DELT LFD Resource Type
Author: Jagged Fel (jaggedfel621@gmail.com)
Site: http://idmr.empirereborn.net
Updated: 2009.10.05
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The DELT resource is image data found in the Concourse and menus. It is a
compressed indexed bitmap, with the color array being defined in a PLTT
resource. Information about the PLTT resource can be found in
Resource_PLTT.txt.

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DELT Structure

The following values are used through this and all of my file definitions
unless otherwise specified:
NAME	LENGTH	DESC
----	------	----
BYTE	1	unsigned 8-bit
CHAR	1	ASCII character
SHORT	2	signed Int16
INT	4	signed Int32

--
0x00	Header
0x10	SHORT	Left
0x12	SHORT	Top
0x14	SHORT	Right
0x16	SHORT	Bottom
0x18	Row[]
	SHORT	Reserved (0)
--

struct Header (size 0x10)
{
	0x0	CHAR[4]	"DELT"
	0x4	CHAR[8]	Name
	0xC	INT	Length
}

struct Row
{
	SHORT	Length
	SHORT	Left
	SHORT	Top
	#if (Length&1 == 0)
	BYTE[Length]	ColorIndexes
	#else
	OpCode[]
	#endif
}

struct OpCode
{
	BYTE	Value
	#if (Value&1 == 0)	(Read)
	BYTE[Value/2]	ColorIndexes
	#else			(Repeat)
	BYTE	ColorIndex
	#endif
}

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DELT Details

As one might expect, the four SHORT values define the outline of the image
data. The Width and Height are simply the difference plus one (to get from
zero-indexed to true size).

-- Row --

Rows read from the top down, left to right. The first SHORT for a given row
gives the number of pixels defined in that row, which can be defined in two
different ways. If Length is even, then the entire set of row definitions are
simply uncompressed indexed values. If Length is odd, then the OpCodes apply
for the image and everything after this paragraph actually applies. In either
case, the Length value is evaluated to (Length >> 1) pixels. The use of
Row.Left is used when "broken" images are used, creating blank spots in the
image when overlaps are required. An example of this would be the galaxy image
in the Tour of Duty screen. On the map itself, there is a blank spot where the
officer's head is. Note that because of blank creations, it is possible to have
two Row declarations with the same Top value. For Rows that do not have blank
spots, the Length is typically the full width of the image and Row.Left will
match Delt.Left. After Left and Top, each pixel in a given Row is assigned an
index value to the palette that has been defined for that image.

There are two different types of OpCodes, which are distinguished by the
first-order bit. Odd OpCodes are Repeat instructions, while even OpCodes
are Read instructions. The OpCode array continues until the number of pixels
processed equals Row.Length.

-- Repeat OpCode --

This operation takes one parameter, ColorIndex. Another OpCode immediately
follows ColorIndex. The number of occurances is calculated by (Value >> 1),
such that
07	Occurs three times
09	Occurs four times
0B	Occurs five times
..

-- Read OpCode --

This OpCode instructs the application to read a given number of pixels and
translate them directly to the image. The number of ColorIndexes to be read is
given by (Value >> 1), such that
04	Reads two pixels
06	Reads three pixels
08	Reads four pixels
...

After the given number of pixels has been read, the next BYTE will be another
OpCode.

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