plinko

Plinko

This program simulates a Plinko board game using SDL2, where balls fall through a grid of obstacles and land in slots at the bottom.

Table of Contents

• Program Overview
• Physics Simulation
• Collision System
• Rendering
• User Interaction
• Compilation and Execution

Demo

Program Overview

The simulation features:

• Balls that fall under gravity
• A grid of fixed obstacles (pins)
• A slotted board at the bottom
• Physics-based collisions
• Two spawning modes:
  • Manual: Click and hold to spawn balls
  • Automatic: 200 balls spawn automatically at the top

Physics Simulation

Key physics components:

• Gravity: Constant downward acceleration (GRAVITY = 800.0)
• Velocity: Each ball maintains vx (horizontal) and vy (vertical) velocities
• Resting State: Balls "snap" in place when velocities drop below threshold
• Friction: Horizontal friction applied when balls hit the floor

Physics update loop:

apply_gravity(&circles, dt);
for (int iter = 0; iter < circles.used / 2; iter++) {
    resolve_walls_and_floor(&circles);
    check_obstacle_collisions(&circles, &obstacles);
    check_line_collisions(&circles);
    resolve_ball_ball_collisions(&circles);
}

Collision System

1. Wall and Floor Collisions

• Floor: Balls stop vertical movement and experience friction
• Walls: Perfectly elastic collisions with restitution (REST_SIDE = 0.7)

2. Ball-Ball Collisions

• Momentum-conserving collisions
• Mass-based response (stationary balls have higher mass)
• Restitution coefficient: REST_BALL = 0.6

3. Obstacle Collisions

• Grid of circular pins arranged in staggered pattern
• Elastic collisions when balls hit pins

4. Slot Collisions

• Vertical lines at bottom form slots
• Special collision detection for line segments
• Horizontal velocity reversal on impact

Rendering

Visual elements:

1. Balls: White filled circles (FillCircle())
2. Obstacles: Grid of white circles
3. Slot: Vertical white lines at bottom (draw_slot())

Rendering pipeline:

SDL_FillRect(surface, NULL, COLOR_BLACK);  // Clear screen
draw_obstacles(surface, &obstacles);      // Draw pins
draw_slot(surface);                       // Draw bottom slots
for (each ball) {
    FillCircle(surface, ball, COLOR_WHITE); // Draw balls
}
SDL_UpdateWindowSurface(window);          // Refresh display

User Interaction

• Mouse Click: Spawn single ball at cursor position
• Mouse Hold: Continuously spawn balls (0.06s interval)
• Automatic Mode: 200 balls spawn from top at 0.15s intervals
• Close Window: Exit program

Compilation and Execution

Dependencies

• SDL2 library
• C compiler (GCC recommended)

Compilation

chmod +x build.sh
./build

Execution

./plinko

Controls

• Click and hold mouse: Spawn balls at cursor
• Automatic mode: Starts immediately (200 balls from top)
• Close window: Exit program

The simulation demonstrates realistic physics interactions between balls, obstacles, and boundaries, creating an engaging Plinko game experience.