Animated flat

Animated flats are a feature of the Doom engine where flats (textures used on floors and ceilings) are made to animate through several frames. One of the most common uses of this is to make a floor resemble a moving liquid, such as water or lava. They are often used to indicate damaging floors.

In the shareware episode of Doom, nukage (or green slime) was the only animated flat, depicting substances the UAC meddled with, while the episodes for the registered version added blue water, as well as blood and lava to give places a more hellish aspect. Doom II added molten rock for devastated and hellish areas and brown slime to represent sewer water and other wastes.

The names of the animated flats are built into the game executable. These can be changed through dehacked string substitutions. Some source ports, most notably Boom, allow the animation tables to be changed and extended more flexibly.

In Doom, the built-in animated flats are:



Technical
At initialization time, the engine looks up each lump named in the "First" column above, and links it with all following lumps, up to and including that named in the "Last" column.

If any of the linked names is used as the floor (or ceiling) of a sector, then the engine changes it to the next linked name about three times a second (wrapping around to the first after using the last).

In the "Version" column above:
 * "All" refers to all versions, including the shareware
 * "R" refers to registered Doom and The Ultimate Doom
 * "2" refers to Doom II

The "Frames" and "Description" columns refer to the graphics distributed with the original games. It is difficult, but possible, to replace the animated flats using a PWAD. (In particular, a PWAD must contain all the flats used, not just those replaced.) In that case, the number of frames, and of course the description, may vary.

The SWATER textures are left over from the Doom press release.

Source

 * Unofficial Doom Specs