A DELT LFD resource codes a static image.
They are generally used as backgrounds for ANIMs, but their most important use is in the briefings 'texts' (the scrollable section of the briefing screen) which are a DELT stored in dfbrief.lfd for each level.

Note: the .dlt extension is a convention adopted by add-on developers when writing conversion programs, there are no real DLT files in DARK FORCES.

DELT_Header IS { OffsX int // X offset OffsY int // Y offset SizeX int // X size - 1 ! SizeY int // Y size - 1 ! }

After the header, a variable number of line descriptors follow.
They are composed of an header and some data.

DELT_Line { SizeAndType int // size and compression of the line StartX int // X position of line start StartY int // Y position of line start }

StartX and StartY indicate the point where to start the drawing. You can start in the middle of a line, and draw a portion of it. Lines need not be in consequential order. You can split one line in more than one section. Portions not covered are, of course, transparent.

Bits 1-15 of SizeAndType indicate the number of pixels described in this section.

If bit 0 of SizeAndType is 0, the byte following the header contains the number of bytes to copy.
Those bytes follow.

If bit 0 is 1, data compressed with RLE follows.
This data may be composed of copy and RLE parts, which is indicated by bit 0 of the count byte.