The DDS file (.dds)

A DDS file holds a texture: an image meant to be wrapped onto 3D objects by a graphics card. Microsoft introduced it with DirectX 7.0 in 1999, and it has become a common texture format for PC games, including the textures in many game mods. Where a PNG stores one picture as compactly as possible on disk, a DDS stores it the way the GPU wants it in memory.

• Block-compressed. Most DDS files use BC compression, which the graphics card decodes in hardware as it draws. A compressed texture takes a quarter or less of the memory, and the game doesn't have to decompress it.
• Mipmaps included. A DDS can carry ready-made smaller copies of the image, each half the size of the previous, which the GPU uses for distant surfaces.
• More than flat images. One file can hold a cube map (six faces of a sky or reflection), a volume texture (a 3D grid of pixels) or an array of textures.

The compression is lossy and tuned for speed of decoding, not for photos on a screen, so a DDS is a poor format for sharing pictures. Browsers don't display it.

Block compression splits the image into 4 × 4 pixel blocks and stores each one in a fixed number of bytes, so the GPU can find and decode any block instantly. BC1 to BC3 come from S3 Texture Compression (S3TC), developed by S3 Graphics, and were known as DXT1 to DXT5 in DirectX 9. Direct3D 10 and 11 added the rest.

In a BC1 block, two colours are stored and two more are blended from them: one-third and two-thirds of the way between. Each pixel picks one of the four with a 2-bit index. If the first colour isn't the larger number, the block switches to three colours plus fully transparent, which is BC1's 1-bit alpha. BC7 uses the same idea with 8 different modes per block and better precision, which is why it looks much better than BC1 at twice the size.

A full mip chain adds about one-third. Because the formats work on 4 × 4 blocks, Direct3D requires the dimensions of a block-compressed texture to be multiples of 4; most game textures are powers of two, such as 512, 1024 or 2048.

After the headers, the data is just surfaces one after another, with no markers between them. A reader works out where each one starts from the size, format and counts in the header:

• Mipmaps: the full-size image first, then each smaller level down to 1 × 1. The header's mipmap count says how many levels there are.
• Cube maps: six faces in the order +X, −X, +Y, −Y, +Z, −Z, each with its own mip chain. Direct3D 10 and later require all six; Direct3D 9 accepted partial cube maps.
• Volume textures: slices of a 3D texture, with the header's depth field set.
• Texture arrays: several textures of the same size and format, possible only with the DX10 header; each array element comes with all its mipmaps.

Since nothing marks where one surface ends, a header with a wrong mipmap count or format makes every later surface garbage. That's the most common cause of textures that look like noise or crash a game.

The original header describes pixel formats with bit masks or a four-character code such as DXT1, DXT5, ATI2. That can't express everything Direct3D 10 and later support: BC6H, BC7, sRGB variants, floating-point formats or texture arrays. So when the code is DX10, a 20-byte extra header follows:

Not every tool reads the DX10 header, and Microsoft recommends the legacy header for uncompressed RGB and BC1 to BC3 when compatibility matters. Use the DX10 header for BC6H, BC7, sRGB or arrays, and check that the game or engine supports it.

A common workflow keeps the artwork as PSD, TGA or PNG and converts it to DDS as a build step. Editing the DDS itself and saving again compresses an already-compressed image a second time and loses a little more quality each time.

• The app can't open it. Older tools only understand legacy DXT1 to DXT5 files. BC7, BC6H and anything with the DX10 header need a current version of GIMP, Paint.NET, texconv or NVIDIA's tools. Windows' built-in imaging component only decodes BC1 to BC3.
• It's upside down. DirectX treats the top row as the first row; some OpenGL-based tools and engines expect the bottom row first. GIMP's DDS export has a "Flip image vertically" option.
• A normal map looks red and green, not blue-purple. BC5 stores only two channels (X and Y); the third is calculated by the shader. That's normal for the file.
• Transparent areas are black, or edges have dark fringes. The alpha mode doesn't match: premultiplied versus straight alpha, or a BC1 file where alpha was expected.
• Colours are too dark or too light in the game. An sRGB texture was saved as linear, or the other way round. Match the format to what the engine expects (_SRGB formats for colour textures in most engines).
• Blocky or banded areas. BC1 artefacts, especially on gradients. Use BC7 for colour textures when the target hardware supports it.
• A mod texture crashes the game or shows noise. Mismatched format, size or mipmap count. Save it in the same format and size as the original texture, with mipmaps if the original had them.

• GIMP (Windows, macOS, Linux) has opened and saved DDS since version 2.10.10. Export offers BC1 to BC5 and BC7, mipmaps, cube maps, volume maps and texture arrays.
• Paint.NET (Windows) opens and saves DDS files.
• Visual Studio's Image Editor opens DDS files, shows their mipmap levels and can generate mipmaps.
• NVIDIA Texture Tools Exporter (Windows, NVIDIA GPU) is a standalone app and a Photoshop plug-in that reads and writes DDS with BC1 to BC7. Photoshop has no DDS support of its own.
• texconv, part of Microsoft's open-source DirectXTex, converts on the command line on Windows.

# PNG to BC7 DDS with a full mip chain (writes texture.dds)
texconv -f BC7_UNORM -m 0 texture.png

# all PNGs in a folder: sRGB in and out, DX10 header, overwrite
texconv -f BC7_UNORM -m 0 -dx10 -srgb -y -o C:\out textures\*.png

# DDS back to PNG
texconv -ft png texture.dds

To check what's in a file, OpenImageIO's oiiotool --info -v texture.dds prints the size, channels and compression. A DDS file is plain image data with no scripts; with game mods, the real risk is in executable files that come with them, not in the textures.