Karl2D: a 2D library for Odin that you copy into your game folder

A library you copy, not a dependency you resolve

Karl2D is described as an Odin 2D game library that is beginner friendly and easy to modify, and its central design choice is stated in the repository description: it strives to minimize the number of dependencies, making you feel in control of the technology stack.

That choice shows up in the install instructions, which are unusual for a game library. There is no package manager step and no version constraint. You install the Odin compiler, create a folder for your game, put the contents of the repository into a subfolder called karl2d inside that folder, and then create your own file next to it.

The reason to care about that is the one that matters in practice: every line of the library is on your disk and editable. A game developer who needs one function to behave differently does not have to fork a package, submit an upstream patch and wait for a release. They change the file. The cost of that freedom is that you own the maintenance of whatever you change, and a project that grows will drift from upstream unless someone keeps merging changes back.

The library is written in Odin, the licence is Zlib, and the API overview is a document file in the repository, karl2d.doc.odin, rather than a website. The homepage is karl2d.com, and the author links a Discord channel for support.

The scope is 2D. Karl2D does not attempt to be an engine in the general sense, it is a layer over the platform: a window, a draw loop, input, audio, textures, fonts and shaders. The examples folder in the tree confirms that reading, with folders for audio, camera, cursors, depth testing, a dual grid tilemap editor, events, fonts, gamepad input, mouse and mouse locking, multitexture, a palette, positional audio, premultiplied alpha, a raylib port, render textures, a minimal hello world in both desktop and web forms, and a folder of scraps.

The update loop is three procedures and a boolean

The core of the library is small enough to show completely. A first program looks like this:

package hello_world

import k2 "karl2d"

main :: proc() {
    k2.init(1280, 720, "Greetings from Karl2D!")

    for k2.update() {
        k2.clear(k2.LIGHT_BLUE)
        k2.draw_text("Hellope!", {50, 50}, 100, k2.DARK_BLUE)
        k2.present()
    }

    k2.shutdown()
}

The shape is worth reading closely. k2.init takes a width, a height and a window title, and it has to come first: the documentation notes that you must call it before using any Karl2D procedure that queries or modifies runtime state, which it lists as window size, input, drawing, audio, textures, fonts and shaders. Pure helper procedures, types and constants can be used before it, which is a small but useful distinction for code that runs during setup.

The loop is a for statement over k2.update(), which returns a boolean, so the loop continues while the game should keep running and exits when it should not. Inside, k2.clear takes a colour, k2.draw_text takes a string, a position, a size and a colour, and k2.present hands the frame to the display. The three calls inside the loop are the whole contract: clear the frame, draw into it, present it. k2.shutdown at the end releases the platform resources.

The library ships its own constants, since LIGHT_BLUE and DARK_BLUE come from the k2 namespace rather than from a separate colour type, and the repository has a palette example that shows what the set contains.

Web builds without Emscripten, and why the loop has to change

The repository description makes a point of the web target: you can create desktop games and web games, with no Emscripten needed. That is possible because the browser build is not an emscripten translation of the whole program, it is a WebAssembly build driven by a different entry shape.

The reason the program has to be restructured is a genuine constraint of the browser environment, and the documentation states it plainly. A web build cannot have a loop like for k2.update(), because the browser decides when to redraw. Instead, the web build calls step when the browser wants new content. So the desktop main procedure stays for desktop builds, and the web build uses the init and step procedures directly. The restructured program moves the library calls into three separate procedures:

package hello_world

import k2 "karl2d"

main :: proc() {
    init()
    for step() {}
    shutdown()
}

init :: proc() {
    k2.init(1280, 720, "Greetings from Karl2D!")
}

step :: proc() -> bool {
    if !k2.update() {
        return false
    }

    k2.clear(k2.LIGHT_BLUE)
    k2.draw_text("Hellope!", {50, 50}, 100, k2.DARK_BLUE)
    k2.present()

    return true
}

shutdown :: proc() {
    k2.shutdown()
}

Note that step returns a bool, so the same procedure can drive both the desktop loop and the browser callback. The desktop main becomes a three-line wrapper.

The build itself is a tool bundled with the library, run from the game folder:

odin run karl2d/build_web -- .

The double dash and the trailing period are both called out in the documentation, and the period means the current folder where the game lives. The output lands in bin/web, and you run the game by opening index.html in that folder.

The build_web tool copies odin.js from the Odin distribution's core/sys/wasm/js directory into bin/web, copies an HTML index file in, creates a build/web folder containing the package it actually builds with a small amount of wrapper code that calls the init and step functions, and produces a main.wasm file that ends up in bin/web as well. The flag -debug can be added after the double dash for better in-browser debug symbols, since the flags after the separator are what gets passed on to the build_web program, and there are -o:speed and -o:size flags for optimization.

Two warnings are worth repeating. If the page appears blank, the browser console may be reporting a cross site policy error, and the fix is to run a local server with python -m http.server in the bin/web folder and open https://localhost:8000. And on Linux and Mac you may need an lld package containing the wasm-ld linker, which is included with Odin on Windows.

Assets have to be baked in for the web build

There is one API difference that will bite anyone porting a desktop game to the web, and the documentation calls it out directly. Procedures such as k2.load_texture_from_file("image.png") do not work on the web, because they depend on there being a file system. The web equivalent is k2.load_texture_from_bytes with the #load call, and #load bakes the image into the executable at compile time.

So a desktop build loads a texture at run time from disk, and a web build has the texture compiled into the binary. The same applies to any other file-based asset.

The documentation suggests the pattern for handling both: if you want to load with k2.load_texture_from_file on desktop and with k2.load_texture_from_bytes plus #load on the web, you create an abstraction. The file listing shows a build tag convention for this, with a desktop variant carrying a #+build !js line at the top, and the platform-specific files are named to match, including file_system_default.odin and file_system_web.odin.

That is the shape of the whole library, and it is a consistent design rather than an accident. Look at the top-level file list and the same pattern appears everywhere: a platform_interface file next to platform_linux, platform_mac, platform_windows and platform_web implementations, a render_backend_interface with render_backend_gl, render_backend_d3d11, render_backend_webgl and render_backend_nil, and an audio_backend_interface with audio_backend_alsa, audio_backend_core_audio, audio_backend_waveout, audio_backend_web_audio and audio_backend_nil. There are chooser files for both the render and the audio backend, so the library picks a backend at run time and has a nil implementation to fall back on.

On Linux the tree lists both platform_linux_window_wayland.odin and platform_linux_window_x11.odin with matching glue files, so the windowing layer is chosen per session rather than being fixed at build time, and there is a separate wayland file for client side decorations. The macOS glue is platform_mac_glue_gl.odin, and Windows has platform_windows_glue_gl.odin alongside platform_windows.odin, with the Windows path also carrying d3d11 support in the render backend.

Where Karl2D is the wrong choice

The first limitation is that Karl2D is a 2D library and nothing more. There is no 3D support in the file listing, no scene graph, no animation system, no entity component framework, and no asset pipeline that turns an art directory into a pack. The examples make that clear: a camera example, a tilemap editor built on the dual grid technique, a render texture example for drawing into a texture and drawing that texture repeatedly, and a Box2D port. The physics example lives at a separate site rather than in the examples folder, which tells you Box2D is not part of the library.

The second is the copy-in installation model. It works, and it is genuinely good for learning, but it has consequences at scale. There is no version pin, so two projects can end up with two different versions of the library and there is no mechanism to say which is which. There is no way to publish a fork to a registry. And any project that wants a package manager workflow, with a lockfile and reproducible dependency resolution, has to wrap the library in its own build tooling.

The third is the web build's asset model. Baking assets into the executable at compile time is simple and works, but it means the web build is a separate artefact with its own compile step and its own flags, and a game with a lot of assets will have a large binary. The build also needs a wasm-ld linker on Linux and Mac, and the blank-page troubleshooting path goes through a local web server, which is a rough edge for something that is meant to be the easy route to a web game.

The fourth is the platform surface itself. The tree shows a Linux, macOS, Windows and web story, and the Linux dependency question is left to the reader, with the documentation suggesting sudo apt install libudev-dev on Ubuntu and Debian and asking what other distributions need. A library that is beginner friendly in the common case and hands you a package manager prompt in the uncommon case is a reasonable trade, but it is a trade.

The fifth is maturity. The published releases are jam special releases and beta builds, with the Karl2D Jam special release and its hotfix both dated 2026-05-01 and beta-3-hotfix-1 from 2026-04-10 fixing macOS audio issues. A 0.x library with a small maintainer base is a fair thing to build a weekend game with and a risky thing to build a product on.

Editorial conclusion

Karl2D fits a developer who wants to ship a small 2D game in Odin without assembling a package graph, since the entire library is a folder you copy in and the API is a single import, with desktop and web builds from the same source and no Emscripten in the picture. It is the wrong choice for a 3D project, for a team that wants a managed dependency, and for anyone who expects a scene graph, an animation system or a physics engine, since the examples show a Box2D port living outside the library. Before committing, copy the karl2d folder into a scratch project, run odin run . and confirm a window opens, read karl2d.doc.odin for the API surface, and check whether the platforms you target are covered, since the last push was on 2026-10-19 and the newest published release is the Karl2D Jam special release from 2026-05-01.

Frequently asked questions

Official sources

1. karl-zylinski/karl2d on GitHub
2. License: Zlib
3. Project website
4. README
5. Releases