Editing WoW:BLP file (section)

== Compression ==
BLP, DXT1, DXT3, DXT5 or Uncompressed 8-bit per channel ARGB.

=== BLP compression ===
This is the BLP specific image format, where the 'compression' is via the use of a palette, and the use of alpha flags where 'alpha' data is omitted if flags indicate that there is no alpha data at the end of the regular image data for a given 'mip' level.

If 'compression' is 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_depth' is > 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.

=== Uncompressed ===
If 'compression' is 3, each mipmap contains plain 32-bit BGRA data.

The 'alpha_depth' flag represents 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.

=== DXTC compression ===
DXT is an industry standard texture compression format, designed originally for efficiency in real time graphics rendering and decompression in hardware. Most current 3D hardware supports loading some form of DXTC data directly. See [[Wikipedia:S3TC|Wikipedia's entry on DXT compression]] for more detail.

If 'compression' is 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.

==== 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_depth' is 0, the reserved color is black. If 'alpha_depth' 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 8, 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.