From Dark Omen Wiki

Contents 1 Overall Structure 2 Sprite File Header 3 Frame Header 3.1 1. Frame Type 3.2 2. Frame Compression 3.3 3. Offset Into the Frame Data

Overall Structure

The overall structure of a sprite file looks a like this (in this order):

SPRITE FILE HEADER (32 bytes)

FRAME HEADER 1
FRAME HEADER 2
.
.
FRAME HEADER number of frames

PALETTE ENTRY 1
PALETTE ENTRY 2
.
.
PALETTE ENTRY number of palette entries

FRAME 0 DATA
FRAME 1 DATA
.
.
FRAME number of frames DATA 

For a description of each part, see below.

Sprite File Header

The file header consists of the first 32 bytes of the file

struct SpriteFileHeader
{
    char ID[4];            // = 'WHDO'
    int  filesize;         // = the size of the file in bytes
    int  version?;         // = 32
    int  frameDataOffset   // = file offset to beginning of the frame data
    int  paletteDataOffset // = file offset to beginning of the palette data
    int  paletteEntryCount // = the total number of palette entries (*)
    int  paletteCount      // = how many unique palettes exist in this sprite
    int  frameCount        // = the number of frames of animation in this sprite   
}

(* This is the total number of colours stored in the file, each of which belongs only to one palette. e.g. there may be 300 colours stored in the file, spread throughout 5 different palettes; the paletteEntryCount is 300, the paletteCount is 5 )

Frame Header

There is one frame header for each animation frame in the sprite:

struct FrameHeader
{
     byte frameType;        // see table below (1)
     byte compressionType;  // 0: not compressed, 1: packbits, 2: unknown (2)
     short colourCount;     // = the number of unique colours in the frame
     signed short X;        // = X position of this frame
     signed short Y;        // = Y position of this frame
     short frameWidth;      // = width of this frame
     short frameHeight;     // = height of this frame
     int dataOffset;        // = offset into the frame data (3)
     int compressedSize;    // = length of the compressed frame data
     int uncompressedSize;  // = length of the uncompressed frame data
     int paletteDataOffset; // = offset into the palette data
     int unknown;           // = 0
}

1. Frame Type

Each frame has a type:

Type	Description
4	This is a normal frame.
5	The frame is empty. i.e. there is no frame or palette data associated with this frame. Width and Height are 0.

2. Frame Compression

Frames can be compressed. Unit sprite files seem to use no compression at all, animations for encounters between battles (filename beginning M_) can be compressed using two methods (or be uncompressed).

Compression Type	Decription
0	Uncompressed - the frame data consists of 1 byte indices in to the associated palette
1	Packbits - the frame is compressed using the 'packbits' compression scheme
2	Unknown - the frame is compressed using an unknown compression algorithm

3. Offset Into the Frame Data

This offset is into the frame data. i.e. this frame's data is at SpriteHeader.frameDataOffset + FrameHeader.dataOffset