Apps similar to idTech4A++ include GZDoom, which is free and open source. Other options are LZDoom and Chocolate Doom. In total there are three First-Person Shooters like idTech4A++ for Mac, Windows and Linux.
ZDoom is a family of enhanced ports of the Doom engine for running on modern operating systems. It runs on Windows, Linux, and OS X, and adds new features not found in the games as originally published by id Software.
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LZDoom is based on an older version of GZDoom. It does not provide all features currently supported by GZDoom but in turn is capable of running the hardware renderer on older hardware which does not support modern OpenGL features.
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1998 saw the public release of the Doom source code by its original authors, id Software. Since then, a large number of Doom "source ports" have been created by programmers, who have extended the code, adding new features and removing some of the limitations of DOS...
