Flat

From ZDoom Wiki

A flat is an image that is drawn on the floors and ceilings of sectors. Flats are very different from wall textures or patches. Flats are a raw collection of pixels with no offset or other dimension information as in patches. The pixels comprise an index into the palette, the same as for regular patches.

ZDoom supports larger flats. Flats can be 64x64 (stock DOOM), 128x128 or 256x256 pixels square. The latter two are called hires or large flats. Images that do not match these dimensions will not look as intended. The image by default is resized to fit the 64x64 size. However, you can use special commands either in ACS or as a line special to display them any size you want.

Larger flats are made by PWAD editing tools such as XWE and DeePsea. You load the large flats between FF_START/FF_END markers in your favorite Wad tool that supports large flats and the creation of empty markers. This is the same procedure as for regular sized flats (64x64).

The FF_START/FF_END markers are two zero sized lumps. Create these according to the method available in the PWAD editing tool of your choice. You can then enter the name of the flat in the sector mode of your level editor - but keep in mind that some editors may not be able to support this type of texture properly, but they usually display at least a "corner".

When using it (the word "FLAT") to define the flat used for the background of the exit text's screen or the intermission's screen, it must be in the CLUSTERDEF x section of the MAPINFO lump:
clusterdef <number>
flat <flatname>

For example:
clusterdef 1
flat brick

If this picture is a highres one, it must be stored in the GFX section of the pwad while using XWE (for example), like any other highres picture, and not in the FLAT section (not between FF_START and FF_END markers), and a new lump named HIRESTEX created.
Here is an example of a HIRESTEX lump:
define ZIGVERT1 128 128
define PAVEROSE 256 256
(a highres picture must be stored with the "Load (RAW DATA)" option of the menu).

ZDOOM also supports image lumps between TX_START/TX_END. These are used for PNG and DOOM graphic format images, not for flats.