def populate_frames(num_frames, animation_data):
    """
    Takes a dictionary of frame_id: data pairs and produces a list of length num_frames with data inserted at indices
    specified by frame_id, and None everywhere else.
    """
    frame_data = {}
    for bone_id, keyframes in animation_data.items():
        frame_data[bone_id] = [None] * num_frames
        for frame_id, frame_value in keyframes.items():
            frame_data[bone_id][frame_id] = frame_value
    return frame_data