DOOM

DOOM Open Source Release

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chocolate-doom

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Chocolate Doom is a Doom source port that is minimalist and historically accurate.

gzdoom

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GZDoom is a feature centric port for all Doom engine games, based on ZDoom, adding an OpenGL renderer and powerful scripting capabilities

Quick Overview

The id-Software/DOOM repository contains the source code for the iconic first-person shooter game DOOM, originally released in 1993. This open-source release allows developers to study, modify, and port the game to various platforms, contributing to its longevity and continued popularity.

Pros

Historical significance: Access to the source code of one of the most influential games in history
Educational value: Provides insights into game development techniques from the early 1990s
Modding potential: Allows for extensive modifications and custom content creation
Cross-platform compatibility: Can be ported to numerous platforms and devices

Cons

Outdated codebase: Written in C, which may be challenging for modern developers accustomed to newer languages
Limited documentation: Lack of comprehensive documentation for the codebase
Complex architecture: May be difficult to understand for beginners due to its age and optimization techniques
Potential licensing issues: Some versions may have restrictions on commercial use

Code Examples

// Example 1: Drawing a line on the screen
void R_DrawColumn (void)
{
    int			count;
    byte*		dest;
    fixed_t		frac;
    fixed_t		fracstep;	

    count = dc_yh - dc_yl;

    // Zero length, column does not exceed a pixel.
    if (count < 0)
	return;
				
#ifdef RANGECHECK
    if ((unsigned)dc_x >= SCREENWIDTH
	|| dc_yl < 0
	|| dc_yh >= SCREENHEIGHT)
	I_Error ("R_DrawColumn: %i to %i at %i", dc_yl, dc_yh, dc_x);
#endif

    // Framebuffer destination address.
    // Use ylookup LUT to avoid multiply with ScreenWidth.
    // Use columnofs LUT for subwindows? 
    dest = ylookup[dc_yl] + columnofs[dc_x];  

    // Determine scaling,
    //  which is the only mapping to be done.
    fracstep = dc_iscale; 
    frac = dc_texturemid + (dc_yl-centery)*fracstep; 

    // Inner loop that does the actual texture mapping,
    //  e.g. a DDA-lile scaling.
    // This is as fast as it gets.
    do 
    {
	// Re-map color indices from wall texture column
	//  using a lighting/special effects LUT.
	*dest = dc_colormap[dc_source[(frac>>FRACBITS)&127]];
	
	dest += SCREENWIDTH; 
	frac += fracstep;
	
    } while (count--); 
}

// Example 2: Playing a sound effect
void S_StartSound
( void*		origin,
  int		sfx_id )
{
    int		rc;
    int		sep;
    int		cnum;
    int		volume;
    int		pitch;
    int		priority;
    sfxinfo_t*	sfx;
    int		i;
    
    mobj_t*	origin_p;

    origin_p = (mobj_t *) origin;

    // check for bogus sound #
    if (sfx_id < 1 || sfx_id > NUMSFX)
        I_Error("Bad sfx #: %d", sfx_id);
    
    sfx = &S_sfx[sfx_id];

    // Initialize sound parameters
    if (sfx->link)
    {
	pitch = sfx->pitch;
	priority = sfx->priority;
	volume = sfx->volume;

	if (origin_p)
	{
	    sep = S_SoundDistance(origin_p);

	    // Separation, that is, orientation/stereo.
	    //  range is: 1 - 256
	    sep += 1;
	}
	else
	{
	    sep = 128;
	}
    }
    else
    {
	// These are set as default for all sounds.
	pitch =

Competitor Comparisons

Quake

4,791

Quake GPL Source Release

Pros of Quake

More advanced 3D graphics engine, supporting true 3D environments
Improved networking capabilities for multiplayer gameplay
Modular design allowing for easier modding and custom content creation

Cons of Quake

Higher system requirements due to more complex graphics
Less accessible to casual players compared to DOOM's simpler gameplay
Darker, more atmospheric art style may not appeal to all players

Code Comparison

DOOM (C):

void P_SpawnMobj(fixed_t x, fixed_t y, fixed_t z, mobjtype_t type)
{
    mobj_t* mobj;
    state_t* st;
    mobjinfo_t* info;

Quake (C):

void SV_SpawnServer (char *server, char *startspot)
{
    edict_t	*ent;
    int		i;
    static char dummy[8] = { 0,0,0,0,0,0,0,0 };

Both codebases use C, but Quake's engine is more complex and modular. DOOM's code focuses on 2D sprite-based object spawning, while Quake handles full 3D entity management and server initialization.

chocolate-doom

1,940

Chocolate Doom is a Doom source port that is minimalist and historically accurate.

Pros of Chocolate Doom

More actively maintained with regular updates and bug fixes
Focuses on accurate source port with minimal changes to original gameplay
Cross-platform support for modern operating systems

Cons of Chocolate Doom

Less feature-rich compared to other source ports
May lack some modern enhancements and quality-of-life improvements
Stricter adherence to original limitations may not appeal to all players

Code Comparison

DOOM (original):

void P_DamageMobj
( mobj_t*	target,
  mobj_t*	inflictor,
  mobj_t*	source,
  int		damage )
{
    unsigned	ang;
    int		saved;
    player_t*	player;
    fixed_t	thrust;
    int		temp;

Chocolate Doom:

void P_DamageMobj(mobj_t *target, mobj_t *inflictor, mobj_t *source, int damage)
{
    unsigned int ang;
    int saved;
    player_t *player;
    fixed_t thrust;

The code structure remains similar, with Chocolate Doom maintaining close compatibility with the original DOOM codebase. Minor differences include updated variable declarations and coding style improvements in Chocolate Doom.

gzdoom

2,419

GZDoom is a feature centric port for all Doom engine games, based on ZDoom, adding an OpenGL renderer and powerful scripting capabilities

Pros of gzdoom

Enhanced graphics and rendering capabilities, including support for modern OpenGL features
Expanded modding support with advanced scripting and customization options
Cross-platform compatibility (Windows, macOS, Linux)

Cons of gzdoom

Larger codebase and more complex architecture, potentially harder to maintain
May require more system resources due to advanced features
Potential compatibility issues with some classic DOOM mods

Code Comparison

DOOM (original):

void R_DrawColumn (void)
{
    int			count;
    byte*		dest;
    fixed_t		frac;
    fixed_t		fracstep;

gzdoom:

void R_DrawColumn(const wallcol_t &column)
{
	const int count = column.length;
	uint8_t *dest = ylookup[column.yl] + column.x + dc_destorg;
	fixed_t frac = column.texturefrac;
	fixed_t fracstep = column.texturestep;

The gzdoom code shows a more modern C++ approach with improved parameter passing and data structures, while the original DOOM code uses a simpler C-style implementation.

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README

Here it is, at long last. The DOOM source code is released for your non-profit use. You still need real DOOM data to work with this code. If you don't actually own a real copy of one of the DOOMs, you should still be able to find them at software stores.

Many thanks to Bernd Kreimeier for taking the time to clean up the project and make sure that it actually works. Projects tends to rot if you leave it alone for a few years, and it takes effort for someone to deal with it again.

The bad news: this code only compiles and runs on linux. We couldn't release the dos code because of a copyrighted sound library we used (wow, was that a mistake -- I write my own sound code now), and I honestly don't even know what happened to the port that microsoft did to windows.

Still, the code is quite portable, and it should be straightforward to bring it up on just about any platform.

I wrote this code a long, long time ago, and there are plenty of things that seem downright silly in retrospect (using polar coordinates for clipping comes to mind), but overall it should still be a usefull base to experiment and build on.

The basic rendering concept -- horizontal and vertical lines of constant Z with fixed light shading per band was dead-on, but the implementation could be improved dramatically from the original code if it were revisited. The way the rendering proceded from walls to floors to sprites could be collapsed into a single front-to-back walk of the bsp tree to collect information, then draw all the contents of a subsector on the way back up the tree. It requires treating floors and ceilings as polygons, rather than just the gaps between walls, and it requires clipping sprite billboards into subsector fragments, but it would be The Right Thing.

The movement and line of sight checking against the lines is one of the bigger misses that I look back on. It is messy code that had some failure cases, and there was a vastly simpler (and faster) solution sitting in front of my face. I used the BSP tree for rendering things, but I didn't realize at the time that it could also be used for environment testing. Replacing the line of sight test with a bsp line clip would be pretty easy. Sweeping volumes for movement gets a bit tougher, and touches on many of the challenges faced in quake / quake2 with edge bevels on polyhedrons.

Some project ideas:

Port it to your favorite operating system.

Add some rendering features -- transparency, look up / down, slopes, etc.

Add some game features -- weapons, jumping, ducking, flying, etc.

Create a packet server based internet game.

Create a client / server based internet game.

Do a 3D accelerated version. On modern hardware (fast pentium + 3DFX) you probably wouldn't even need to be clever -- you could just draw the entire level and get reasonable speed. With a touch of effort, it should easily lock at 60 fps (well, there are some issues with DOOM's 35 hz timebase...). The biggest issues would probably be the non-power of two texture sizes and the walls composed of multiple textures.

I don't have a real good guess at how many people are going to be playing with this, but if significant projects are undertaken, it would be cool to see a level of community cooperation. I know that most early projects are going to be rough hacks done in isolation, but I would be very pleased to see a coordinated 'net release of an improved, backwards compatable version of DOOM on multiple platforms next year.

Have fun.

John Carmack 12-23-97

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