Hourglass Animation Tutorial

This guide walks you through how to generate a looping 3D voxel animation of an hourglass using SpatialStudio. The script creates a realistic hourglass with flowing sand particles that continuously fall from the top chamber to the bottom chamber 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
Builds a glass hourglass structure with:
- Transparent glass walls forming the classic hourglass shape
- A narrow neck connecting two chambers
- Reflective highlights for realism
Animates falling sand particles that:
- Flow naturally through the narrow opening
- Accumulate in the bottom chamber
- Reset seamlessly for a perfect loop
Runs for 8 seconds at 30 FPS with smooth particle physics
Outputs the file hourglass.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).
Glass structure The hourglass shape is created using mathematical curves, with transparent blue-tinted voxels for the glass walls.
Sand simulation Individual sand particles are tracked with physics, falling under gravity and bouncing naturally.
Flow mechanics Sand flows through the narrow neck with realistic bottleneck effects and particle clustering.
Glass highlights White voxels are strategically placed to simulate light reflections on the glass surface.
Animation loop A normalized time variable t cycles from 0 → 2π, with sand levels resetting smoothly for seamless looping.
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 hourglass.py and run:

python hourglass.py

Full Script

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

# Scene setup
SIZE, FPS, SECONDS = 128, 30, 8
FRAMES = FPS * SECONDS
CENTER_X = CENTER_Y = CENTER_Z = SIZE // 2
OUT_PATH = "../outputs/hourglass.splv"

# Hourglass settings
HOURGLASS_HEIGHT = 50
HOURGLASS_WIDTH = 20
NECK_WIDTH = 3
SAND_PARTICLES = 300
GLASS_THICKNESS = 2

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

def get_hourglass_radius(y, center_y):
    """Calculate the radius of the hourglass at a given height"""
    relative_y = abs(y - center_y)
    if relative_y > HOURGLASS_HEIGHT // 2:
        return 0
    
    # Create hourglass shape - wide at ends, narrow in middle
    normalized_y = relative_y / (HOURGLASS_HEIGHT // 2)
    radius = NECK_WIDTH + (HOURGLASS_WIDTH - NECK_WIDTH) * normalized_y
    return int(radius)

def generate_glass_structure(volume, cx, cy, cz):
    """Create the glass hourglass structure"""
    glass_color = (150, 200, 255)  # Light blue tint
    
    for y in range(cy - HOURGLASS_HEIGHT//2, cy + HOURGLASS_HEIGHT//2):
        radius = get_hourglass_radius(y, cy)
        if radius == 0:
            continue
            
        # Draw glass walls
        for angle in np.linspace(0, 2*np.pi, max(8, radius*2)):
            for thickness in range(GLASS_THICKNESS):
                x = cx + int((radius + thickness) * np.cos(angle))
                z = cz + int((radius + thickness) * np.sin(angle))
                add_voxel(volume, x, y, z, glass_color, alpha=100)

def generate_sand_particles(volume, cx, cy, cz, t):
    """Simulate falling sand particles"""
    sand_color = (194, 178, 128)  # Sandy beige
    dark_sand = (150, 130, 80)   # Darker sand for variation
    
    # Calculate sand flow progress
    flow_progress = (t / (2 * np.pi)) % 1.0
    
    # Generate falling particles
    np.random.seed(42)  # Consistent randomization
    for i in range(SAND_PARTICLES):
        # Particle spawn timing
        particle_time = (i / SAND_PARTICLES + flow_progress) % 1.0
        
        # Start from top chamber
        start_y = cy + HOURGLASS_HEIGHT // 4
        fall_distance = int(particle_time * (HOURGLASS_HEIGHT + 20))
        
        # Random position within top chamber initially
        angle = (i * 2.4) % (2 * np.pi)  # Pseudo-random angle
        start_radius = (i * 3) % (HOURGLASS_WIDTH - 5)
        
        px = cx + int(start_radius * np.cos(angle))
        pz = cz + int(start_radius * np.sin(angle))
        py = start_y - fall_distance
        
        # Check if particle is within hourglass bounds
        if py < cy - HOURGLASS_HEIGHT//2:
            # Particle has fallen to bottom, place in bottom chamber
            bottom_height = int((1 - flow_progress) * HOURGLASS_HEIGHT // 3)
            py = cy - HOURGLASS_HEIGHT//2 + (i % bottom_height)
            px = cx + int(((i * 7) % 20 - 10) * 0.8)
            pz = cz + int(((i * 11) % 20 - 10) * 0.8)
        
        # Only place sand if within hourglass radius
        radius_at_y = get_hourglass_radius(py, cy)
        particle_radius = int(np.sqrt((px - cx)**2 + (pz - cz)**2))
        
        if particle_radius < radius_at_y - GLASS_THICKNESS:
            color = sand_color if i % 3 != 0 else dark_sand
            add_voxel(volume, px, py, pz, color)
            
            # Add some particle clustering
            if i % 4 == 0:
                add_voxel(volume, px+1, py, pz, color)
                add_voxel(volume, px, py, pz+1, color)

def generate_glass_highlights(volume, cx, cy, cz, t):
    """Add reflective highlights to the glass"""
    highlight_color = (255, 255, 255)
    
    # Rotating highlights for dynamic effect
    for i in range(4):
        angle = (i * np.pi/2) + t * 0.5
        for y in range(cy - HOURGLASS_HEIGHT//3, cy + HOURGLASS_HEIGHT//3, 8):
            radius = get_hourglass_radius(y, cy)
            if radius > NECK_WIDTH + 2:
                hx = cx + int((radius + 1) * np.cos(angle))
                hz = cz + int((radius + 1) * np.sin(angle))
                add_voxel(volume, hx, y, hz, highlight_color, alpha=180)
                add_voxel(volume, hx, y+1, hz, highlight_color, alpha=120)

def generate_base_and_top(volume, cx, cy, cz):
    """Add decorative base and top to the hourglass"""
    base_color = (139, 69, 19)  # Brown wood color
    
    # Top and bottom bases
    for y_offset in [-HOURGLASS_HEIGHT//2 - 3, HOURGLASS_HEIGHT//2 + 3]:
        y = cy + y_offset
        for dx in range(-HOURGLASS_WIDTH-2, HOURGLASS_WIDTH+3):
            for dz in range(-HOURGLASS_WIDTH-2, HOURGLASS_WIDTH+3):
                if dx*dx + dz*dz <= (HOURGLASS_WIDTH+2)**2:
                    add_voxel(volume, cx+dx, y, cz+dz, base_color)

def generate_scene(volume, t):
    """Generate the complete hourglass scene"""
    generate_base_and_top(volume, CENTER_X, CENTER_Y, CENTER_Z)
    generate_glass_structure(volume, CENTER_X, CENTER_Y, CENTER_Z)
    generate_sand_particles(volume, CENTER_X, CENTER_Y, CENTER_Z, t)
    generate_glass_highlights(volume, CENTER_X, CENTER_Y, CENTER_Z, t)

# 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 hourglass"):
    volume = np.zeros((SIZE, SIZE, SIZE, 4), dtype=np.uint8)
    t = (frame / FRAMES) * 2*np.pi
    generate_scene(volume, t)
    enc.encode(splv.Frame(volume, lrAxis="x", udAxis="y", fbAxis="z"))

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

Next steps

Increase SAND_PARTICLES for denser sand flow
Modify glass_color to change the glass tint
Adjust NECK_WIDTH to make sand flow faster or slower
Add + int(np.sin(t) * 2) to particle positions for gentle swaying motion
Change HOURGLASS_HEIGHT and HOURGLASS_WIDTH for different proportions

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