Blender Motion Capture

---
name: blender-motion-capture
description: >-
  Automate motion capture and tracking workflows in Blender with Python. Use
  when the user wants to import BVH or FBX mocap data, retarget motion to
  armatures, track camera or object motion from video, solve camera motion,
  clean up motion capture data, or script any tracking pipeline in Blender.
license: Apache-2.0
compatibility: >-
  Requires Blender 3.0+. Video tracking needs ffmpeg (bundled).
  Run: blender --background --python script.py
metadata:
  author: terminal-skills
  version: "1.0.0"
  category: automation
  tags: ["blender", "motion-capture", "tracking", "mocap", "animation"]
---

# Blender Motion Capture

## Overview

Import, process, and retarget motion capture data in Blender using Python. Work with BVH/FBX mocap files, track camera and object motion from video footage, solve 3D camera paths, and clean up animation data — all scriptable from the terminal.

## Instructions

### 1. Import BVH motion capture files

```python
import bpy

bpy.ops.import_anim.bvh(
    filepath="/path/to/mocap.bvh",
    target='ARMATURE',
    global_scale=1.0,
    frame_start=1,
    use_fps_scale=False,
    rotate_mode='NATIVE',
    axis_forward='-Z', axis_up='Y'
)

armature = bpy.context.active_object
action = armature.animation_data.action
print(f"Imported: {armature.name}, Bones: {len(armature.data.bones)}, Frames: {action.frame_range}")
```

### 2. Import FBX with animation

```python
bpy.ops.import_scene.fbx(
    filepath="/path/to/mocap.fbx",
    use_anim=True,
    ignore_leaf_bones=True,
    automatic_bone_orientation=True,
    primary_bone_axis='Y', secondary_bone_axis='X'
)
```

### 3. Retarget motion between armatures

```python
from mathutils import Matrix

def retarget_motion(source_armature, target_armature, bone_mapping):
    """Retarget animation using a bone name mapping: {target_bone: source_bone}"""
    source_action = source_armature.animation_data.action
    frame_start, frame_end = int(source_action.frame_range[0]), int(source_action.frame_range[1])

    if not target_armature.animation_data:
        target_armature.animation_data_create()
    new_action = bpy.data.actions.new(f"{source_action.name}_retarget")
    target_armature.animation_data.action = new_action

    for frame in range(frame_start, frame_end + 1):
        bpy.context.scene.frame_set(frame)
        for tgt_name, src_name in bone_mapping.items():
            src = source_armature.pose.bones.get(src_name)
            tgt = target_armature.pose.bones.get(tgt_name)
            if not src or not tgt:
                continue
            tgt.rotation_quaternion = src.rotation_quaternion
            tgt.keyframe_insert(data_path="rotation_quaternion", frame=frame)
            # Copy location for root bone only
            if src_name == list(bone_mapping.values())[0]:
                tgt.location = src.location
                tgt.keyframe_insert(data_path="location", frame=frame)

# Example Mixamo → Rigify mapping
mapping = {
    "spine": "mixamorig:Hips", "spine.001": "mixamorig:Spine",
    "spine.004": "mixamorig:Neck", "spine.006": "mixamorig:Head",
    "upper_arm.L": "mixamorig:LeftArm", "forearm.L": "mixamorig:LeftForeArm",
    "upper_arm.R": "mixamorig:RightArm", "forearm.R": "mixamorig:RightForeArm",
    "thigh.L": "mixamorig:LeftUpLeg", "shin.L": "mixamorig:LeftLeg",
    "thigh.R": "mixamorig:RightUpLeg", "shin.R": "mixamorig:RightLeg",
}
```

### 4. Clean up motion capture data

```python
def decimate_fcurve(fcurve, factor=0.5):
    """Remove keyframes to reduce data while keeping shape."""
    points = fcurve.keyframe_points
    total = len(points)
    keep_every = max(1, int(1.0 / factor))
    remove_indices = [i for i in range(total) if i % keep_every != 0 and i != 0 and i != total - 1]
    for i in reversed(remove_indices):
        points.remove(points[i])

armature = bpy.context.active_object
action = armature.animation_data.action
for fcurve in action.fcurves:
    decimate_fcurve(fcurve, factor=0.5)
    fcurve.update()
```

### 5. Video motion tracking and camera solve

```python
# Load footage
clip = bpy.data.movieclips.load("/path/to/footage.mp4")
scene = bpy.context.scene
scene.active_clip = clip

# Configure tracking
tracking = clip.tracking
settings = tracking.settings
settings.default_pattern_size = 21
settings.default_search_size = 71
settings.default_motion_model = 'AFFINE'

# Camera settings for solving
camera = tracking.camera
camera.sensor_width = 36.0
camera.focal_length = 50.0

# Solve camera motion
bpy.ops.clip.solve_camera()
solve_error = tracking.reconstruction.average_error
print(f"Solve error: {solve_error:.4f} px ({'Good' if solve_error < 0.5 else 'Needs refinement'})")

# Set up scene from solved data
bpy.ops.clip.setup_tracking_scene()
```

### 6. Apply tracked motion to objects

```python
obj = bpy.data.objects["MyObject"]
constraint = obj.constraints.new(type='FOLLOW_TRACK')
constraint.clip = clip
constraint.track = tracking.tracks["Marker_01"]
constraint.use_3d_position = True
constraint.camera = scene.camera

# Bake constraint to keyframes
bpy.context.view_layer.objects.active = obj
obj.select_set(True)
bpy.ops.nla.bake(
    frame_start=1, frame_end=clip.frame_duration,
    only_selected=True, visual_keying=True,
    clear_constraints=True, bake_types={'OBJECT'}
)
```

### 7. Export animation data

```python
# Export as BVH
bpy.ops.export_anim.bvh(
    filepath="/tmp/output_mocap.bvh",
    frame_start=int(action.frame_range[0]),
    frame_end=int(action.frame_range[1]),
    rotate_mode='NATIVE'
)

# Export as FBX with baked animation
bpy.ops.export_scene.fbx(
    filepath="/tmp/output_anim.fbx",
    use_selection=True, bake_anim=True,
    bake_anim_use_all_bones=True, add_leaf_bones=False
)
```

## Examples

### Example 1: Batch scan mocap library

**User request:** "Import all BVH files from a folder, list bone counts and frame ranges"

```python
import bpy, glob, os

for filepath in sorted(glob.glob("/path/to/mocap_library/*.bvh")):
    bpy.ops.object.select_all(action='SELECT')
    bpy.ops.object.delete()
    bpy.ops.import_anim.bvh(filepath=filepath, target='ARMATURE', global_scale=0.01, frame_start=1)
    arm = bpy.context.active_object
    if arm and arm.animation_data:
        action = arm.animation_data.action
        duration = (action.frame_range[1] - action.frame_range[0]) / bpy.context.scene.render.fps
        print(f"{os.path.basename(filepath)}: {len(arm.data.bones)} bones, {duration:.1f}s")
```

Run: `blender --background --python scan_mocap.py`

### Example 2: Apply mocap to character and render

**User request:** "Import a BVH file, apply it to my rigged character, and render a preview"

```python
import bpy

bpy.ops.wm.open_mainfile(filepath="/path/to/character.blend")
char_armature = bpy.data.objects["Armature"]

bpy.ops.import_anim.bvh(filepath="/path/to/walk_cycle.bvh", target='ARMATURE', global_scale=0.01)
mocap_armature = bpy.context.active_object
mocap_action = mocap_armature.animation_data.action

# Transfer action (works when bone names match)
if not char_armature.animation_data:
    char_armature.animation_data_create()
char_armature.animation_data.action = mocap_action

# Remove temp armature, set frame range, add camera, render
bpy.data.objects.remove(mocap_armature)
scene = bpy.context.scene
scene.frame_start, scene.frame_end = int(mocap_action.frame_range[0]), int(mocap_action.frame_range[1])
scene.render.filepath = "/tmp/mocap_preview/frame_"
bpy.ops.render.render(animation=True)
```

## Guidelines

- BVH is simplest (plain text with hierarchy + motion). FBX supports richer data (blend shapes, multiple takes).
- Scale matters: BVH files often use centimeters. Set `global_scale=0.01` for cm-based files.
- Bone name matching is critical for retargeting. Build a mapping dictionary for each source format.
- For retargeting, copy rotations for all bones but only location for the root/hip bone.
- Clean up imported mocap by decimating keyframes — raw mocap has every-frame keys, making editing difficult.
- Camera solve quality depends on marker count and distribution. Use 8+ well-distributed markers, keep error below 0.5px.
- Use `bpy.ops.nla.bake()` to convert constraints to keyframes for export.
- Always export with `bake_anim=True` in FBX to flatten NLA strips and constraints.