GZDoom

GZDoom is a ZDoom-based port which is maintained by Christoph Oelckers (Graf Zahl). It was first released on August 30, 2005 and runs on Linux, Windows and MacOS. Its development was temporarily halted on April 14, 2010, with development resuming in 2014.

History
On January 19, 2008, development of the port was put on hold until another official release of ZDoom is made, due to highly extensive changes to the ZDoom renderer (which consequently require extensively changing the rendering code for compatibility) coming more frequently than Graf Zahl could catch up. An official release of ZDoom was made on February 14, 2008, and version 1.1.0 of GZDoom was released shortly afterward incorporating the changes from that version.

On April 14 2010, Graf Zahl announced on the Doomworld forums that development of GZDoom had halted. He stated his reasons as having no time to work on it, and that he was frustrated by numerous complaints about the port's lack of compatibility with ATI video cards. Graf Zahl took down the website and all its subfolders, making GZDoom temporarily unavailable except from its mirrors. The site has since been restored.

In 2014, work on GZDoom's renderer resumed. Upon release of 2.0 on August 2, 2014, OpenGL 2.x support was dropped, as the renderer was rewritten to use OpenGL 3.x and OpenGL 4.4, albeit in compatibility mode. After some time of having dual 1.9.x and 2.1.x releases, support for OpenGL 2.0 was reinstated with GZDoom 2.2.0 in 2016, but with fewer features than before in that legacy mode.

Beginning in December 2016, following ZDoom's official halt in development, GZDoom's development is now detached from ZDoom and the port is the first-level upstream for other forks (namely QZDoom and Zandronum), without looking back at ZDoom.

The current primary direction of development is ZScript, with several other things developed by contributors (for example, OpenGL renderer enhancements by dpJudas). Starting with version 4.3.2, support for the WAD files from the Doom Classic Unity port was added.

Features

 * All ZDoom features
 * Rudimentary support for Doom Legacy (To run existing Legacy mods like Ni'mRoD:IXNAY on the HOMBRE)
 * ZScript
 * OpenGL renderer, allowing the following features:
 * Full 3D floors (including slopes)
 * Reflective floors
 * Dynamic lights, brightmaps, glowing flats, custom hardware shaders
 * '/'-style skyboxes in addition to regular ZDoom skyboxes
 * Optional High-quality (HQnX) rescaling filters for graphics, sprites and textures
 * MD2, MD3 and DMD model support

Merged features from QZDoom
After the QZDoom codebase got merged with GZDoom, the latter gained several new features, courtesy of Magnus Norddahl (dpJudas) and Rachael Alexanderson (Eruanna). These include:
 * Polygonal, true-3D software renderer and truecolor software renderer
 * Postprocessing framework (SSAO, bloom, blur, custom post process shaders, etc.)
 * Vulkan renderer backend
 * HDR monitor support
 * ZScript JIT bytecode compiler
 * Proper windowed mode support (i.e. you can freely resize the game window)
 * Softpoly software renderer
 * Correct ZDoom software light mode for the hardware renderer
 * High resolution correct version of the software fuzz, for both software and hardware renderers
 * Improved dynamic light math, shadowmaps and spotlights
 * Materials (Classic, normal and specular maps and PBR textures), but not environment maps

Trivia

 * GZDoom was originally based on PrBoom, with some added code from Doom Legacy. This version was never released.