRACKCOM], create one light per selected link as child of it.

camera:¶

Create use bpy.ops.forge.create_camera(camera_type='<ENUM>', camera_mode='<ENUM>') optionally based on selected existing link object(s).

• camera_type in [PERSPECTIVE, ORTHOGRAPHIC].
• Select none objects create world camera at 3d cursor location, camera_mode in [FIXED, TRACK, TRACKCOM].
• Select 1 link: camera_mode in [TARGETBODY, TARGETBODYCOM, FIXED, TRACK, TRACKCOM]; when camera_mode is [TARGETBODY, TARGETBODYCOM] create single world camera at 3d cursor location targeting the selected link, other modes create child camera of it.
• Select 2 links: when camera_mode is [TARGETBODY, TARGETBODYCOM] create single camera, first selected is camera position, second is target; other modes create one camera per selected link as child of it.
• Select 3+ links: camera_mode restricted to [FIXED, TRACK, TRACKCOM], create one camera per selected link as child of it.

frame:¶

Create use bpy.ops.forge.create_frame() optionally based on selected existing link | frame | replicate object(s).

• Select none objects create at 3d cursor location.
• Select objects create one frame per selected object as child of it.
• Shift + Click: reverse parent, selected objects become child of new frame.

replicate:¶

Create use bpy.ops.forge.create_replicate(count=0, sep='', offset=(0, 0, 0), euler=(0, 0, 0)) based on selected existing camera | light | visual | collider | site | link | frame | attach | replicate objects.

• Parameter count: replicate count.
• Parameter sep: separator string.
• Parameter offset: offset vector between copies.
• Parameter euler: euler rotation between copies.
• Create single replicate object, selected objects as its source.
• A replicate owns its source: the selected objects move into the replicate's own viewport-hidden collection and cannot be selected afterwards. Replicate objects that no later operation still needs to select.

attach:¶

Create use bpy.ops.forge.create_attach() optionally based on selected existing link | frame | replicate object.

• Need select exactly 1 object or none.
• Choose file in dialog via Forge_Temp_Props: tmp_file, tmp_model, tmp_prefix, tmp_link.
• Import the model from external mjcf file into scene, then create attach reference.
• Requires Forge Library asset library (Preferences -> File Paths -> Asset Libraries).
• The file must not be one already included in the scene — attach and include take separate files. Sharing one reports Attach fail: list index out of range.

include:¶

Create use bpy.ops.forge.create_include() only when nothing selected.

• Choose file in dialog via Forge_Temp_Props.tmp_include.
• Only add include tag when export, content of the file will not be exported.

Material creation rules:¶

Materials are PBR datablocks from the bundled Forge library — not mjcf model objects; they attach to visual, hfield or plane geometry.

material:¶

Use MCP forge_create_material(category, material, use_existing=True, select=[...]), which wraps bpy.ops.forge.create_material(category='<str>', material='<str>'), based on selected visual, hfield or plane object(s):

• Requires a non-empty selection where every object is type VISUAL, HFIELD or PLANE — poll fails when nothing is selected or any other type is selected.
• Parameter category in [Crystal, Human, Liquid, Manmade, Metal, Organic, Plastic] (~110 materials total).

• Parameter material is the exact library name inside the category — e.g. Gold, Water, Skin I, Titanium (Anodized); names carry parenthesized variants, match them exactly. Enumerate at runtime:

• Builds a Principled BSDF material datablock named after the library entry (base color is linear, set directly), non-render fields (density, viscosity, ...) stored as custom ID properties.

• Parameter use_existing (default True): when an existing datablock with the same name is present it is assigned as-is — user tweaks are preserved, it is NOT rebuilt from the library. Pass use_existing=False to always build a fresh datablock from the library (Blender suffixes the taken name .001).
• Assigns to the first object-level material slot of every selected object (creating the slot when empty). This applies equally to mesh and primitive visual, hfield, and plane objects; shape/material modifier sockets are not edited by the operator.

Enumerate categories / names from Python running inside Blender:

import bpy
import importlib

# Resolve both legacy and extension-managed installs without assuming the
# extension repository is named vscode_development or user_default.
module_name = next(
    (name for name in bpy.context.preferences.addons.keys() if name == "Forge" or name.endswith(".Forge")),
    None,
)
if module_name is None:
    raise RuntimeError("Enable the Forge add-on before inspecting its material library")
materials = importlib.import_module(f"{module_name}.core.materials")

materials.get_categories()  # ['Crystal', 'Human', 'Liquid', ...]
materials.get_material_names("Metal")  # ['Aluminum', 'Brass', 'Chromium', ...]