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 Microsoft's Direct X as a DXT1 picture (eg) 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;        //OPTIONAL
 Tagg     Taggs[...];       //OPTIONAL
 Palette  Palette[...];
 MipMap   MipMaps[...];
 ushort   Always0;
};

TypeOfPaa (optional)

If the file does not start with one of these Paa types, the image data is stored as indices to a color palette and the file starts with Tagg structure instead.

• 0xFF01 DXT1 texture (may have 1 bit alpha map, check MSDN documentation for details)
• 0xFF02 DXT2 texture (Oxygen 2)
• 0xFF03 DXT3 texture
• 0xFF04 DXT4 texture (Oxygen 2)
• 0xFF05 DXT5 texture
• 0x4444 RGBA 4:4:4:4 texture
• 0x1555 RGBA 5:5:5:1 texture
• 0x8888 RGBA 8:8:8:8 texture (Oxygen 2)
• 0x8080 Luminosity/Alpha map. Actual color of texture is derived from AVGCTAGG tag?

Taggs

Tagg 
{
 char   signature[4]; //always "GGAT"
 char   name[4];      //name of the tagg in reversed order (for example: "CGVA")
 ulong  dataLen;
 byte   data[dataLen];
}

Taggs are read until no "GGAT" signature is encountered. Palette structure follows.

AVGCTAGG

Average Colour

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

{
  char  "GGATCGVA";
  ulong len; // 4 bytes
  ulong RGBA;//FF443D39
}

FLAGTAGG

{
  char  "GGATGALF";
  ulong len;   // 4 bytes
  ulong range; // 0 to 2
}

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.

MAXCTAGG

{
  char  "GGATCXAM";
  ulong len;   // 4 bytes
  ulong  data; // FFFFFFFFF no other value seen so far
}

Contains color of brightest pixel in texture?

SWIZTAGG

{
  char  "GGATZIWS";
  ulong len;   // 4 bytes
  ulong  data; // 0x05040203
}

Swizzle is apparently used to modify texture components processing like swizzle modifiers in pixel shaders. For example ArmA sky texture has green channel stored in alpha channel and inversed to take advantage from feature that in DXT5 64 bits are used for alpha channel in each block and 64 bits for RBG, giving double the accuracy to green channel as opposed to storing texture just normally.

Exact format of swizzle data is still unknown. swizzle data:

char channelSwizzleA; 
char channelSwizzleR; 
char channelSwizzleG; 
char channelSwizzleB;

format of swizzle char:

bits 7-4 = 0;
bit 3 - "1" flag. All channel data must be set to 0xff;
bit 2 - "negate flag". Channel data must be negated. 
bit 1-0 - number of color channel:
00 - Alpha channel
01 - Red channel
10 - Green channel
11 - Blue channel

for example (*_nohq.paa textures)

swizzle data is:

0x05 - Alpha, Negated, stored in Red; 
0x04 - Red, Negated, stored in Alpha; 
0x02 - Green, Stored as is; 
0x03 - Blue, Stored as is;

PROCTAGG

{
  char  "GGATCORP";
  ulong len;   //  strlen(text)
  char  text   // NOT ASCIIz
}

this is a non asciiz string (not zero terminated)

example:

x = ((u+1)*0.5);
y = ((v+1)*0.5);

if (y<0.5) then
{
 sharpOut = 20;
 sharpIn = 60;
 
 offset = x-0.5;
 if (offset<0) then
 {
   edge = (1+offset)^sharpOut;
 }
 else
 {
   edge = (1-offset)^sharpIn;
 };
 res = edge*y*2;
}
else
{
 sharpOut = 20-40*(y-0.5);
 sharpIn = 60-125*(y-0.5);
 
 offset = x-0.5;
 if (offset<0) then
 {
   edge = (1+offset)^sharpOut;
 }
 else
 {
   edge = (1-offset)^sharpIn;
 };
 res = edge;
 sdCoef = y * 2 - 1;
 shoreDisappear = 1 - sdCoef^2;
 res = res * shoreDisappear;
};
a = 1;
r = res;
g = res;
b = res;

OFFSTAGG

{
  char  "GGATSFFO";
  ulong len;         // 16 * sizeof(ulong)
  ulong offsets[16];
}

Example:
= 6 entries. last 10 unused
256 x 128 Size=16384
128 x 64 Size=4096
64 x 32 Size=1024
32 x 16 Size=256
16 x 8 Size=64
8 x 4 Size=16

MipMap data is presented in 'blocks'. One or more 'blocks' exist in a paa file.

This tag declares where each of these blocks are in the file, relative to start of file.

The location of each block is already known, relative to the size of the previous block (if any). So, although almost always present in paa files, it's use, is redundant.

This tag always contains 16 ULONG offsets. Each one is a hard offset to actual mipmap data relative to start of file.

Not all entries are used (obvuously) since most paa files contain less than 16 mipmaps. Unused offsets contain the value 0x00000000. There are no known examples of splattered offset entries. All offsets after the first 0 entry are 0 as well.

Palette

Palette
{
 ushort nPaletteTriplets;                // always 0 except for index palette type
 bytes  BGR_Palette[nPaletteTriplets][3]; // only exists if nPaletteTriplets > 0
}

palette triplets when they exist, consist of BGR values

Mipmap

Mipmaps are in contiguous blocks that extend to end of file.

Mipmap
{
	ushort	width;		// width of this mipmap
	ushort	height;		// height of this mipmap
       if (width && height)
       {
	 byte	size[3];	// size of texture data in file. this is 24-bit unsigned integer. 
	 byte	data[size];	// actual texture data compressed or otherwise
       }
};

The last mipmap is a dummy consisting of zero width and height with NO FURTHER DATA

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

Tag OFFSTAGG (if supplied) contains (up to) 16 offsets. Each of those offsets point succsessive mipmap blocks (relative to start of the file).

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

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

DXT1 textures are stored "as is" ( see ->DXTn compress/decompress).

Index Palette Compression

If the file doesn't start with a known Paa type the data array of the mipmap contains the indices to the color palette. But a very basic "compression", which even can make the data slightly bigger, is used. The data is splitted in blocks of this structure:

struct block
{
 byte flag;
 byte data[...];
}

The flag can be of two different types. If the most significant bit of flag is zero, it tells you how much bytes to read. If flag is 0x05 for example you have to read 6 bytes (always one more then the value of flag). If the most significant bit of flag is one it tells you how often the next byte is repeated. If flag is 0x82 for example the value after the flag byte is repeated 3 times (always one more than (flagvalue - 0x80)).

Addenda

Decompression Code

/*
by Flea
*/
int LZSSDecode(unsigned char * in,unsigned char * out,int szin,int szout)
{
        szin = szin > 0? szin: 0x7fffffff;
	int  i, j, k, r = 0, pr, pi = 0,po = 0;
	unsigned int  flags = 0;
        unsigned char buf[0x100F], c;
	for (i = 0; i < 0x100F; buf[i] = 0x20, i++);
	while (pi < szin && po < szout)
        {
		if (((flags >>= 1) & 256) == 0)
                {
                        if(pi >= szin)break;
        		c = in[pi++];
			flags = c | 0xff00;
		}
		if (flags & 1)
                {
                        if(pi >= szin || po >= szout)break;
                        c = in[pi++];
                        out[po++] = c;
                        buf[r++] = c;
                        r &= 0xfff;
		} else
                {
                        if(pi + 1 >= szin)break;
			i = in[pi++];
			j = in[pi++];
			i |= (j & 0xf0) << 4;
                        j  = (j & 0x0f) + 2;
                        pr = r;
 			for (k = 0; k <= j; k++)
                        {
				c = buf[(pr - i + k) & 0xfff];
                                if(po >= szout)break;
        			out[po++] = c;
                                buf[r++] = c;
                                r &= 0xfff;
			}
		}
	}
	return pi;
}

HexDump

The 0x4747 format mentioned in this picture indicates index palette format, because 0x4747 is the "GG" of "GGAT" signature.

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
Squish Compression DXTn compress/decompress