Saturday, August 20, 2022

Porting a Duke3D map to Shadow Warrior

Almost six years ago, I wrote a blog post about Duke Nukem 3D, the underlying BUILD engine and my own total conversion that consists of 22 maps and a variety of interesting customizations.

Between 1997 and 2000, while I was still in middle school, I have spent a considerable amount of time developing my own maps and customizations, such as modified monsters. In the process, I learned a great deal about the technical details of the BUILD engine.

In addition to Duke Nukem 3D, the BUILD engine is also used as a basis for many additional games, such as Tekwar, Witchaven, Blood, and Shadow Warrior.

In my earlier blog post, I also briefly mentioned that in addition to the 22 maps that I created for Duke Nukem 3D, I have also developed one map for Shadow Warrior.

Last year, in my summer holiday, that was still mostly about improvising my spare time because of the COVID-19 pandemic, I did many interesting retro-computing things, such as fixing my old computers. I also played a bit with some of my old BUILD engine game experiments, after many years of inactivity.

I discovered an interesting Shadow Warrior map that attempts to convert the E1L2 map from Duke Nukem 3D. Since both games use the BUILD engine with mostly the same features (Shadow Warrior uses a slightly more advanced version of the BUILD engine), this map inspired me to also port one of my own Duke Nukem 3D maps, as an interesting deep dive to compare both game's internal concepts.

Although most of the BUILD engine and editor concepts are the same in both games, their game mechanics are totally different. As a consequence, the porting process turned out to be very challenging.

Another reason that it took me a while to complete the project is because I had to put it on hold in several occasions due to all kinds obligations. Fortunately, I have managed to finally finish it.

In this blog post, I will describe some of the things that both games have in common and the differences that I had to overcome in the porting process.

BUILD engine concepts

As explained in my previous blog post, the BUILD-engine is considered a 2.5D engine, not a true 3D engine due to the fact that it had to cope with all kinds of technical limitations of home computers commonly used at that time.

In fact, most of the BUILD-engine concepts are two-dimensional -- maps are made out two-dimensional surfaces called sectors:

The above picture shows a 2-dimensional top-level view of my ported Shadow Warrior map. Sectors are two dimensional areas surrounded by walls -- the white lines denote solid walls and red lines the walls between adjacent sectors. Red walls are invisible in 3D mode.

The purple and cyan colored objects are sprites (objects that typically provide some form of interactivity with the player, such as monsters, weapons, items or switches). The "sticks" that are attached to the sprites indicate in which direction the sprite is facing. When a sprite is purple, it will block the player. Cyan colored sprites allow a player to move through it.

You can switch between 2D and 3D mode in the editor by pressing the Enter-key on the numeric key pad.

In 3D mode, each sector's ceiling and floor can be given its own height, and we can configure textures for the walls, floors and ceilings (by pointing to any of these objects and pressing the 'V' key) giving the player the illusion to walk around in a 3D world:

In the above screenshot, we can see the corresponding 3D view of the 2D grid shown earlier. It consists of an outdoor area, grass, a lane, and the interior of the building. Each of these areas are separate 2D sectors with their own custom floor and ceiling heights, and their own textures.

The BUILD engine has all kinds of limitations. Although a world may appear to be (somewhat) 3-dimensional, it is not possible to stack multiple sectors on top of each other and simultaneously see them in 3D mode, although there are some tricks to cope with that limitation.

(As a sidenote: Shadow Warrior has a hacky feature that makes it possible for a player to observe multiple rooms stacked on top of each other, by using specialized wall/ceiling textures, special purpose sprites and a certain positioning of the sectors themselves. Sectors in the map are still separated, but thanks to the hack they can be visualized in such a way that they appear to be stacked on top of each other).

Moreover, the BUILD engine can also not change the perspective when a player looks up or down, although there is the possibility to give a player that illusion by stretching the walls. (As a sidenote: modern source ports of the BUILD engine have been adjusted to use Polymost, an OpenGL rendering extension, which actually makes it possible to provide a true 3D look).

Monsters, weapons, items, and most breakable/movable objects are sprites. Sprites are not really "true 3D" objects. Normally, sprites will always face the player from the same side, regardless of the position or the perspective of the player:

As can be seen, the guardian sprite always faces the player from the front, regardless of the angle of the camera.

Sprites can also be flattened and rotated, if desired. Then they will appear as a flat surface to the player:

For example, the wall posters in the screenshot above are flattened and rotated sprites.

Shadow warrior uses a slightly upgraded BUILD engine that can provide a true 3D experience for certain objects (such as weapons, items, buttons and switches) by displaying them as voxels (3D pixels):

The BUILD engine that comes with Duke Nukem 3D lacks the ability to display voxels.

Porting my Duke Nukem 3D map to Shadow Warrior

The map format that Duke Nukem 3D and Shadow Warrior use are exactly the same. To be precise: they both use version 7 of the map format.

At first, it seemed to look relatively straight forward to port a map from one game to another.

The first step in my porting process was to simply make a copy of the Duke Nukem 3D map and open it in the Shadow Warrior BUILD editor. What I immediately noticed is that all the textures and sprites look weird. The textures still have the same indexes and refer to textures in the Shadow Warrior catalog that are completely different:

Quite a bit of my time was spent on fixing all textures and sprites by looking for suitable replacements. I ended up replacing textures for the rocks, sky, buildings, water, etc. I also had to replace the monsters, weapons, items and other dynamic objects, and overcome some limitations for the player in the map, such as the absence of a jet pack. The process was a labourious, but straight forward.

For example, this is how I have fixed the beach area:

I have changed the interior of the office building as follows:

And the back garden as follows:

The nice thing about the garden area is that Shadow Warrior has a more diverse set of vegetation sprites. Duke Nukem 3D only has palm trees.

Game engine differences

The biggest challenge for me was porting the interactive parts of the game. As explained earlier, game mechanics are not implemented by the engine or the editor. BUILD-engine games are separated into an engine and game part in which only the former component is generalized.

This diagram (that I borrowed from a Duke Nukem 3D code review article written by Fabien Sanglard) describes the high-level architecture of Duke Nukem 3D:

In the above diagram, the BUILD engine (on the right) is general purpose component developed by Ken Silverman (the author of the BUILD engine and editor) and shipped as a header and object code file to 3D Realms. 3D Realms combines the engine with the game artifacts on the left to construct a game executable (DUKE3D.EXE).

To configure game effects in the BUILD editor, you need to annotate objects (walls, sprites and sectors) with tags and add special purpose sprites to the map. To the editor these objects are just meta-data, but the game engine treats them as parameters to create special effects.

Every object in a map can be annotated with meta data properties called Lotags and Hitags storing a 16-bit numeric value (by using the Alt+T and Alt+H key combinations in 2D mode).

In Shadow Warrior, the tag system was extended even further -- in addition to Lotags and Hitags, objects can potentially have 15 numerical tags (TAG1 corresponds to the Hitag, and TAG2 to the Lotag) and 11 boolean tags (BOOL1-BOOL11). In 2D mode, these can be configured with the ' and ; keys in combination with a numeric key (0-9).

We can also use special purpose sprites that are visible in the editor, but hidden in the game:

In the above screenshot of my Shadow Warrior map, there are multiple special purpose sprites visible: the ST1 sprites (that can be used to control all kinds of effects, such as moving a door). ST1 sprites are visible in the editor, but not in the game.

Although both games use the same principles for configuring game effects, their game mechanics are completely different.

In the next sections, I will show all the relevant game effects in my Duke Nukem 3D map and explain how I translated them to Shadow Warrior.

Differences in conventions

As explained earlier, both games frequently use Lotags and Hitags to create effects.

In Duke Nukem 3D, a Lotag value typically determines the kind of effect, while an Hitag value is used as a match tag to group certain events together. For example, multiple doors can be triggered by the same switch by using the same match tag.

Shadow Warrior uses the opposite convention -- a Hitag value typically determines the effect, while a Lotag value is often used as a match tag.

Furthermore, in Duke Nukem 3D there are many kinds of special purpose sprites, as shown in the screenshot above. The S-symbol sprite is called a Sector Effector that determines the kind of effect that a sector has, the M-symbol is a MUSIC&SFX sprite used to configure a sound for a certain event, and a GPSPEED sprite determines the speed of an effect.

Shadow Warrior has fewer special purpose sprites. In almost all cases, we end up using the ST1 sprite (with index 2307) for the configuration of an effect.

ST1 sprites typically combine multiple interactivity properties. For example, to make a sector a door, that opens slowly, produces a sound effect and that closes automatically, we need to use three Sector Effector sprites and one GPSPEED sprite in Duke Nukem 3D. In Shadow Warrior, the same is accomplished by only using two ST1 sprites.

The fact that the upgraded BUILD engine in Shadow Warrior makes it possible to change more than two numerical tags (and boolean values), makes it possible to combine several kinds of functionality into one sprite.

Co-op respawn points

To make it possible to play a multiplayer cooperative game, you need to add co-op respawn points to your map. In Duke Nukem 3D, this can be done by adding seven sprites with texture 1405 and setting the Lotag value of the sprites to 1. Furthermore, the player's respawn point is also automatically a co-op respawn point.

In Shadow Warrior, co-op respawn points can be configured by adding ST1 sprites with Hitag 48. You need eight of them, because the player's starting point is not a co-op start point. Each respawn point requires a unique Lotag value (a value between 0 and 7).

Duke match/Wang Bang respawn points

For the other multiplayer game mode: Duke match/Wang Bang, we also need re-spawn points. In both games the process is similar to their co-op counterparts -- in Duke Nukem 3D, you need to add seven sprites with texture 1405, and set the Lotag value to 0. Moreover, the player's respawn point is also a Duke Match respawn point.

In Shadow Warrior, we need to use ST1 sprites with a Hitag value of 42. You need eight of them and give each of them a unique Lotag value between 0-7 -- the player's respawn point is not a Wang Bang respawn point.

Underwater areas

As explained earlier, the BUILD engine makes it possible to have overlapping sectors, but they cannot be observed simultaneously in 3D mode -- as such, it is not possible to natively provide a room over room experience, although there are some tricks to cope with that limitation.

In both games it is possible to dive into the water and swim in underwater areas, giving the player some form of a room over room experience. The trick is that the BUILD engine does not render both sectors. When you dive into the water or surface again, you get teleported from one sector to another sector in the map.

Although both games use a similar kind of teleportation concept for underwater areas, they are configured in a slightly different way.

In both games, you need the ability to sink into the water in the upper area. In Duke Nukem 3D, the player automatically sinks by giving the sector a Lotag value of 1. In Shadow Warrior, you need to add a ST1 sprite with a Hitag value of 0, and a Lotag value that determines how much the player will sink. 40 is typically a good value for water areas.

The underwater sector in Duke Nukem 3D needs a Lotag value of 2. In the game, the player will automatically swim when it enters the sector and the colors will be turned blue-ish.

We also need to determine from what position in a sector a player will teleport. Both the upper and lower sector should have the same 2 dimensional shape. In Duke Nukem 3D, teleportation can be specified by two Sector Effector sprites having a Lotag 7. These sprites need to be exactly in the same position in the upper and lower sectors. The Hitag value (match tag) needs to be the same:

In the screenshot above, we should see a 2D grid with two Sector Effector sprites having a Lotag of 7 (teleporter) and unique match tags (110 and 111). Both the upper and underwater sectors have exactly the same 2-dimensional shape.

In Shadow Warrior, teleportation is also controlled by sprites that should be in exactly the same position in the upper and lower sectors.

In the upper area, we need an ST1 sprite with a Hitag value of 7 and a unique Lotag value. In the underwater area, we need an ST1 sprite with an Hitag value of 8 and the same match Lotag. The latter ST1 sprite (with Hitag 8) automatically lets the player swim. If the player is an under water area where he can not surface, the match Lotag value should be 0.

In Duke Nukem 3D the landscape will automatically look blue-ish in an underwater area. To make the landscape look blue-ish in Shadow Warrior, we need to adjust the palette of the walls, floors and ceilings from 0 to 9.

Garage doors

In my map, I commonly use garage/DOOM-style doors that move up when you touch them.

In Duke Nukem 3D, we can turn a sector into a garage door by giving it a Lotag value of 20 and lowering the ceiling in such a way that it touches the floor. By default, opening a door does not produce any sound. Moreover, a door will not close automatically.

We can adjust that behaviour by placing two special purpose sprites in the door sector:

  • By adding a MUSIC&SFX sprite we can play a sound. The Lotag value indicates the sound number. 166 is typically a good sound.
  • To automatically close the door after a certain time interval, we need to add a Sector Effector sprite with Lotag 10. The Hitag indicates the time interval. For many doors, 100 is a good value.

In the above screenshot, we can see what the garage door looks like if I slightly move the ceiling up (normally the ceiling should touch the floor). There is both a MUSIC&SFX (to give it a sound effect) as well as a Sector Effector sprite (to ensure that the door gets closed automatically) in the door sector.

In Shadow Warrior, we can accomplish the same thing by adding an ST1 sprite to the door sector with Hitag 92 (Vator). A vator is a multifunctional concept that can be used to move sectors up and down in all kinds of interesting ways.

An auto closing garage door can be configured by giving the ST1 sprite the following tag and boolean values:

  • TAG2 (Lotag). Is a match that that should refer to a unique numeric value
  • TAG3 specifies the type of vator. 0 indicates that it is operated manually or by a switch/trigger
  • TAG4 (angle) specifies the speed of the vator. 350 is a reasonable value.
  • TAG9 specifies the auto return time. 35 is a reasonable value.
  • BOOL1 specifies whether the door should be opened by default. Setting it to 1 (true) allows us to keep the door open in the editor, rather than moving the ceiling down so that it touches the floor.
  • BOOL3 specifies whether the door could crush the player. We set it to 1 to prevent this from happening.

By default, a vator moves a sector down on first use. To make the door move up, we must rotate the ST1 sprite twice in 3D mode (by pressing the F key twice).

We can configure a sound effect by placing another ST1 sprite near the door sector with a Hitag value of 134. We can use TAG4 (angle) to specify the sound number. 473 is a good value for many doors.

In the above screenshot, we should see what a garage door looks like in Shadow Warrior. The rotated ST1 sprite defines the Vator whereas the regular ST1 provides the sound effect.


Another prominent feature of my Duke Nukem 3D map are lifts that allow the player to reach the top or roofs of the buildings.

In Duke Nukem 3D, lift mechanics are a fairly concept -- we should give a sector a Lotag value of 17 and the sector will automatically move up or down when the player presses the use key while standing in the sector. The Hitag of a MUSIC&SFX sprite determines the stop sound and a Lotag value the start sound.

In Shadow Warrior, there is no direct equivalent of the same lift concept, but we can create a switch-operated lift by using the Vator concept (the same ST1 sprite with Hitag 92 used for garage doors) with the following properties:

  • TAG2 (Lotag) should refer to a unique match tag value. The switches should use the exact same value.
  • TAG3 determines the type of vator. 1 is used to indicate that it can only be operated by switches.
  • TAG4 (Angle) determines the speed of the vator. 325 is a reasonable value.

We have to move the ST1 sprite to the same height where the lift should arrive after it was moved up.

Since it is not possible to respond to the use key while the player is standing in the sector, we have to add switches to control the lift. A possible switch is sprite number 575. The Hitag should match the Lotag value of the ST1 sprite. The switch sprite should have a Lotag value of 206 to indicate that it controls a Vator.

The above screenshot shows the result of my porting effort -- switches have been added, the MUSIC&SFX sprite was replaced by an equivalent ST1 sprite. The ST1 sprite that controls the movement is not visible because it was moved up to the same height as the adjacent upper floor.

Swinging doors

In addition to garage doors, my level also contains a number of swinging doors.

In Duke Nukem 3D, a sector can be turned into a swinging door by giving it a Lotag of 23 and moving the floor up a bit. We also need to add a Sector Effector with Lotag 11 and a unique Hitag value that acts as the door's pivot.

As with garage doors, they will not produce any sound effects or close automatically by default, unless we add a MUSIC&SFX and a Sector Effector sprite (with Lotag 10) to the door sector.

In Shadow Warrior, the rotating door concept is almost the same. We need to add an ST1 sprite with Hitag 144 and a unique Lotag value to the sector that acts as the door's pivot.

In addition, we need to add an ST1 sprite to the sector that configures a rotator:

  • TAG2/Lotag determines a unique match tag value that should be identical to the door's pivot ST1 sprite.
  • TAG3 determines the type of rotator. 0 indicates that it can be manually triggered or by a switch.
  • TAG5 determines the angle move amount. 512 specifies that it should move 90 degrees to the right. -512 is moving the door 90 degrees to the left.
  • TAG7 specifies the angle increment. 50 is a good value.
  • TAG9 specifies the auto return time. 35 is a good value.

As with garage doors, we also need to add an ST1 sprite (with Hitag 134) to produce a sound. TAG4 (the angle) can be used to specify the sound number. 170 is a good value for rotating doors.

Secret places

My map also has a number of secret places (please do not tell anyone :-) ). In Duke Nukem 3D, any sector that has a Lotag value of 32767 is considered a secret place. In Shadow Warrior the idea is the same -- any sector with a Lotag of 217 is considered a secret place.

Puzzle switches

Some Duke Nukem 3D maps also have so-called puzzle switches requiring the player to find the correct on-and-off combination to unlock something. In my map they are scattered all over the level to unlock the final key. The E2L1 map in Duke Nukem 3D shows a better example:

We can use the Hitag value to determine whether the switch needs to be switched off (0) or on (1). We can use the Lotag as a match tag to group multiple switches.

In Shadow Warrior, each switch uses a Hitag as a match tag and a Lotag value to configure the switch type. Giving a switch a Lotag value of 213 makes it a combo switch. TAG3 can be used set to 0 to indicate that it needs to be turned off and 1 that it needs to be turned on.

Skill settings

Both games have four skill levels. The idea is that the higher the skill level is, the more monsters you will have to face.

In Duke Nukem 3D you can specify the minimum skill level of a monster by giving the sprite a Lotag value that corresponds to the minimum skill level. For example, giving a monster a Lotag value of 2 means that it will only show up when the skill level is two or higher (Skill level 2 corresponds to: Let's rock). 0 (the default value) means that it will show up in any skill level:

In Shadow Warrior, each sprite has its own dedicated skill attribute that can be set by using the key combination ' + K. The skill level is displayed as one of the sprite's attributes.

In the above screenshot, the sprite on the left has a S:0 prefix meaning that it will be visible in skill level 0 or higher. The sprite on the right (with a prefix: S:2) appears from skill level 2 or higher.

End switch

In both games, you typically complete a level by touching a so-called end switch. In Duke Nukem 3D an ending switch can be created by using sprite 142 and giving it a Lotag of 65535. In Shadow Warrior the idea is the same -- we can create an end switch by using sprite 2470 and giving it a Lotag of 116.


In this blog post, I have described the porting process of a Duke Nukem 3D map to Shadow Warrior and explained some of the properties that are common and different in both games.

Although this project was a pretty useless project (the game is quite old, from the late 90s), I had a lot of fun doing it after not having touched this kind of technology for over a decade. I am quite happy with the result:

Despite the fact that this technology is old, I am still quite surprised to see how many maps and customizations are still being developed for these ancient games. I think this can be attributed to the fact that these engines and game mechanics are highly customizable and still relatively simple to use due to the technical limitations at the time they were developed.

Since I did most of my mapping/customization work many years before I started this blog, I thought that sharing my current experiences can be useful for others who intend to look at these games and creating their own customizations.