The Compress sample demonstrates how to load the DDS file format into a compressed texture surface. DDS textures can be created using the DxTex program included with the DirectX SDK.
Source: (SDK root)\Samples\Multimedia\D3dim\Src\Compress
Executable: (SDK root)\Samples\Multimedia\D3dim\Bin
Press F1 to see available commands.
The file format is called DDS because it encapsulates the information in a DirectDrawSurface. The data can be read directly into a surface of a matching format.
The ReadDDSTexture function demonstrations how a DDS surface is read from a file.
A DDS file has the following format:
| DWORD dwMagic | (0x20534444, or "DDS ") |
| DDSURFACEDESC2 ddsd | Information about the surface format |
| BYTE bData1[] | Data for the main surface |
| [BYTE bData2[] | Data for attached surfaces, if any, follow |
This format is easy to read and write, and supports features such as alpha and multiple mip levels, as well as DXTn compression. If it uses DXTn compression, it may be one of 5 compressed types. See Compressed Texture Surfaces.
After the texture is read in, a pixel format must be chosen that is supported by the renderer. In the sample, the supported pixel formats are enumerated and stored in a linked list. After the pixel formats are collected, the list is searched for a best match, using the FindBestPixelFormatMatch function.
Some Direct3D devices such as the reference rasterizer and some hardware devices can render compressed textures directly. But for renderers that don't directly support this, the compressed surface must be blitted to a non-compressed surface. The function BltToUncompressedSurface demonstrates how this is done.