Programming language: Crystal
License: MIT License
Tags: GUI Development    

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Crystal bindings to Dear ImGui, through cimgui


This library lets you use Dear ImGui with the Crystal programming language.

Dear ImGui is an immediate-mode library for programming and rendering graphical user interfaces. But note that just by itself it won't directly let you do those things, it needs to be wired up to code that actually handles rendering and input. You can write these yourself or use a library ("backend"):

  • crystal-imgui-sfml - backend using SFML (assumed through CrSFML).
    Please refer to its instructions primarily, as it takes over large parts of the installation.


API documentation is currently missing any actual descriptions, but it cross-links to the well-commented imgui.h header file.

For guidance specific to Crystal and crystal-imgui, see Usage.

Examples of most possible usages of the library are covered in the extensive [src/demo.cr](src/demo.cr) (entry point [examples/demo_sfml.cr](examples/demo_sfml.cr); note that it requires crystal-imgui-sfml).

There's also a small self-contained example from crystal-imgui-sfml.


Following the installation process of crystal-imgui-sfml is recommended instead (it even provides libcimgui.so automatically), but continue here if you want an alternate route.

Repeat: do not follow these instructions if you're using crystal-imgui-sfml (which in the current state of things you probably are).

crystal-imgui actually binds to the cimgui library. The supported version is bundled as a submodule (for other versions you can try your luck by re-generating sources). This repository needs to have been cloned with submodules:

git clone --recursive https://github.com/oprypin/crystal-imgui

Then you can build cimgui:

cd cimgui
cmake --build .
ln -s cimgui.so libcimgui.so  # or .dylib on macOS

Yes, the flag is required (just makes the library play much nicely with Unicode and Crystal), and the symlink is also required, because the library ends up as cimgui.so but somehow is referred by both that name and libcimgui.so.

Building a project

Try the example from inside the folder of crystal-imgui:

crystal run examples/test.cr

Prior to that you'll need to add cimgui to the library search path:

export LD_LIBRARY_PATH="$(pwd)/cimgui"
export LIBRARY_PATH="$(pwd)/cimgui"

For your own project, crystal-imgui will be in a subdirectory, so adjust this accordingly.


Using crystal-imgui is agnostic of the backend for the most part, you invoke those just at the beginning of a frame, to process inputs, and at the end, to do the actual rendering (generally imgui just builds up the things to be drawn until the end).

The API surface follows Dear ImGui itself very closely. Main differences are:

  • Functions are lowercased: ImGui::InputFloat3() becomes ImGui.input_float3.
  • Types are all in the module as well: ImGuiIO becomes ImGui::ImGuiIO.
    • But you're welcome to "unpack" those as an opt-in, by running include ImGui::TopLevel.
  • Enums are namespaced.
  • Important: To faithfully preserve the APIs of ImGui where its functions accept pointers that are used both for input and output (and for lack of an obvious alternative), crystal-imgui also requires pointers to be passed to it in the same way.
    • However, since Crystal requires very prohibitive circumstances to be able to take an actual pointer, crystal-imgui actually wraps these functions into macros, for which pointerof(foo.bar) is just a fake syntax that will be transformed into a call to foo.bar for reading and a call to foo.bar=(value) for writing. As another example, pointerof(arr[0]) also works - rewritten as arr[0] = value.
    • If the input kind expects N values, then a Slice of N values should be passed directly instead.
    • For editable strings, one should pass an ImGui::TextBuffer instead.
  • String buffers (particularly begin+end pairs) are abstracted as Slice or ImGui::TextBuffer.
    • For the latter, crystal-imgui even takes the liberty to implement the buffer resize callback for unbounded text inputs.
  • ImVector is discouraged. Usually you can just work with Array or Slice.
  • ImColor is even more discouraged, in favor of ImVec4. See some color helper functions in src/imgui.cr.


Note that most source files of crystal-imgui are auto-generated from outputs of cimgui.

If you're editing files, make sure they wouldn't be overwritten by generate.cr - in that case that is the file that you should be editing instead. In general, to generate the source code, run this:

crystal generate.cr  # assumes cimgui/ is populated, writes to src/
crystal tool format src


crystal-imgui was made by Oleh Prypin. It uses and is based on Dear ImGui and cimgui.

crystal-imgui is [licensed](LICENSE.md) under the terms and conditions of the MIT license.

*Note that all licence references and agreements mentioned in the crystal-imgui README section above are relevant to that project's source code only.