Magnum.engine is described as 'Lightweight and modular C++11/C++14 graphics middleware for games and data visualization' and is an app in the Development category. There are five alternatives to Magnum.engine for a variety of platforms, including Mac, Windows, Linux, Online / Web-based and Android. The best alternative is Qt, which is both free and Open Source. Other great apps like Magnum.engine are GTK+ (Free, Open Source), Uno Platform (Free, Open Source), JUCE (Free Personal) and CrossBrowdy (Free, Open Source).
Qt is a cross-platform application framework that is widely used for developing application software that can be run on various software and hardware platforms with little or no change in the underlying codebase, while having the power and speed of native applications.
GTK+ is a multi-platform toolkit for creating graphical user interfaces. Offering a complete set of widgets, GTK+ is suitable for projects ranging from small one-off tools to complete application suites.
Uno is the platform for building native mobile, desktop and WebAssembly apps with C#, XAML from single codebase. It's a Universal Windows Platform Bridge that allows UWP-based code (C# and XAML) to run on iOS, Android, and WebAssembly.