Converting Blizzard BLP Textures to PNG: Warcraft III & WoW Asset Decoding

For more than two decades, Blizzard Entertainment games—including Warcraft III: Reign of Chaos, The Frozen Throne, and World of Warcraft—have stored character models, spell icons, terrain tilesets, and user interface graphics in the proprietary BLP (Blizzard Picture) container format.

Because mainstream image editors and web browsers cannot view or open .blp files directly, extracting and converting them into standard 32-bit RGBA PNG files is an essential step for game modders, web UI developers, and digital archivists.

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1. BLP1 vs BLP2 Evolution

Blizzard's texture format evolved across two major generations:

1. BLP1 (Warcraft III): Focused on 256-color paletted textures with dedicated 1-bit, 4-bit, or 8-bit alpha channels, as well as embedded JPEG-compressed graphics.
2. BLP2 (World of Warcraft): Expanded to incorporate DirectX texture compression (DXT1/BC1, DXT3/BC2, DXT5/BC3) and raw 32-bit BGRA uncompressed mipmapped textures.

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2. BLP Header Anatomy

Both BLP variants begin with a 4-byte ASCII signature followed by metadata offsets:

| Field | Type | Description | |---|---|---| | Magic | 4 bytes | BLP1 or BLP2 | | Type | uint32 | 0 = JPEG, 1 = Direct / Paletted | | Compression / Alpha | uint8 / uint32 | Color encoding flags and alpha bit depth | | Width | uint32 | Texture width in pixels | | Height | uint32 | Texture height in pixels | | Mipmap Offsets | 16 x uint32 | Byte offsets into the file for each mipmap level | | Mipmap Lengths | 16 x uint32 | Byte sizes of each mipmap level payload |

In paletted files, a 1024-byte palette follows the header, consisting of 256 four-byte BGRA color entries.

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3. Paletted Color and Alpha Channel Extraction

In Warcraft III BLP1 paletted images, pixel color indices and alpha transparency values are stored separately:

1. Color Indices: width * height sequential bytes indexing into the 256-entry BGRA palette.
2. Alpha Channel:
   • 8-bit Alpha: 1 byte per pixel providing full 256-step alpha blending.
   • 4-bit Alpha: 4 bits per pixel (packed 2 pixels per byte), scaled to 0..255.
   • 1-bit Alpha: 1 bit per pixel (packed 8 pixels per byte) for binary cutout transparency.

convrtr combines the palette RGB colors with the extracted alpha values into an un-premultiplied 32-bit RGBA pixel array.

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4. Hardware DXT Decompression

For World of Warcraft BLP2 textures compressed with S3TC/DXT:

• DXT1: 4x4 pixel blocks compressed into 8 bytes, interpolating between two 16-bit 5:6:5 RGB endpoints.
• DXT5: 4x4 blocks compressed into 16 bytes, with 8 bytes dedicated to 8-step interpolated alpha.

convrtr decompresses these blocks directly in TypeScript, synthesizing transparent 32-bit RGBA PNG files in memory with zero external dependencies.