Tic Tac Toe

Demonstration project — provided as an example of what the PixelRoot32 Game Engine can do. It is not a product: parts may be incomplete, experimental, or deliberately simplified to keep one idea in focus.

Three-in-a-row against a heuristic AI on a 3×3 board, drawn entirely with Renderer primitives — no sprites, no tilemap, no asset headers. The single idea it is about is the custom 16-colour palette: the whole neon look comes from one setCustomPalette() call remapping the engine's named colours, so the game code keeps saying Color::LightRed and gets neon pink.

Language: C++17
Engine: gperez88/PixelRoot32-Game-Engine@^1.9.0
Environments: native, esp32dev
Category: Games

Requirements (build flags)

Set in lib/platformio.ini's [base] template, so every environment inherits them:

• PIXELROOT32_ENABLE_AUDIO=1 — required. The background loop, the win jingle, and the four AudioEvent cues all go through MusicPlayer / AudioEngine. Building without it is not a compile error — it is silence.
• PIXELROOT32_ENABLE_PHYSICS=0 — the board is a Player[3][3] array and every rule is an array lookup. There is nothing to simulate.
• PIXELROOT32_ENABLE_PARTICLES=0 — not used.
• PIXELROOT32_ENABLE_UI_SYSTEM=0 — the status line and the instructions line are Renderer::drawTextCentered calls, not UI widgets.

Display size is 240×240 in the project platformio.ini (see PHYSICAL_DISPLAY_*). The board is BOARD_SIZE * CELL_SIZE = 150 px and is centred from DISPLAY_WIDTH / DISPLAY_HEIGHT at reset, so a different resolution re-centres it instead of clipping it.

Platforms

Environment	Display	Audio backend
native	SDL2, 240×240	SDL2_AudioBackend in src/platforms/native.h
esp32dev	ST7789 240×240	ESP32_I2S_AudioBackend in src/platforms/esp32_dev.h

Pin choices (ST7789 SPI, I2S audio, D-pad + two buttons) are in src/platforms/esp32_dev.h — edit there if your wiring differs.

Controls

Action	native (keyboard)	esp32dev (GPIO)
Move cursor	Arrow keys	D-pad
Place mark	Space	Button A
Restart (from Game Over)	Space	Button A

The cursor wraps at the board edges in both axes, so nine cells are always two presses apart at most. After a game ends the input is ignored for 500 ms so the press that ended the game cannot immediately restart it.

What it demonstrates

• Custom palette remapping. CUSTOM_NEON_PALETTE is a plain uint16_t[16] of RGB565 values handed to graphics::setCustomPalette() in init(). Nothing else in the scene knows about it: drawX still asks for Color::LightRed, and the palette decides that means neon pink. Swapping the look is one array, not a sweep through the draw code.
• Two-layer audio. A looping MusicTrack built from makeNote / makeRest with the INSTR_TRIANGLE_PAD preset runs as background music, while placement, loss, and draw are one-shot AudioEvents played straight on the AudioEngine. Win stops the loop and plays a non-looping MusicTrack instead — the two paths never fight over the same voice.
• Heuristic AI with a tunable error rate. computeAIMove() is the classic four-step ladder: win if you can, block if you must, take the centre, take a corner. DEFAULT_AI_ERROR_CHANCE (0.25) then rerolls that choice a quarter of the time, which is what keeps a perfect-information 3×3 game losable.
• Edge-detected input without an input queue. Cursor movement compares the current isButtonDown against last frame's, so a held direction moves one cell rather than nine. The inputReady latch additionally swallows the button that was still down when the scene started.
• Zero assets, zero allocation. Everything on screen is a line, a rectangle, or text. The two status strings are fixed char buffers filled with snprintf; nothing in update() or draw() allocates.

Project layout

src/
├── GameConstants.h      board size, cell size, button IDs, AI error chance
├── TicTacToeScene.h/.cpp  the whole game: state, AI, audio, drawing
├── main.cpp             platform selector
└── platforms/           native.h, esp32_dev.h — backend wiring per target

Engine documentation

• Audio API
• Core API
• Graphics / Renderer
• Input API

Build

From games/tic_tac_toe:

pio run -e native
pio run -e esp32dev

Upload (ESP32)

pio run -e esp32dev --target upload

License

Source code: MIT.

Asset	Author	License	Source
All audio cues	This repository	CC0 1.0 (public domain)	Original work, synthesised at runtime from AudioEvent / MusicTrack definitions in src/

This demo ships no art and no generated asset headers — the board, the marks, and the cursor are Renderer primitives resolved through the custom palette, so there is nothing else here to attribute.

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Source code: https://github.com/PixelRoot32-Game-Engine/PixelRoot32-Demo-Projects/tree/main/games/tic_tac_toe