osGUI

A tiny immediate-mode GUI library written in C++, structured exactly like Dear ImGui: a portable core that emits draw lists of vertices/indices, a platform backend that feeds input, and a renderer backend that draws the geometry.

This is not Dear ImGui — it's a small homage that follows the same architecture and visual style (the "Dark" theme colors, the famous (0.45, 0.55, 0.60) clear color, ProggyClean-ish monospace text).

File layout (mirrors Dear ImGui)

osGUI	Dear ImGui equivalent
osgui.h / osgui.cpp	imgui.h / imgui.cpp (core)
osgui_demo.cpp	imgui_demo.cpp (the demo window)
osgui_impl_win32.cpp	imgui_impl_win32.cpp (platform backend)
osgui_impl_opengl2.cpp	imgui_impl_opengl2.cpp (renderer backend)
main.cpp	examples/example_win32_opengl3/main.cpp

Build (Windows / MSVC)

Everything links against libraries that ship with Windows (user32, gdi32, opengl32) — no GLFW / GLEW / GLAD / external downloads.

build.bat

or manually from a Developer Command Prompt:

cl /EHsc /O2 /MD main.cpp osgui.cpp osgui_demo.cpp ^
   osgui_impl_win32.cpp osgui_impl_opengl2.cpp ^
   /Fe:osgui_demo.exe ^
   /link user32.lib gdi32.lib opengl32.lib /SUBSYSTEM:WINDOWS

MinGW / g++

g++ main.cpp osgui.cpp osgui_demo.cpp osgui_impl_win32.cpp osgui_impl_opengl2.cpp \
    -o osgui_demo.exe -lopengl32 -lgdi32 -luser32 -mwindows

Usage (the immediate-mode loop)

og::CreateContext();
OG_ImplWin32_Init(hwnd);
OG_ImplOpenGL2_Init();

while (running) {
    OG_ImplOpenGL2_NewFrame();
    OG_ImplWin32_NewFrame();
    og::NewFrame();

    og::Begin("Hello, world!");
    if (og::Button("Click me")) counter++;
    og::SameLine();
    og::Text("counter = %d", counter);
    og::SliderFloat("float", &f, 0.0f, 1.0f);
    og::End();

    og::Render();
    glClear(GL_COLOR_BUFFER_BIT);
    OG_ImplOpenGL2_RenderDrawData(og::GetDrawData());
    SwapBuffers(hdc);
}

What's implemented

• Core: context, draw list (vertex/index buffers + clip-rect command list), font atlas, ID stack with ## label hashing, hover/active interaction model.
• Windows: draggable title bar, collapse arrow, close button, resize grip, mouse-wheel scrolling + scrollbar, focus / z-ordering, SetNextWindowPos/Size.
• Widgets: Text / TextDisabled / TextColored / BulletText, Button, SmallButton, Checkbox, RadioButton, SliderFloat, SliderInt, CollapsingHeader, TreeNode/TreePop, Separator, Spacing, Indent/Unindent, SameLine, ProgressBar, PlotLines, PlotHistogram.
• Platform backend (Win32): mouse/wheel/keyboard/char input, timing, display size, GDI font-atlas baking.
• Renderer backend (OpenGL 1.x): ortho projection, per-command scissor, textured triangle draw via client arrays.

Scope / honesty

Dear ImGui is ~60k lines with 15+ backends (GLFW, SDL2/3, DX9–12, Vulkan, Metal, WebGPU, OSX, Android…). This is a focused subset that demonstrates the same architecture with one platform backend and one renderer backend — the canonical Win32 + OpenGL pair.

Adding another backend follows the exact same interface:

• A new renderer (e.g. osgui_impl_dx11.cpp) only needs to implement CreateFontsTexture() (upload og::GetFontAtlas().pixels) and RenderDrawData() (walk og::DrawData -> per DrawCmd: set scissor from clip_rect, bind tex_id, draw elem_count indices from idx_offset).
• A new platform (e.g. osgui_impl_sdl2.cpp) only needs to fill og::IO (mouse, wheel, keys, chars, display_size, delta_time) each frame.

Not implemented vs. real ImGui: docking/viewports, tables, full text editing (InputText caret/selection), combo/menu popups, drag-drop, multi-viewport, TrueType atlas baking via stb_truetype (osGUI bakes via GDI instead).