07 Jul 2025

Building Pong in Zig with Raylib – Part 3: Edge Collisions, Scoring & Player Input

In this part, we round out the core gameplay of our Pong clone in Zig using raylib. By the end, we’ve got a working game - with a bouncing ball, paddle controls, and basic scoring.

🧱 Edge Collisions

We already had paddle collision working, but we needed the ball to bounce off the top and bottom edges of the screen. That meant checking if the ball’s y-position was outside the visible range and inverting its vertical velocity accordingly:

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pub fn checkEdgeCollisions(self: *Ball, screen_height: f32) void {
    if (self.pos.y < self.r or self.pos.y > screen_height - self.r) {
        self.vel.y *= -1;
    }
}

There was a small detour where I realised screen_height wasn’t giving the expected value - using GetScreenHeight() directly from raylib helped resolve that.

🏁 Scoring System

Once edge collisions were in place, it was time to detect goals. If the ball passed the left or right edge of the screen, the opposing player got a point. We also need a reset() method to the Ball struct to return it to the centre after each goal.

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pub fn reset(self: *Ball) void {
    self.pos = self.home;
    self.vel = .{ .x = 250, .y = -50 };
}

For now, let’s print the scores via std.debug.print, but this sets the stage for UI integration.

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if (ball.pos.x > screenWidth) {
    left_paddle.score += 1;
    std.debug.print("scores: l: {d}, r: {d}", .{ left_paddle.score, right_paddle.score });
    ball.reset();
}

if (ball.pos.x < 0) {
    right_paddle.score += 1;
    std.debug.print("scores: l: {d}, r: {d}", .{ left_paddle.score, right_paddle.score });
    ball.reset();
}

⌨️ Player Input

Finally, we wired up keyboard input to allow paddle movement. We used IsKeyDown from raylib, and made sure movement was frame-rate independent by scaling it with dt:

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pub fn move(self: *Paddle, y: f32, dt: f32) void {
    self.pos.y += y * dt;
}

Left paddle uses W/S, right paddle uses E/D. Simple and responsive.

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if (rl.isKeyDown(.w)) {
    left_paddle.move(-100, dt);
}

if (rl.isKeyDown(.s)) {
    left_paddle.move(100, dt);
}

if (rl.isKeyDown(.e)) {
    right_paddle.move(-100, dt);
}

if (rl.isKeyDown(.d)) {
    right_paddle.move(100, dt);
}

✅ What’s Working

By the end of this episode, we’ve got:

Ball bounces off top and bottom edges
Scoring when the ball passes a paddle
Ball reset after each goal
Both paddles are fully controllable

All that’s missing now is a visible score, a win condition, and maybe a simple menu.