PAA File Format

PAA texture file structure

Introduction

Of the many image file formats 'out there', such as jpeg, such as gif. BI choose to use a specially developed file format (paa) as the base texture file for all engine types.

The reason for this is the raw data within the file can be passed directly to Miscrosft's Direct X as a DCT1 picture without further massaging.

While all engines except Elite also support JPG files, PAA files can result in much better performance.

Main Format

Overall structure of a Paa file is

struct overall
{
 ushort TypeOfPaa;
 struct HeaderTags {...};
 ushort EndofTags;               // always zero
 struct MipMap_DataBlocks{....};
 byte   EndOfMaps[6];            // always zero
};

Every PAA file starts with a TypeOfPaa

ushort TypeOfPaa; // type of texture, known values are 
		   // 0xFF01 DXT1 compressed texture (may have 1 bit alpha map, check MSDN documentation for details)
		   // 0x1555 Uncompressed RGBA 5:5:5:1 texture
	           // 0x4444 Uncompressed RGBA 4:4:4:4 texture
                  // 0x4747 Uncompressed Index Palette texture
		   // 0x8080 Uncompressed Luminosity/Alpha map. Actual color of texture is derived from AVGCTAGG tag?
		   // 0xFF05 XBox version only. Most likely DXT5 compressed texture

followed by one or more header tags

struct PAA_Tag {
	byte    name[8];	// name of tag is actually reversed when written in file, 
				// so OFFSTAGG would be written as GGATSFFO. See  below for known tags.
	ULONG	tag_size;	// size of this tag
	byte    data[tag_size];	// size * bytes of actual data
}

followed (obviously) by a EndofTags ushort of zero, indicating, no more tags !!

Known header tags

There are several knowns header tags.

OFFSTAGG

This tag has always 16 ULONG entries which contain pointers to actual mipmap data inside file. Note that while 16 offsets are always stored, actual number of mipmaps might be smaller. In this case, residual offset data is marked as 0x00000000. Most (but not all) paa files contain this tagg. For those that aren't, the offset to the next block of data is developed from the size of the previous one(s).

AVGCTAGG

This tag contains average color of texture, probably used in rendering 8:8 luminosity/alpha textures.

FLAGTAGG

Marks if texture contains transparency. Value 1 means basic transparency, 2 means alpha channel is not interpolated. This flag should be always present in LOD textures with 1-bit alpha with value of 2 or there will be "ghost outlines" on LOD textures when viewed from distance. Note that this flag must be present in texture file when binarizing model, because Binarize stores information about how to render textures in actual P3D file.

If format not 0x4747 then end of header tag data is marked by UWORD 0x0000, else this is a `size of palette` and next block it`s a palette(sizeof = `size of palette` * 2)

Header tags found in Operation Flashpoint: Elite

Purpose of these headers is not yet known.

MAXCTAGG Contains color of brightest pixel in texture?

SWIZTAGG Found in detail map textures. Seems to always contain 4 bytes of data.

Mipmap data

After end-of-header marker, actual mipmap data follows. Tag OFFSTAGG (which is, obviously, a mandatatory tag) contains (up to) 16 offsets. Each offset points to the following struct type (relative to start of the file).

struct Mipmap_Data {
	UWORD	width;		// width of this mipmap
	UWORD	height;		// height of this mipmap
	UCHAR	size[3];	// size of compressed texture data. this is 24-bit unsigned integer. 
	UCHAR	data[size];	// actual texture data 
};

Note that the size reflects the size of data in the file not the actual size if that data if is compressed.

The size of output data for Dct1 format is (width*height / 2)

Size of output data for Dct5 format is (width*height)

Texture types 0x4444,0x4747,0x1555 and 0x8080 are always stored in compressed format. See Pbo_File_Format#Data Compression for details. DXT1 textures are stored "as is".

After last mipmap, there are six (6) bytes set to 0x00 to mark end of texture data.

Alpha channel interpolation

These two images visualize difference between alpha channel interpolation (FLAGTAGG header tag value).

FLAGTAGG = 1, interpolated alpha channel (default behaviour)

FLAGTAGG = 2, alpha channel interpolation disabled

Bibliography

Feersum's original posting on BIS forums: Paa/pac texture format documentation
MSDN documentation on DXT1 textures: DirectX: Opaque and 1-Bit Alpha Textures