What you're actually building when you make a game engine
A game engine is software that handles the repetitive work every game needs: drawing graphics to the screen, playing sounds, detecting when objects collide, responding to keyboard and mouse input, and running the game loop that updates everything 60 times per second. Instead of writing all of that from scratch for each game, you build it once and reuse it.
Most people starting out should not build an engine. If you want to make a game, use an existing engine like Unity, Unreal, or Godot. But if you want to understand how games work at a fundamental level, or you need something the existing engines do not do, building your own teaches you more than using someone else's ever will.
The smallest working engine is about 500 lines of code. A production engine used by studios is millions of lines. This guide covers what goes into a basic one you can actually finish.
Key Takeaways
- A game engine needs a window to draw in, a way to handle input, a graphics renderer, a physics system, and a game loop that runs 60 times per second.
- Start with a single language and graphics library — C++ with OpenGL or SDL2 is a common beginner path, or Python with Pygame if you want faster iteration.
- Build the core loop first (window, input, drawing a rectangle), then add systems one at a time rather than trying to build everything at once.
- You will spend more time on the renderer than anything else, because drawing things correctly is harder than it looks.
- Do not add features like networking, advanced physics, or a visual editor until the basic engine runs a simple game without crashing.
The core loop: the heartbeat of every engine
Every game engine runs the same basic loop, thousands of times per second. It looks like this: clear the screen, read input, update game state, draw everything, repeat. This loop is what makes the game feel alive instead of frozen.
In pseudocode, it looks like this:
while game is running: handle input from keyboard and mouse update positions and states check for collisions draw everything to screen wait until 1/60th of a second has passed
The wait at the end is crucial. Without it, your loop runs as fast as the computer can manage, which is different on every machine. By locking the frame rate to 60 frames per second, every player sees the same speed. Modern engines often target 144 or 240 frames per second on high-end hardware, but 60 is a safe starting point.
Getting this loop right is harder than it sounds. If you update the game state before reading input, the player's button press arrives one frame late. If you draw before updating, you see the old positions. The order matters, and the order is always: input, update, draw.
Graphics: getting pixels on the screen
The renderer is the part that actually draws things. You have two main choices: use a graphics library that handles the hard part for you, or write directly to the graphics card using OpenGL or Vulkan.
For a first engine, use a library. SDL2 is a C library that gives you a window and lets you draw rectangles and images. SFML is similar but slightly higher-level. Pygame is the Python equivalent. All three let you draw a colored rectangle in about 20 lines of code.
If you want to learn how graphics actually work, use OpenGL. It is lower-level — you write code that talks directly to the GPU — but it teaches you how modern games really render. The learning curve is steep. You will spend a week just getting a triangle on the screen. But once you understand shaders and vertex buffers, you understand something real.
Start by drawing static images and rectangles. Once that works, add sprite animation (cycling through different images to make something look like it is moving). Then add a camera system so you can zoom and pan around the world. Only after that add lighting or particle effects.
Input handling: reading the keyboard and mouse
Input is simpler than graphics but easy to get wrong. You need to know which keys are pressed right now, which keys were just pressed this frame, and which keys were just released.
Most graphics libraries give you an event system. Every frame, you ask "what happened?" and the library tells you: the user pressed W, the user moved the mouse to position 500,300, the user closed the window. You store which keys are currently held down in a data structure (usually an array or a dictionary), and update it as events arrive.
The mistake beginners make is checking "is the spacebar pressed?" once per frame and assuming it means the player pressed it this frame. But the player might have held it down for three frames. You need to track "was it pressed last frame?" and "is it pressed this frame?" so you can detect the transition from unpressed to pressed.
Physics and collision: making objects interact
Physics is optional for simple games. A game about moving a square around does not need physics. A game where objects fall, bounce, and push each other does.
Start with the simplest possible collision detection: axis-aligned bounding boxes. Every object is a rectangle. Two rectangles collide if they overlap. This is fast and works for most 2D games. You can add circles and polygons later.
For physics, you have two paths. Write it yourself: store velocity and acceleration for each object, update position each frame, and check for collisions. This takes a few hundred lines of code and teaches you how physics works. Or use a library like Box2D (2D) or Bullet (3D), which handles gravity, friction, and realistic collisions. Using a library is faster but you learn less.
Most beginner engines skip physics entirely and just move objects based on input and simple rules. A player presses right, the player moves right. An enemy follows a path. A bullet travels in a straight line until it hits something. This is enough for many games.
Sound: audio playback and management
Sound is the easiest system to add and the easiest to skip. Many first engines have no sound at all.
If you want sound, use a library. SDL_mixer works with SDL2. SFML has built-in audio. OpenAL is lower-level but more powerful. All of them let you load a WAV or MP3 file and play it with one function call.
The hard part is not playing sound — it is managing it. If the player shoots 10 bullets in one frame, do you play the gun sound 10 times? (Usually no.) Do you limit how many of the same sound can play at once? (Usually yes.) Do you fade out music when the player enters a building? (Depends on the game.) These are design questions, not technical ones, but they take time to get right.
Organizing your code: structure before you scale
A 500-line engine can be one file. A 5,000-line engine needs structure. Separate your code into modules: a graphics module, an input module, a physics module, a sound module. Each module has a public interface (the functions other parts of the engine call) and private implementation (the details nobody else needs to know).
Use a scene or world system to organize game objects. Instead of a global list of everything, each scene contains the objects in that scene. When the player moves to a new level, you unload the old scene and load the new one. This keeps memory usage reasonable and makes it easy to restart or reload.
Store configuration in files, not in code. Frame rate, window size, asset paths, physics constants — put these in a config file so you can change them without recompiling. This saves hours of time during development.
Building your first engine: a realistic path
Do not try to build everything at once. Follow this order:
- Create a window and clear it to a color each frame.
- Draw a rectangle and make it move when you press arrow keys.
- Draw a second rectangle and detect when they collide.
- Load and draw an image instead of a rectangle.
- Add a second scene and switch between them.
- Add sound effects when objects collide.
- Add a simple enemy that moves on its own.
- Build a complete small game (a player, enemies, a goal, a win condition).
Each step should take a few hours, not days. If you are stuck on one step for more than a day, you chose the wrong step. Go back and do something simpler first.
The first complete game does not need to be good. It needs to run without crashing and prove that all the systems work together. A game where you move a square to touch other squares is enough. Once you have that, you understand what you are doing, and adding features becomes much faster.
Frequently Asked Questions
Should I use C++, Python, or something else?
C++ is the industry standard and teaches you how computers actually work, but it is hard to learn. Python is faster to write and easier to debug, but slower to run. For your first engine, choose based on what you already know. If you know Python, start there. If you want to learn C++, start there. The concepts are the same in both.
Do I need to understand graphics cards and shaders?
Not for a basic 2D engine. You can draw rectangles and images without knowing how the GPU works. If you build a 3D engine or want to do advanced effects, you will need to learn shaders. Start without them and add them when you hit a wall.
How long does it take to build a working engine?
A basic 2D engine that can run a simple game takes 2 to 4 weeks of part-time work. A 3D engine takes 2 to 3 months. A production engine that studios use takes years. The time depends on how many features you add and how much you optimize.
What if I get stuck and do not know how to fix a bug?
Print out the values of variables to see what is actually happening. If the player is not moving, print the input value and the position each frame. If objects are not colliding, print their positions and sizes. Most bugs are obvious once you see the actual numbers. Use a debugger if your language has one, but print statements work too.
Is it worth building an engine if I just want to make a game?
No. Use Unity, Unreal, or Godot. They are free, they work, and they save you months of work. Build an engine only if you want to learn how games work or if you need something existing engines do not do.