Convert a C3d and TRC files to BVH

Question

I have two files one .bvh and the other is .c3d and I want to convert them to .bvh files if anyone could please advise how to do this as I need to work on bvh only.

Answer 1

You can import .c3d file in blender like this:

https://stackoverflow.com/questions/20499320/how-to-import-c3d-files-into-blender

The problem is it won't be an armature, but animated locations of points:

Some points will be dead in the center (marked).

1. Adjust the frame range to fit the mocap data
2. Delete the dead markers

3. Re-build the armature - add armature to scene

   • looks like we have markers from both sides for each leg - that means the bones will be somewhere in between markers

   • Building the armature:

     • select a marker (or two if I want position in between) and align 3D cursor with 'Shift-S > Cursor to Selected'
     • then enter edit-mode of armature and select a joint and 'Shift-S > Selection to Cursor'

     • this way I can place all the bones (joints) precisely

       

     • also recalculate the Roll of the bones with 'Ctrl+N > Cursor' with 3D cursor on some reference marker

   • Name the bones properly

   • Constraining location of root bones:

     • use copy_location constraint
     • or two of them with second on 50% influence to get in-between position of two markers
     • similar for center of 3 markers, you get the idea

   • Constraining bones direction

     • use damped_track constraint

     • also multiple of them with adjusted influence for center of group of markers

       

4. Bake the animation to Action with Pose > Animation

   Bake Action: visual_keying_ON, clear_constraints_ON

5. Export armature as .bvh from the blender main menu (you might need to enable BVH addon).

These steps will result in having a .c3d file converted to a .bvh file. A sample .blend file with rigged markers here:

Answer 2

Created a script to create a hierarchal armature and add constraints to animate.

Firstly import the c3d with a prefix, I've chosen "GT_", this makes it easier to clean up.

import bpy
context = bpy.context
scene = context.scene
prefix = "GT_"
# select all imported with prefix GT_
obs = [o for o in scene.objects if o.name.startswith(prefix)]
root = "RootDog"

# clean up unwanted objects could look for flat action sum.

text = bpy.data.texts.get("C3D_bones")
if not text:
    text = bpy.data.texts.new("C3D_bones")

text.clear()

bones = {}    
for o in obs:
    if o.location.length < 0.0000001:
        scene.objects.unlink(o)
        bpy.data.objects.remove(o)
    else:
        name = o.name[3:]
        print(bones)
        bones[name] = {"parent":None,
                       "tail":root}

names = sorted(bones.keys())
text.write("%s\n" % root)
for name in names: 
    if name == root:
        continue    
    text.write("\t%s\n" % name)   

It creates a text object with a simple structure. Tabs must be used to indent. Like

RootDog
    LBackElbow
    LBackLeg
    LBackPaw

Which was quite simple to edit into, and I renamed to "C3DBones.txt" to avoid overwriting with previous script.

RootDog
    RFrontWaistDog
        RBackWaistDog
            RBackElbow
                RBackLeg
                    RBackPaw
    LFrontWaistDog
        LBackWaistDog
            LBackElbow
                LBackLeg
                    LBackPaw
    LowBackDog
        MidbackDog
            TopSpineDog
                RFShoulderDog
                    RElbowDog
                        RForeLeg
                            RPaw
                LFhoulderDog
                    LElbowDog
                        LForeLeg
                            LPaw                    
                MidDoghead
                    LeftDogHead
                    RightDogHead

Lastly, the hierarchy is used to create a rig and add constraints.

import bpy
from mathutils import Vector
context = bpy.context
scene = context.scene

text = bpy.data.texts.get("C3D_bones.txt")
prefix = "GT_"
parent_level = 0
parents = [None] * 16


arm = bpy.data.armatures.new("Armature")

rig = bpy.data.objects.new("C3DRig", arm)
scene.objects.link(rig)
scene.objects.active = rig
scene.update()
bpy.ops.object.mode_set(mode='EDIT')

for l in text.lines:

    body = l.body
    level = 0
    while body.startswith("\t"):
        body = body[1:]
        level += 1
    print(l.body, level)
    body = "%s%s" % (prefix, body)
    bone = arm.edit_bones.new(body)

    o = scene.objects.get(body)
    if not o:
        print("No %s" % body)
        continue

    bone.tail = o.location
    parent_level = max(level-1, 0)
    parents[level] = o
    parent = parents[parent_level]

    bone.parent = arm.edit_bones.get(parent.name)
    if bone.parent:
        bone.head = bone.parent.tail

bone = arm.edit_bones.get(parents[0].name)
if bone:    
    bone.head = bone.tail + Vector([0, 0, 1])
    bpy.ops.object.mode_set(mode='OBJECT')
else:
    print("no root bone")    

# do something with pose bones

for pb in rig.pose.bones:
    con = pb.constraints.new('COPY_LOCATION')
    if not pb.parent:
        con.target = bpy.data.objects.get(pb.name) 
        continue

    con.target = bpy.data.objects.get(pb.parent.name)
    con = pb.constraints.new('TRACK_TO')
    con.target = bpy.data.objects.get(pb.name)