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BLP files are Blizzard's texture format, used for many games. World of Warcraft uses the BLP2 format in particular. The BLP file structure consists of a header, up to 16 mipmaps of the texture, and a palette. Texture sizes must be powers of two, though the two dimensions do not have to be equal; 512x256 is valid, but 512x200 is not. The first mipmap (mipmap #0) is the full size image; each subsequent mipmap halves both dimensions. The final mipmap should be 1x1.

It should be noted that World of Warcraft does ship a small number of BLP files with dimensions that are not powers of two; 768x128 is used for many cube map textures, for example. In this case, the final mipmap may not be 1x1.

The .BLP version 2 header

struct blp2header {
  uint8_t    ident[4];           // "BLP2" magic number
  uint32_t   type;               // Texture type: 0 = JPG, 1 = S3TC
  uint8_t    compression;        // Compression mode: 1 = raw, 2 = DXTC
  uint8_t    alpha_bits;         // 0, 1, 4, or 8
  uint8_t    alpha_type;         // 0, 1, 7, or 8
  uint8_t    has_mips;           // 0 = no mips levels, 1 = has mips (number of levels determined by image size)
  uint32_t   width;              // Image width in pixels
  uint32_t   height;             // Image height in pixels
  uint32_t   mipmap_offsets[16]; // The file offsets of each mipmap, 0 for unused
  uint32_t   mipmap_lengths[16]; // The length of each mipmap data block
  uint32_t   palette[265];       // A set of 256 ARGB values used as a color palette
} blp2header;

World of Warcraft does not use JPG textures. A discussion of BLP files using JPG compression is beyond the scope of this article. The type flag should always be 1, which indicates the use of either RAW, DXT1, DXT3, or DXT5 compression. Each compression type will be covered separately.

The .BLP color palette

The header is always followed by a 256-entry color table. Each entry a 32-bit BGRA 8888 value. This table is only used for RAW images, but is present in all BLPs regardless.

RAW compression

RAW1

If compression is set to 1, each mipmap is stored as an array of 8-bit values, one per pixel, left to right, top to bottom. Each value is an index to the palette.

If alpha_bits is greater than 0, an alpha channel will immediately follow the image data, and comes in 1, 4, and 8 bit varieties. The 1 and 4 bit versions have multiple values packed into a single byte, with the least significant bit belonging to the first packed value.

RAW3

With the compression set to 3, each mipmap contains what appears to be 32 bit BGRA data. alpha_bits seems to represent a set of bit flags rather than depth, as all images of this type seem to have 4 bytes per pixel regardless of depth, and it has been seen to exceed 8. Their meaning is unknown.

DXTn compression

If compression is set to 2, each mipmap is composed of 4×4 blocks of pixels. The blocks and the pixels within each block are ordered from left to right, top to bottom.

See Wikipedia's entry on DXT compression for the technical details.

DXT1

If alpha_type is 0, then DXT1 compression is used.

Each block is 64 bits and begins with two 16 bit values, and are used to derived a 4 color palette.

The values are interpreted as 565 RGB colors, with the least significant bits corresponding to blue, to create the first two colors in the palette.

If the first value is less than or equal to the second, the final entry of the palette is reserved. If alpha_bits is 0, the reserved color is black. If alpha_bits is 1, the reserved color is transparent.

The remaining colors are created by interpolating between the first two colors in the palette.

The remaining 32 bits are 16 2-bit values acting as a lookups to specify the colors in the block.

DXT3

If alpha_type is 1, then DXT3 compression is used.

Each block is 128 bits and begins identically to DXT1, except that no special color is reserved in the palette.

It is followed by 16 4-bit values corresponding to the alpha values for each of the pixels in the block.

DXT5

If alpha_type is 7, then DXT5 compression is used. This format was first used for Burning Crusade images.

Each block is 128 bits and begins with two 8-bit values to create an 8 element lookup table for alpha values.

The first two elements in the lookup table are copies of those values.

If the first value is less than or equal to the second, the final two entries of the lookup table are reserved for transparent and opaque.

The remaining entries are created by interpolating between the first two entries in the lookup table.

The next 48 bits make up 16 3-bit values acting as lookups specifying the alpha values for each of the pixels in the block.

The remaining 64 bits are identical to DXT1, except that no special color is reserved in the palette.

Sample Files

Below is a list of BLP files, each with some unique characteristic.

File Encoding Features
GLUES\LoadingBar\Loading-BarGlow.blp RAW1 No alpha
CURSOR\Attack.blp RAW1 1-bit alpha
CURSOR\Buy.blp RAW1 8-bit alpha
Icons\Trade_Alchemy.blp DXT1 No alpha
AuctionFrame\BuyoutIcon.blp DXT1 1-bit alpha
Icons\INV_Fishingpole_02.blp DXT3 4-bit alpha
Icons\Ability_Rogue_Shadowstep.blp DXT5 8-bit alpha
BUTTONS\UI-PaidCharacterCustomization-Button.blp RAW3 BGRA color

The following files require an MPQ viewer to access, as they aren't exposed by Blizzard's AddOn Kit.

File Encoding Features
Character\Tauren\Female\TAURENFEMALESKIN00_01_EXTRA.blp RAW1 4-bit alpha
Environments\Stars\HellFireSkyNebula03.blp DXT5 No alpha
Textures\SunGlare.blp RAW3 alpha_bits=136
TILESET\Terrain Cube Maps\TCB_CrystalSong_A.blp DXT1 width=768
TILESET\Terrain Cube Maps\oilslickenvA.blp RAW3 alpha_bits=1

Conversion tools

There exist BLP tools to convert both from and to the BLP format. They can be found in UI Authors resource list.

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