3D Voxel YoYo Animation Tutorial

This guide walks you through how to generate a looping 3D voxel animation of a yo-yo using SpatialStudio. The script creates a colorful yo-yo that bounces up and down with a realistic string inside a cubic 3D space, then saves the animation to a .splv file.

What this script does

Creates a 3D scene of size 128×128×128
Spawns 1 yo-yo with:
- Two circular disc halves forming the yo-yo body
- A realistic string that extends and retracts
- Metallic highlights for a glossy finish
- Smooth up-and-down bouncing motion
Animates the yo-yo for 6 seconds at 30 FPS with realistic physics
Outputs the file yoyo.splv that you can play in your viewer

How it works (simplified)

Voxel volume Each frame is a 3D grid filled with RGBA values (SIZE × SIZE × SIZE × 4).
Yo-yo body The yo-yo consists of two disc-shaped halves with a narrow connecting axle in the middle.
String physics The string length changes dynamically based on the yo-yo's position, simulating realistic string extension and retraction.
Bounce motion Uses sine wave functions to create smooth up-and-down motion with acceleration and deceleration.
Highlights Metallic highlights are added to the yo-yo discs to simulate light reflection and give it a realistic appearance.
Animation loop A normalized time variable t cycles from 0 → 2π, ensuring the motion loops smoothly with the yo-yo returning to its starting position.
Encoding Frames are passed into splv.Encoder, which writes them into the .splv video file.

Try it yourself

Install requirements first:

pip install spatialstudio numpy tqdm

Then copy this script into yoyo.py and run:

python yoyo.py

Full Script

import numpy as np
from spatialstudio import splv
from tqdm import tqdm

# Scene setup
SIZE, FPS, SECONDS = 128, 30, 6
FRAMES = FPS * SECONDS
CENTER_X = CENTER_Y = CENTER_Z = SIZE // 2
OUT_PATH = "../outputs/yoyo.splv"

# Yo-yo settings
YOYO_RADIUS = 12
YOYO_THICKNESS = 4
AXLE_RADIUS = 3
MAX_STRING_LENGTH = 40
BOUNCE_AMPLITUDE = 30

def add_voxel(volume, x, y, z, color):
    if 0 <= x < SIZE and 0 <= y < SIZE and 0 <= z < SIZE:
        volume[x, y, z, :3] = color
        volume[x, y, z, 3] = 255

def generate_yoyo_disc(volume, cx, cy, cz, radius, thickness, color, t):
    """Generate one half of the yo-yo (disc shape)"""
    for dx in range(-radius, radius+1):
        for dy in range(-thickness//2, thickness//2+1):
            for dz in range(-radius, radius+1):
                distance = np.sqrt(dx*dx + dz*dz)
                if distance <= radius:
                    # Add subtle texture variation
                    texture = int(np.sin(dx*0.2 + dz*0.2 + t*0.3) * 3)
                    brightness = 1.0 + texture * 0.05
                    final_color = tuple(min(255, int(c * brightness)) for c in color)
                    add_voxel(volume, cx+dx, cy+dy, cz+dz, final_color)

def generate_yoyo_axle(volume, cx, cy, cz, radius, height, color):
    """Generate the central axle of the yo-yo"""
    for dx in range(-radius, radius+1):
        for dy in range(-height//2, height//2+1):
            for dz in range(-radius, radius+1):
                if dx*dx + dz*dz <= radius*radius:
                    add_voxel(volume, cx+dx, cy+dy, cz+dz, color)

def generate_string(volume, start_x, start_y, start_z, end_y, t):
    """Generate the yo-yo string with slight swaying motion"""
    string_color = (139, 69, 19)  # Brown string
    string_length = abs(end_y - start_y)
    
    for i in range(int(string_length)):
        progress = i / string_length if string_length > 0 else 0
        # Add slight swaying motion
        sway = np.sin(t * 3.0 + progress * 8) * 2
        x = start_x + int(sway * progress * 0.3)
        y = start_y + i
        z = start_z + int(np.sin(t * 2.0 + progress * 5) * 0.5)
        add_voxel(volume, x, y, z, string_color)

def generate_yoyo_highlights(volume, cx, cy, cz, t):
    """Add metallic highlights to the yo-yo"""
    highlight_color = (255, 255, 255)
    
    # Top disc highlights
    for dx in range(-3, 4):
        for dz in range(-3, 4):
            if dx*dx + dz*dz <= 9:
                add_voxel(volume, cx-6+dx, cy-YOYO_THICKNESS+1, cz-4+dz, highlight_color)
    
    # Bottom disc highlights  
    for dx in range(-3, 4):
        for dz in range(-3, 4):
            if dx*dx + dz*dz <= 9:
                add_voxel(volume, cx-6+dx, cy+YOYO_THICKNESS-1, cz-4+dz, highlight_color)

def generate_yoyo_body(volume, cx, cy, cz, t):
    """Generate the complete yo-yo body"""
    # Yo-yo colors (red and blue discs)
    top_color = (220, 20, 60)    # Crimson red
    bottom_color = (30, 144, 255)  # Dodger blue
    axle_color = (169, 169, 169)   # Dark gray
    
    # Generate top disc
    generate_yoyo_disc(volume, cx, cy - YOYO_THICKNESS//2, cz, 
                      YOYO_RADIUS, YOYO_THICKNESS, top_color, t)
    
    # Generate bottom disc
    generate_yoyo_disc(volume, cx, cy + YOYO_THICKNESS//2, cz, 
                      YOYO_RADIUS, YOYO_THICKNESS, bottom_color, t)
    
    # Generate axle
    generate_yoyo_axle(volume, cx, cy, cz, AXLE_RADIUS, YOYO_THICKNESS * 2, axle_color)
    
    # Add highlights
    generate_yoyo_highlights(volume, cx, cy, cz, t)

def calculate_yoyo_position(t):
    """Calculate yo-yo position based on realistic bouncing motion"""
    # Use a combination of sine waves to create realistic yo-yo motion
    # The yo-yo accelerates down and decelerates up
    bounce_cycle = np.sin(t) * 0.5 + 0.5  # Normalize to 0-1
    
    # Apply easing for more realistic motion
    if bounce_cycle < 0.5:
        # Going down - accelerating
        eased = 2 * bounce_cycle * bounce_cycle
    else:
        # Going up - decelerating  
        eased = 1 - 2 * (1 - bounce_cycle) * (1 - bounce_cycle)
    
    y_offset = int(BOUNCE_AMPLITUDE * eased)
    return y_offset

def generate_scene(volume, t):
    """Generate the complete scene for one frame"""
    # Calculate yo-yo position
    y_offset = calculate_yoyo_position(t)
    yoyo_y = CENTER_Y - BOUNCE_AMPLITUDE//2 + y_offset
    
    # Generate yo-yo body
    generate_yoyo_body(volume, CENTER_X, yoyo_y, CENTER_Z, t)
    
    # Generate string from hand (top) to yo-yo
    hand_y = CENTER_Y - BOUNCE_AMPLITUDE//2 - 20  # Hand position above yo-yo
    generate_string(volume, CENTER_X, hand_y, CENTER_Z, yoyo_y - YOYO_THICKNESS, t)
    
    # Add a simple "hand" at the top
    hand_color = (255, 220, 177)  # Skin color
    for dx in range(-2, 3):
        for dy in range(-3, 1):
            for dz in range(-2, 3):
                if dx*dx + dz*dz <= 4:
                    add_voxel(volume, CENTER_X+dx, hand_y+dy, CENTER_Z+dz, hand_color)

# Initialize encoder
enc = splv.Encoder(SIZE, SIZE, SIZE, framerate=FPS, outputPath=OUT_PATH, motionVectors="off")

# Generate animation frames
for frame in tqdm(range(FRAMES), desc="Generating yo-yo animation"):
    volume = np.zeros((SIZE, SIZE, SIZE, 4), dtype=np.uint8)
    t = (frame / FRAMES) * 2*np.pi  # Normalized time for smooth looping
    generate_scene(volume, t)
    enc.encode(splv.Frame(volume, lrAxis="x", udAxis="y", fbAxis="z"))

enc.finish()
print(f"Created {OUT_PATH}")

Next steps

Adjust BOUNCE_AMPLITUDE to change how far the yo-yo travels
Modify YOYO_RADIUS and YOYO_THICKNESS to create different yo-yo sizes
Change the disc colors by editing top_color and bottom_color
Add spin rotation by modifying the highlight positions over time
Create multiple yo-yos by calling generate_yoyo_body() with different positions
Experiment with different easing functions in calculate_yoyo_position() for varied motion styles

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