A flat is the picture a floor or ceiling is covered with. It has no header and no compression: 4096 bytes, 64 rows of 64 palette indexes, top row first. Its size is fixed, so the lump’s length is all a reader needs.
0Row 0 64 palette indexes64 bytes64Row 164 bytes…Rows 2 to 633968 bytes
Where flats are
Flats lie between the markers F_START and F_END, and the
engine takes every lump between them as a flat, in order. Sectors name their floor and
ceiling flats, and the sky’s flat is F_SKY1. Animated floors, such as
water, are runs of flats whose first and last names the engine knows; it shows each one
in turn. A PWAD’s flats go between FF_START and FF_END, for a
source port or a merging tool.
Flats are laid on the map on a grid of 64 units from the map’s origin, and cannot be moved or turned in Doom.
Raw full-screen pictures
Heretic and Hexen keep some full-screen pictures, such as the title screen, the same way: 64,000 bytes, 200 rows of 320 palette indexes, with no header. Doom keeps its screens in the picture format instead. The explorer takes a lump of exactly 64,000 bytes that is not a picture as a 320 × 200 screen.