Building a Game Engine From Scratch: What You Actually Need to Do

A game engine is software that handles the core systems your game runs on — graphics rendering, physics, sound, input, and the game loop that ties them together

You do not need to build one from nothing. Most people use existing engines like Unity, Unreal Engine, or Godot because they already have these systems built and tested. But if you are building one yourself — either to learn how games work or because you need something specialized — you are writing code that repeatedly does the same things: draws graphics to the screen, checks what the player pressed, moves objects, detects collisions, and runs that cycle 60 or 120 times per second.

Building an engine means deciding what you will write yourself and what you will use from libraries. A graphics library like SDL2 or SFML handles drawing to the screen. A physics library like Bullet or Rapier handles collisions and movement. You write the code that connects them — the game loop, the entity system, the input handler — and the code specific to your game.

Key Takeaways

• A game engine needs a game loop that runs 60+ times per second, checking input, updating object positions, detecting collisions, and drawing the screen in that order.
• You will use graphics libraries (SDL2, SFML, or a graphics API like OpenGL) rather than writing graphics code from scratch.
• Physics libraries like Bullet or Rapier handle collisions and movement; you do not need to write collision detection yourself unless you have a specific reason.
• An entity system — a way to organize and update all the objects in your game — is the first real architecture decision you make.
• Start with a 2D engine if you are learning; 3D adds rendering complexity that obscures the core concepts.

Choose a Programming Language and Graphics Library

Your language choice determines what libraries are available and how fast your code runs. C++ is the standard for performance-critical engines and has the most mature libraries. C# with MonoGame or FNA gives you faster development with slightly lower performance. Python with Pygame is slowest but easiest to learn. Rust with Bevy is newer but growing fast and enforces safer code patterns.

Your graphics library is what actually puts pixels on the screen. SDL2 and SFML are the most beginner-friendly; they handle windowing, input, and 2D drawing. OpenGL and Vulkan are lower-level graphics APIs that give you more control but require more code. For 3D, you will likely use OpenGL or a higher-level wrapper. Do not write your own graphics code — the graphics card has its own language and the driver is thousands of lines of optimized code.

If you are learning, start with C++ and SDL2, or C# and MonoGame. Both have clear documentation and examples. If you want to move faster, Python and Pygame will get you a working engine in less time, though it will be slower.

Build the Game Loop

The game loop is the heartbeat of your engine. It runs in this order: check for input, update all game objects, detect collisions, draw everything, then repeat. This happens 60 times per second (or 120, or 144, depending on your target). If any step takes too long, the frame rate drops and the game feels sluggish.

A basic loop in pseudocode looks like this: while the game is running, get input from the keyboard and mouse, move all objects based on their velocity and the time since the last frame, check which objects are touching each other, draw all objects to the screen, then measure how long that took and sleep if necessary to hit your target frame rate.

The time measurement is critical. If you move an object by a fixed amount every frame, it will move at different speeds on different computers. Instead, measure how much time passed since the last frame (called delta time) and multiply movement by that. An object moving at 100 pixels per second will move 1.67 pixels in a 16-millisecond frame, regardless of the computer.

Create an Entity System to Organize Your Game Objects

An entity system is a way to store and update all the things in your game — the player, enemies, projectiles, walls, anything with position and behavior. The simplest approach is a list of objects, each with an update function that runs every frame. A more scalable approach is the entity-component-system (ECS) pattern, where each object is an entity, each entity has components (position, velocity, sprite, health), and systems run code on all entities with certain components.

Start straightforward: a list of objects, each with an update function. When you need to add features — like running physics only on objects with a physics component, or rendering only visible objects — you can refactor into ECS. Most game engines use some version of this pattern because it scales from a small game to a large one without rewriting everything.

Your entity system needs to handle creation and destruction. When the player shoots, you create a projectile entity. When it hits something, you destroy it. If you destroy an entity while iterating through the list, you will crash. Use a queue: mark entities for deletion, then remove them after the update loop finishes.

Add Physics and Collision Detection

Physics means movement with acceleration and gravity. Collision detection means checking which objects are touching. You can use a library like Bullet (C++) or Rapier (Rust) that handles both, or you can write straightforward collision detection yourself and use a physics library just for gravity and velocity.

For a 2D game, axis-aligned bounding box (AABB) collision is straightforward: each object has a rectangle, and you check if two rectangles overlap. For circles, you check if the distance between centers is less than the sum of the radii. These are fast enough for most 2D games. For 3D or complex shapes, use a physics library.

Collision response — what happens when two objects collide — depends on your game. In a platformer, the player stops when hitting a wall. In a shooter, a bullet disappears. In a physics puzzle, objects bounce. Your collision system needs to detect the collision and tell your game code what to do about it.

Handle Input and Connect It to Game Logic

Input means keyboard, mouse, and gamepad. Your graphics library handles reading input; your engine code decides what to do with it. Check if the player pressed jump, and if so, add upward velocity to the player entity. Check if they pressed shoot, and create a projectile entity at the player's position.

Separate input handling from game logic. Do not write "if key W is pressed, move player forward" inside the player update function. Instead, read input once per frame, store which keys are pressed, then have the player update function check that state. This makes it easier to add rebinding, controller support, and replay systems later.

For gamepads, use a library like SDL_GameControllerDB or your graphics library's built-in gamepad support. Map buttons to actions (jump, shoot, move left) rather than reading raw button numbers. This makes it easier to support different controller layouts.

Optimize for Performance on Your Target Hardware

Performance optimization means measuring where your code is slow, then making it faster. Use a profiler — a tool that measures how much time each function takes. Most IDEs have one built in. Profile on the hardware you are targeting: a laptop, a console, a phone. A game that runs at 60 frames per second on your desktop might run at 20 on a phone.

Common bottlenecks: drawing too many objects (reduce by culling objects off-screen), checking collisions between every pair of objects (use spatial partitioning like quadtrees), and allocating memory every frame (reuse objects instead of creating new ones). Do not optimize before you have a working engine — premature optimization makes code harder to understand and usually does not help much.

If your game is 3D, graphics rendering is usually the bottleneck. Use a graphics profiler to see how much time the GPU spends drawing. Reduce the number of objects drawn, use lower-resolution textures, or use simpler shaders. If your game is 2D, physics and collision detection are usually the bottleneck.

Frequently Asked Questions

Should I use an existing engine instead of building my own?

Yes, unless you are learning how engines work or need something very specialized. Unity, Unreal, and Godot are free, have thousands of tutorials, and handle graphics, physics, and input for you. Building an engine teaches you a lot but takes months. Use an existing engine to make games faster.

What is the difference between 2D and 3D engines?

A 2D engine draws flat sprites on a 2D screen. A 3D engine renders 3D models in 3D space, which requires more complex graphics code and more GPU power. Start with 2D; the core concepts are the same, and you will finish faster. You can always add 3D later.

Do I need to write my own graphics code?

No. Use a graphics library like SDL2 or SFML for 2D, or OpenGL for 3D. These handle talking to the graphics card. Writing graphics code from scratch requires learning the graphics API, the shader language, and how the GPU works — that is a separate skill from engine building.

How do I make my engine run on multiple platforms?

Use libraries that work on Windows, Mac, and Linux: SDL2, SFML, OpenGL. Write your engine code in a way that does not depend on the operating system. Keep platform-specific code (file paths, window creation) in one place. Test on each platform as you go, not at the end.

What should I build first — graphics, physics, or input?

Start with the game loop and input. Get something on screen that responds to keyboard presses. Then add physics and collision. Graphics can be straightforward rectangles at first. This order lets you test each system as you add it, rather than building everything and debugging a broken mess.