Dear ImGui from C++

Already have a Dear ImGui app? Keep it. BUI runs the real Dear ImGui (1.93.1 WIP), so your code compiles unchanged. You swap the platform and renderer backend for one header, and your windows, sliders and tables come out in Blender’s colours and font, drawn by BUI on the CPU or the GPU.

A backend, in ImGui terms, is the glue that feeds ImGui input and draws its output. BUI’s is imgui_impl_bui.h. It replaces the usual pair (say, imgui_impl_win32 plus imgui_impl_dx11) on every platform BUI supports.

Hello, ImGui on BUI

#include "imgui.h"
#include "imgui_impl_bui.h"

static void frame(void*) { ImGui::ShowDemoWindow(); }

int main() {
  ImGui::CreateContext();
  ImGui_ImplBUI_App* app = ImGui_ImplBUI_CreateApp("Dear ImGui on BUI", 1280, 800, frame, nullptr);
  ImGui_ImplBUI_RunApp(app);       // BUI's message loop until the window closes
  ImGui_ImplBUI_DestroyApp(app);
  ImGui::DestroyContext();
}

That’s the whole program. Notice what’s missing: no NewFrame(), no Render(), no swap chain. You hand BUI a frame function, and the backend calls ImGui::NewFrame(), your function and ImGui::Render() itself, whenever a frame is due.

• ImGui_ImplBUI_CreateApp(title, width, height, frame, user) opens a native window whose whole client area is ImGui. It needs a current ImGui context. An optional last argument takes a config (below).
• ImGui_ImplBUI_RunApp(app) runs BUI’s message loop until the window closes.
• ImGui_ImplBUI_DestroyApp(app) shuts the backend down.
• user is passed to frame unchanged: point it at your app’s state.

Build it with CMake

Add BUI to your project and link bui::imgui. It contains Dear ImGui’s core, its demo, the BUI backend and the Blender style.

cmake_minimum_required(VERSION 3.20)
project(myapp C CXX)

add_subdirectory(BUI)
add_executable(myapp main.cpp)
target_link_libraries(myapp PRIVATE bui::imgui)

The BUI_BUILD_IMGUI option is on whenever a C++ compiler is present. To try BUI’s own demo first, build the bui_imgui_demo target from a clone of BUI:

cmake -S . -B build
cmake --build build --target bui_imgui_demo

Run it as it is for a window, or with --screenshot out.png to render a few frames headless and save a PNG. Add --gpu to render that screenshot on the GPU.

On the GPU

Set BUI_GPU=1 before you start the app, and BUI draws ImGui with Direct3D 11 (Windows) or OpenGL 3.3 (Linux) instead of the software raster. Your code doesn’t change.

Configure the backend

CreateApp and Init take an optional ImGui_ImplBUI_Config:

ImGui_ImplBUI_Config cfg;
cfg.BlenderStyle = false;   // keep your own ImGuiStyle (it's still scaled)
cfg.FontSize = 15.0f;       // pixels at scale 1
ImGui_ImplBUI_App* app = ImGui_ImplBUI_CreateApp("Tool", 1280, 800, frame, &state, &cfg);

The Blender style is also a function: ImGui_ImplBUI_StyleBlender(&style, scale) applies it to any ImGuiStyle. ImGui_ImplBUI_AddBlenderFont(size) adds Inter on its own, and ImGui_ImplBUI_BlenderFontSize(scale) tells you the size that matches Blender’s text.

Idle costs nothing

Most ImGui backends render 60 frames a second forever. BUI’s doesn’t. Input asks for a frame; after each frame the backend asks for another only while something can still change: a few settle frames after input, frames while the output keeps changing (animations, a held repeat button), timer wakes for the text cursor blink and tooltip delays. Then nothing runs, and an idle window uses no CPU. A frame whose draw data didn’t change doesn’t even redraw.

If your app changes something the UI shows (a download finishes, a file reloads), ask for a frame yourself:

ImGui_ImplBUI_RequestFrames();       // one frame now (or n back to back)
ImGui_ImplBUI_RequestFrameIn(500);   // one frame in about 500 ms

ImGui inside a BUI interface

ImGui doesn’t have to own the window. It can live in one custom node of a larger BUI interface, next to the kit’s native widgets:

BKNode* node = BK_node_new("custom");
BK_node_add(column, node, -1);
BK_set_num(node, "fill", 1);
ImGui::SetCurrentContext(my_ctx);
ImGui_ImplBUI_Init(node, frame, user);   // the backend runs NewFrame/frame/Render itself

Several nodes can each host their own ImGui context. Because the backend runs inside BK_render, it works in every BUI host: the native windows (Win32, X11, Cocoa), the Tk and Qt hosts, and headless rendering into a raster. The C kit page explains nodes and contexts.

What the backend supports

• Input: mouse (three buttons, held together), precise vertical and horizontal wheel, keyboard, typed UTF-8 text, modifiers and focus.
• Platform: mouse cursor shapes, the host’s clipboard, DPI scale, time.
• Rendering: every BUI renderer (software, Direct3D 11, OpenGL 3.3), per-command clip rectangles, large meshes (VtxOffset), dynamic textures. ImTextureID is a BUI texture id from BUI_texture_create, so ImGui::Image works with your own pixels.

Not supported yet: IME candidate window placement, gamepads and multi-viewports Coming soon. The full list of small gaps is on the coverage page.

Next steps