About

dhewm3 is a source port of the original Doom3 (not Doom3 BFG, for that you may want to try RBDoom3BFG). It’s known to work on Windows, Linux, macOS, FreeBSD, OpenBSD and AROS, but it should work on (or be easily portable to) any system that supports OpenGL 1.4 with ARB shaders, SDL and OpenAL.
Compared to the original version of Doom3, dhewm3 has many bugfixes, supports EAX-like sound effects on all operating systems and hardware (via OpenAL Softs EFX support), has much better support for widescreen resolutions and has 64bit support.

It only supports old Mods if they either don’t require their own game DLL or have been ported to dhewm3 - see the Mods page for more information.

Note that while the Doom3 source code has been released under GPL, you still need to legally own the game and provide dhewm3 the game data to play. See the How to Install section for more information.

News

2024-03-29: dhewm3 1.5.3

Just in time for the Easter weekend, dhewm3 1.5.3 is done!

This brings (the often-requested) support for gamepads, as long as they’re supported by SDL2. This includes XBox Controllers (and compatible ones), Playstation 3-5 controllers, Nintendo Switch Pro Controllers, many thirdparty controllers for those consoles, and lots of other gamepads for PC.
See the Configuration documentation for how to make make it work (well, basically you can just plug in your gamepad and configure bindings in the Controls menu, but I also provide configs with default bindings that you could use).
Furthermore, dhewm3 now supports taking screenshots in different formats: TGA (like before), BMP, JPEG and PNG.
Last but not least, there were several bugfixes and other small improvements, see the list below for details.

You can download dhewm3 1.5.3 at Github (incl. builds for Windows and 64bit Linux for both dhewm3 and the supported mods).

Changes since 1.5.2:

• Support for gamepads (based on code from Quadrilateral Cowboy, but heavily expanded). See Configuration.md for more information.
• Support different file formats for screenshots by setting the r_screenshotFormat CVar (0 = TGA, still the default, 1 = BMP, 2 = PNG, 3 = JPG). r_screenshotJpgQuality and r_screenshotPngCompression allow configuring how JPG/PNG are compressed. Thanks eezstreet (Nick Whitlock)!
• Fixed problems with lights after loading a savegame (#495)
• Fix volume of some weapon sounds, like chaingun being too quit (#326)
• Increase stack size on Windows to 8MB (instead default of 1MB) to make loading huge models work
• Fixed crash in Radiant Model Preview Dialog (#496)
• Fix MD3 model support
• Several new CMake options:
  • To enable Clang/GCC Address Sanitizer and Undefined Behavior Sanitizer
  • Hardlink the game code into the executable (instead of using game DLLs, only supports base or d3xp then; needed for Undefined Behavior Sanitizer)
  • Force colored diagnostic output from GCC or Clang (esp. useful when building with ninja)
• Fix several compiler warnings
• Added build instructions for Linux (and similar systems) to README.md
• Added documentation for dhewm3-specific settings (mostly CVars): Configuration.md
• Updated stb_image and stb_vorbis
• Updated minizip (from zlib/contrib) to latest upstream code
• Added in_namePressed CVar to print currently pressed key/button (useful for binding keys in the console or configs). Thanks Biel Bestué de Luna!
• Probably more bugfixes I forgot to list here..
• Updated bundled libs in build for Windows: OpenAL Soft 1.23.1, SDL 2.30.0, cURL 8.6.0, zlib 1.3.1
• Linux binary build now requires SDL 2.0.12 or newer (I provide an updated libSDL2-2.0.so.0 though).
  If you compile dhewm3 yourself, older SDL2 versions (and SDL1.2) are still supported (however, SDL1.2 doesn’t support gamepads)

2024-03-19: dhewm3 1.5.3 Release Candidate 2

The second release candidate for 1.5.3 fixes a bug with gamepad support (sometimes the gamepad buttons were mixed up after using ingame UI panels) and updates the integrated minizip library.

Furthermore, all the mods have been updated for gamepad support and the bugfixes since 1.5.2.

Download dhewm3 1.5.3 RC2 at Github (incl. builds for Windows and 64bit Linux)!

2024-02-04: dhewm3 1.5.3 Release Candidate 1

A first Release Candidate of the upcoming 1.5.3 release is available!

2022-06-13: dhewm3 1.5.2

dhewm3 1.5.2 is done.

You can download it at Github (incl. builds for Windows and 64bit Linux for both dhewm3 and the supported mods)

Since dhewm3 1.5.1, there have been a lot of enhancements, like better compatibility with Wayland, or applying gamma and brightness in shaders, which means that those settings work better on all platforms and are visible in screenshots (to demonstrate this, the screenshot above has been taken with r_gamma 2.0 and r_brightness 1.9).
Furthermore, both keyboard- and mouse-input have been improved and now there are multiple Quicksave slots (that are cycled through), so if you accidentally saved in a bad situation, you can still load the previous Quicksave.
Last but not least, there were lots of bugfixes and the (sadly still Windows-only) tools have been improved again with nice new features like High-DPI support, 64bit-compatibility and the Script Debugger you might remember from Quake4!

See the following list for more details.

Changes since 1.5.1:

• Gamma and Brightness are now applied in the shaders instead of by setting hardware gamma.
  Can be disabled (so hardware gamma is used again) with r_gammaInShaders 0
• Improvements for (Windows-only) MFC-based tools:
  • Added the script debugger! (thanks HarrievG!)
    Original Doom3 didn’t have it (Quake4 did), but the Doom3 GPL source contained most of it. HarrievG implemented the missing parts and we added some new features. It can even be used over the network and while the client part (the debugger GUI) is Windows-only, the server can run on all supported platforms, so you can debug a game running on Linux or macOS, for example.
    Relevant CVars for network debugging are: com_enableDebuggerServer and com_dbgClientAdr and com_dbgServerAdr.
    To debug the running game on the same PC, just enter debugger in the console.
  • All tools can now be built in 64bit (thanks raynorpat!)
  • HighDPI support (thanks HarrievG!)
  • PDAEditor works now
  • Additional bugfixes
• Cycle through multiple Quicksave slots instead of immediately overwriting the last Quicksave. The com_numQuicksaves CVar allows setting the number of QuickSaves (#392)
• Make r_locksurfaces work (#357)
  It doesn’t do exactly what its description and name suggests: it renders everything that is currently visible from the position/view the player had when setting r_locksurfaces 1. Originally it was supposed to render exactly the surfaces that were visible then, but I couldn’t get that to work.
  This is pretty similar, but there may be differences with opened doors and such.
• Keyboard input improvements (mostly SDL2-only):
  • Support (hopefully) all keyboard keys on all kinds of keyboard layouts by using scancodes for otherwise unknown keys
  • Support typing in non-ASCII characters, if supported by Doom3 (it supports ISO-8859-1)
  • Support the clipboard also on non-Windows platforms
    You can paste text from the clipboard into the console or other edit fields with Shift+Insert
  • Explicit support for Right Ctrl, Alt and Shift keys
    (can be bound to different actions than their left counterparts)
  • Added in_grabKeyboard CVar to make sure dhewm3 gets all keyboard input
    Prevents the Windows-key or Alt-Tab or whatever from taking focus from the game
  • Added in_ignoreConsoleKey - if set to 1, the console is only opened with Shift+Esc, and the “console key” (that key between Esc, 1 and Tab) can be freely bound to an action (and its char can be typed in the console without closing it).
  • Added (SDL2-only) “auto” option for in_kbd: When not disabling the console key, dhewm3 will try to automatically detect it if in_kbd is set to “auto” (now default)
• Reworked mouse-input and -grabbing code, using absolute mouse mode in fullscreen GUIs (except for the PDA, because it’s implemented weirdly).
  This made releasing the mouse in the main menu possible, as now the ingame cursor is at the same position as the system cursor.
• s_alReverbGain CVar to reduce EFX reverb effect intensity (#365)
• Pause (looped) sounds when entering menu (#330)
• Fixes for looped sounds (#390)
• Replace libjpeg with stb_image and libogg/libvorbis(file) with stb_vorbis
  • Now the only required external dependencies should be OpenAL, SDL, zlib and optionally libCURL (and of course the C and C++ runtimes)
• (Optionally) use libbacktrace on non-Windows platforms for more useful backtraces in case of crashes (usually linked statically)
• Fixed a deadlock (freeze) on Windows when printing messages from another thread
• Fixed endless loop (game locking up at startup) if graphics settings couldn’t be applied (#386)
• Fixed some warnings and uninitialized variables (thanks turol!)
• Work around dmap bug caused by GCC using FMA “optimizations” (#147)
• Prevent dhewm3 from being run as root on Unix-like systems to improve security
• Replaced most usages of strncpy() with something safer to prevent buffer overflows (remaining cases should be safe).
  • Just a precaution, I don’t know if any of them could actually be exploited, but there were some compiler warnings in newer GCC versions.
• Console output is now logged to dhewm3log.txt (last log is renamed to dhewm3log-old.txt)
  • On Windows it’s in My Documents/My Games/dhewm3/
  • On Mac it’s in $HOME/Library/Application Support/dhewm3/
  • On other Unix-like systems like Linux it’s in $XDG_DATA_HOME/dhewm3/ (usually $HOME/.local/share/dhewm3/)
• Improved compatibility with Wayland (#426)
• Work around assertion in AlphaLabs4 due to “ride_of_death” yeeting the dead “monster_zsec_shotgun_12” into the void (#409)
• Support loading some mods known to need fs_game_base d3xp via Mods menu (currently, The Lost Mission and LibreCoop d3xp are supported)
• Disable assertion in idSampleDecoderLocal::DecodeOGG() that triggered when starting a new Classic Doom3 game (#461)

2022-05-29: dhewm3 1.5.2 Release Candidate 2

The second Release Candidate of the upcoming 1.5.2 release is available!

2022-05-16: dhewm3 1.5.2 Release Candidate 1

A first Release Candidate of the upcoming 1.5.2 release is available!

2021-12-20: Miscellaneous News

Some things of interest (that I should’ve written about much earlier) happened since the 1.5.1 release:

idDevNet

dhewm3.org now hosts a mirror of the (defunct) iddevnet.com:
https://iddevnet.dhewm3.org

It’s still a very valuable resource for modding Doom3/Quake4/idTech4.

It was reconstructed from archive.org and private backups that several people luckily made while the original page was still up. This mirror is (as good as) complete, even including downloads!
To make sure it doesn’t get lost again (and for offline viewing), the whole page source is available on Github.

Hard Corps

The Doom 3: Hard Corps mod, which supports dhewm3 exclusively, has been released. It’s an updated/remastered version of the well-known HardQore2 mod and turns Doom3 into a side-scroller.

You can get it at ModDB; our Mods page also has some information and screenshots.

dhewm3 progress

There have been a lot of improvements to dhewm3 since 1.5.1.
Several concern the (sadly still Windows-only) tools, like full 64bit support, High-DPI support and the Script Debugger you might remember from Quake4!
Furthermore, dhewm3 now applies brightness and gamma in shaders, which means that those settings work better on all platforms (no more changing gamma of the whole screen) and are visible in screenshots (so those now look the same as the game itself). Other improvements include multiple Quicksave slots (that are cycled through), improved keyboard support, fewer external dependencies and of course bugfixes.
The Changelog has more details.

I hope I’ll get around to releasing a fresh build soon-ish.

2021-03-14: dhewm3 1.5.1

dhewm3 1.5.1 is done.

Apart from lots of bugfixes, the biggest changes since 1.5.0 are support for the (Windows-only) editing tools and for the Doom3 Demo gamedata.
See below for a more detailed changelog.

Furthermore, there now is a coop mode for dhewm3: LibreCoop, see the dhewm3 mods page for more details. Thanks a lot to Stradex for creating it!
Another Mod that is now supported is The Lost Mission, see also here - thanks to Arl90 for making this possible!

You can download dhewm3 1.5.1 at Github (incl. builds for Windows and 64bit Linux)

Changes since 1.5.0:

• The (Windows-only) integrated editing tools of Doom3 are back!
  • They can only be built with non-Express versions of Visual Studio (tested Community Editions of VS2013 and VS2017) and can be disabled via CMake
  • Official dhewm3 Windows binaries are built with tools enabled, of course.
  • Only supports 32bit builds, because in contrast to the rest of dhewm3’s code, the tool code is not 64bit compatible at all.
  • Based on Code from the dhewm3 branch of SteelStorm2, thanks to Motorsep for donating that code!
  • Has some bugfixes over the state in Doom3 1.3.1, like selecting a material in the Particle Editor doesn’t break the viewport of the game any more.
  • Thanks to Tommy Hanusa for testing and reporting some issues (that were subsequently fixed)!
• While prior dhewm3 releases for Windows have been built with Visual Studio 2010, this is built with Visual Studio 2017, so if it doesn’t start on your system make sure you have the Visual C++ 2017 Redistributable installed - but chances are good you already have it.
  • Also updated some DLLs that are bundled with the Windows release:
    SDL2 2.0.14, OpenAL-soft 1.21.1 and curl 7.70.0
• Update savegame format (see #303 and #344)
  • old savegames still work, but new savegames can’t be loaded with older versions of dhewm3!
• Uploaded updated builds of Mod DLLs (esp. Dentonmod should run a lot more stable now).
  Added Mod DLLs of LibreCoop and The Lost Mission.
  See the Mods page for more details.
• dhewm3 now supports the Doom3 Demo gamedata
  • See below for installation instructions
  • This is based on Gabriel Cuvillier’s code for D3Wasm, which ports dhewm3 to web browsers, thanks!
• Create the game window on the display the cursor is currently on (when using more than one display)
• Added r_fullscreenDesktop CVar to set if fullscreen mode should be “classic” or “Desktop” which means a borderless window at desktop resolution
• Fullscreen modes that are not at the current desktop resolution should work better now
  • including nvidia DSR / AMD VSR; for that you might have to use the supplied dhewm3_notools.exe, as DSR/VSR seem to be incompatible with applications that use MFC (the GUI framework used for the Doom3 tools like the D3Radiant)
• Several sound-related bugfixes:
  • Lags in starting to play a sound which for example caused the machinegun or plasmagun sounds to stutter have been eliminated (#141)
  • Trying to reset disconnected OpenAL devices, this esp. helps with display audio on Intel GPUs on Windows, when switching to fullscreen (#209)
  • Looping .wav sounds with leadin now work (#291)
  • The game still works if no sound devices are available at all (#292)
  • Make “idSoundCache: error unloading data from OpenAL hardware buffer” a Warning instead of an Error so it doesn’t terminate game (by Corey O’Connor, #235)
• Restore “Carmack’s Reverse” Z-Fail stencil shadows; use glStencilOpSeparate() if available
  • That bloody patent finally expired last October: https://patents.google.com/patent/US6384822B1/en
  • This neither seems to make a visual nor performance difference on any hardware I tried (including Raspberry Pi 4), so this is mostly out of principle
  • Based on Code by Leith Bade and Pat Raynor.
  • The r_useCarmacksReverse and r_useStencilOpSeparate CVars allow switching both things on/off for comparison
• New CVar g_hitEffect: If set to 0, the player camera damage effects (like double-vision and extreme tilt) when being hit are disabled (by dobosken, #279).
• (On Windows) stdout.txt and stderr.txt are not saved next to the binary anymore, but in My Documents/My Games/dhewm3/, like save games, because the binary dir might not be writable and dhewm3 wouldn’t start properly then
• Fix lingering messages in HUD after loading savegame
  • Sometimes the “Game saved…” message didn’t go away after loading a savegame (when having saved while it still was showing from last save)
• Fixed clipping bug in delta1 which sometimes occured and made climbing some ladders impossible (see #328)
• Improve compatibility with some custom scripts (“t->c->value.argSize == func->parmTotal” Assertion; see #303)
• Registering multiplayer servers at id’s master-server fixed, so they can be found in the multiplayer menu (by Stradex, #293)
• Support for reproducible builds by setting the CMake option REPRODUCIBLE_BUILD.
• Should build on recent versions of macOS, also on Apple Silicon (thanks Dave Nicolson and Petter Uvesten).
• Proper handling of paths with dots in directory names (#299, #301)
  • Some string functions that are intended to find/cut off/replace/… file extensions did cut off the whole path at dots..
  • Especially fixes loading and saving maps from such paths in the builtin D3Radiant level editor
• idFileSystemLocal::ListMods() doesn’t search / or C:\ anymore (it did so if one of the paths, like fs_cdpath, was empty)
• Don’t use translation in Autosave filenames (see #305)
  • In the Spanish translation all the Alpha Lab autosaves got the same name, now the autosave name is based on the mapename instead which is distinct

2021-02-23: dhewm3 1.5.1 Release Candidate 3

Finally, the third (and hopefully last) Release Candidate of the upcoming 1.5.1 release is available!

2020-07-21: dhewm3 1.5.1 Release Candidate 2

The second Release Candidate of the upcoming 1.5.1 release is available!

2020-07-13: dhewm3 1.5.1 Release Candidate 1

A first Release Candidate of the upcoming 1.5.1 release is available!

2019-03-11: A first prerelease of dhewm3 1.5.1

There are no immediate plans for the final 1.5.1 release, but the current status might be interesting :-)

Click to see the rest of this (outdated) newspost

Changes since 1.5.0:

• The (Windows-only) integrated editing tools of Doom3 are back!
  • They can only be built with non-Express versions of Visual Studio (tested Community Editions of VS2013 and VS2017) and can be disabled via CMake
  • The dhewm3_1.5.1pre1_win32.zip available on Github has them enabled, of course
    • While prior dhewm3 releases have been built with Visual Studio 2010, this is built with Visual Studio 2013, so if it doesn’t start on your system make sure you have the Visual C++ 2013 Redistributable installed - but chances are good you already have it.
  • Only tested in 32bit builds, in contrast to the rest of dhewm3’s code, the tool code might not be 64bit-clean
  • Based on Code from the dhewm3 branch of SteelStorm2, thanks to Motorsep for donating that code!
  • Has some bugfixes over the state in Doom3 1.3.1, like selecting a material in the Particle Editor doesn’t break the viewport of the game any more.
• dhewm3 now supports the Doom3 Demo gamedata (D3Demo.exe or doom3-linux-1.1.1286-demo.x86.run)
  • On Windows, just run D3Demo.exe to install the demo, on Linux (and probably others) you can extract the relevant file from the .run installer with sh doom3-linux-1.1.1286-demo.x86.run --tar xf demo/
  • This is based on Gabriel Cuvillier’s code for D3Wasm, which ports dhewm3 to web browsers, thanks!
• (On Windows) stdout.txt and stderr.txt are not saved next to the binary anymore, but in My Documents/My Games/dhewm3/, like save games, because the binary dir might not be writable and dhewm3 wouldn’t start properly then

2018-12-16: dhewm3 1.5.0 released

The final version of 1.5.0 release is now available!
You can download it (incl. builds for Windows and 64bit Linux) at Github

Changes since 1.4.1:

• Support for some Mods via custom SDK: Classic Doom3, Fitz Packerton, HardQore2, Denton’s Enhanced Doom3 and Rivensin (formerly known as Ruiner).
  • dhewm3-mods_1.5.0_win32.zip contains prebuild Windows game DLLs for those Mods (except Rivensin) and a README that tells you where to get the game data
  • See the Mods page for more information.
  • This has also broken backwards compatibility with 1.4.x game DLLs, that’s why this version will be 1.5.0 and not 1.4.2.
• Supports High DPI displays on Windows now
• Scale menus, fullscreen videos and the PDA to 4:3 (with black bars left/right) on widescreen displays so they don’t look stretched/distorted. Can be disabled with r_scaleMenusTo43 0.
  No, this unfortunately can’t be done for the HUD (except for the crosshair), because it also handles fullscreen effects (for example when receiving damage), and those would look bad with black/empty bars on left/right.
• Commandline option to display some help on supported commandline arguments: -h or --help or -help or /?
• (Experimental) uncapped framerate, enable by entering com_fixedTic -1 in the console (can be set back with com_fixedTic 0). (this turned out to be broken)
• Providing binaries for Linux amd64 (x86_64) now
  • Should work on any halfway-recent distro, needs libSDL2-2.0.so.0, libopenal.so.1 and libcurl.so.4 installed
• Updated some libraries bundled in the Win32 build: SDL2 2.0.9, OpenAL-soft 1.19.1 and curl 7.62.0
• Support for the AROS and OpenBSD operating systems
• Several bugfixes

2018-07-11: dhewm3.org launched

dhewm3 now has an official homepage!

I guess it makes sense to be a bit less dependent on Github (especially as it has been bought by Microsoft, but also in general), so I decided to create this homepage with a domain I control.
Another advantage is that it has some screenshots and more user-centric documentation than the README.md.
We’ll continue to use Github for hosting the code and issues (and indeed this page via github pages), but if we need to migrate to another hoster some day, dhewm3.org will still be reachable and will link to the new code repo and issue tracker instead.

2016-06-18: dhewm3 1.4.1 released

Fixes/improvements since 1.4.0:

• Fixed some (kinda rare) crashes due to assertion errors, especially observed in the last boss fights of both doom3 and the Resurrection of Evil Addon.
• Improved compatibility with AZERTY keyboards (the row of keys with 1…9, 0 is now usable)
• Fixed a crash (at least on FreeBSD) when loading Resurrection of Evil’s last level
• Compatibility with Microsoft Visual Studio 2015
• Video resolutions in menu now sorted, added 2880x1800
• Support for up to 8 mouse buttons (on Linux this needs SDL2 2.0.4 or newer to work)

How to Install

Getting the Doom3 Game Data

You’ll need the game data from a Doom3 installation patched to 1.3.1. Specifically, you’ll need the following .pk4 files for the main game:

… and (optionally) these .pk4 files for the Resurrection of Evil addon:

.. or alternatively you can use the demo00.pk4 from the free Demo version, see below for more information.

If you’re using an official binary release of dhewm3 (for Win32 or Linux amd64), you just put the .pk4 files into the corresponding directories (base/ and d3xp/) in dhewm3/.

There are also platform-specific places where dhewm3 looks for the game data, see the FAQ for more details.

Alternatively you can just tell dhewm3 where to find the directory that contains base/ and d3xp/ with the .pk4s, with the following commandline argument:

dhewm3 +set fs_basepath /path/to/Doom3/

(On Windows it might look like dhewm3.exe +set fs_basepath C:\Games\Doom3\)

The following sections explain how to obtain the .pk4 files.

… from CD/DVD + Patch

If you bought the game on CDs/DVD, base/pak000.pk4 - pak004.pk4 and d3xp/pak000.pk4 can be copied from the disks, the other files are from the patch.

On Linux (and probably other Unix-like systems and maybe even Windows with a mingw shell) you can extract the needed files from the official 1.3.1 patch for Linux with:

sh /path/to/doom3-linux-1.3.1.1304.x86.run --tar xvf --wildcards base/pak* d3xp/pak*

On Windows you can just install the game and the official 1.3.1 patch for Windows and then get the files from the installation directory (or copy dhewm3 in there).

… from Steam

First you need to buy Doom3 of course - it also includes the Resurrection of Evil Addon (and the BFG Edition re-release of the game, which is not supported by dhewm3).

Thankfully the game from Steam is already fully patched to 1.3.1.

On Windows, just install it and copy the game data from there (see above for what files are needed).

Same on Linux, thanks to Steam Play / Proton, though you might have to enable it:
In Steam in the Steam ➔ Settings menu, under Steam Play, check both:

☑ Enable Steam Play for supported titles
☑ Enable Steam Play for all other titles

On macOS unfortunately you can’t just install the game (unless you run Windows Steam in Wine), because it’s Windows-only on Steam and Steam (currently?) doesn’t use Proton on Mac.

However, you can still download it with SteamCMD.
See this description for macOS or here for Linux on how to install SteamCMD on your system. You won’t have to create a new user.

Then you can download Doom3 with

./steamcmd.sh +@sSteamCmdForcePlatformType windows +login <YOUR_STEAM_LOGIN_NAME> +force_install_dir ./doom3/ +app_update 9050 validate +quit

(replace <YOUR_STEAM_LOGIN_NAME> with your steam login name)
When it’s done you should have the normal windows installation of doom3 in ./doom3/ and the needed files in ./doom3/base/
It should be possible to get the Resurection of Evil data in the same way, by replacing 9050 with 9070; the files should be in ./doom3/d3xp/ then.

By the way, that number is the “AppID” of Doom3/RoE; if you wanna use this to get the data of other games you own, you can look up the AppID at SteamDB.

… from GOG

GOG.com offers the same bundle as Steam (Doom3 + Resurrection of Evil + BFG Edition): https://www.gog.com/game/doom_3

I haven’t tried the GOG version yet, but you should be able to download it without installing any launcher. At GOG you usually get Windows Installers that use Inno Setup and can be extracted with innoextract on all relevant operating systems.
See also https://constexpr.org/innoextract/#gog

I assume that the GOG version of the game is also fully patched to 1.3.1.

Using the Doom3 Demo gamedata

First you need to download the Doom3 Demo, of course.

For Windows, you can get D3Demo.exe at Fileplanet and just run the installer.

For Linux (and other operating systems that have a POSIX-compatible shell and tar) you can download doom3-linux-1.1.1286-demo.x86.run from Holarse or some other page that has that file.
Then you can extract the relevant file from the .run installer with

sh doom3-linux-1.1.1286-demo.x86.run --tar xf demo/

You’ll need demo/demo00.pk4 for playing, either in exactly that directory, or you can copy it into base/.
Note that you don’t need the patches of the full version, adding their pk4s when using Demo gamedata breaks the game.

demo00.pk4 has a size of 462MB but there are minimal differences between the one for Linux and Windows, so they have different md5sums:
Windows: md5sum bd410abbb649b9512d65b794869df9fe size 483534533 Bytes
Linux: md5sum 70c2c63ef1190158f1ebd6c255b22d8e size 483535485 Bytes

Both work on all platforms though, but the Linux demo had been released two months after the Demo for Windows and contains at least one tiny bugfix: A missing string for " or " which is used when binding multiple keys to the same action - the Windows Demo shows a nonsense string instead.

Getting dhewm3 executables

On Windows you can just download the Win32 binaries from the Download page.
You can either extract them into your Doom3 installation directory, or into a fresh directory and copy the needed game data (see above) in there.
The official Windows binaries are built with Visual Studio 2017, so if it doesn’t start on your system make sure you have the Visual C++ 2017 Redistributable for X86 installed.

Similarly, you can find x86_64 (amd64) Linux binaries on the Download page - these expect that your system has libSDL2-2.0.so.0,libopenal.so.1, libz.so.1 and libcurl.so.4 installed (on debian, Ubuntu and similar distros the corresponding packages are called libsdl2-2.0-0, libopenal1, zlib1g and libcurl4).

If you’re using macOS, MacSourcePorts.com provides signed and notarized dhewm3 binaries for 64bit Intel and Apple Silicon.

On Linux and *BSD it might be that your package manager provides a dhewm3 package/port that you can install; please refer to that packages documentation on where to put the game data.
You could also try telling dhewm3 where to find the game data with:

dhewm3 +set fs_basepath /home/HansWerner/Games/doom3

(adjust the path to your needs)

If your package manager does not provide dhewm3, or you want to use the very latest code, you’ll have to compile it yourself. It’s not super hard, see the README for details on compiling and the FAQ on where to put the game data.