FLTK Lightweight GUI Toolkit

FLTK (Fast, Light Toolkit) is a cross-platform C++ graphical user interface toolkit designed for creating lightweight, fast, and portable applications. Originally developed by Bill Spitzak in 1998 and derived from the FORMS library, FLTK has been continuously developed by a community of contributors. Key features: extremely small footprint with the entire library (including all widgets) typically under 1MB when statically linked, making it ideal for embedded systems and applications where binary size matters. Self-drawn widgets using FLTK built-in drawing routines rather than native OS widgets, ensuring consistent appearance across platforms and avoiding platform-specific widget behavior. Over 80 widget classes including buttons, inputs, outputs, browsers (lists), valuator widgets (sliders, dials, counters), menus, windows, groups, tabs, and specialized widgets like spreadsheets and help views. FLUID (Fast Light User Interface Designer) visual GUI builder for designing interfaces interactively and generating C++ source code. Built-in OpenGL support via Fl_Gl_Window for embedding 3D graphics in applications. Image support for BMP, JPEG, PNG, PNM, GIF, and SVG formats. Drawing API with lines, rectangles, circles, polygons, text, and images. Internationalization support with UTF-8 and custom font loading. High-DPI awareness for retina and 4K displays. Threading model with Fl::lock(), Fl::unlock(), and Fl::awake() for safe GUI updates from worker threads. Cross-platform on Windows (via GDI), macOS (via Quartz), and Linux/X11 (via Xlib). Extremely fast performance with minimal overhead per widget. Small memory footprint per widget. Used in scientific visualization, audio applications, embedded HMIs, and specialized tools. Open source under LGPL with static linking exception.