Creating Compressed Textures

To describe a compressed texture surface in the DDSURFACEDESC2 structure when creating the surface, you must include the following steps:

• Specify the DDSCAPS_TEXTURE flag in the dwFlags member of the DDSCAPS structure, just as you would for any texture.
• Set the dwFourCC member of the DDPIXELFORMAT structure to one of the DXT codes described later.
• Include DDPF_FOURCC in the dwFlags member of DDPIXELFORMAT. Do not set the DDPF_RGB flag.
• Specify a width and height that are a multiple of 4 pixels.

There are two ways to load image data into a compressed texture surface:

• Create a regular RGB or ARGB surface and load a normal bitmap into it, then use IDirectDrawSurface4::Blt or IDirectDrawSurface4::BltFast to blit from the uncompressed surface to the compressed surface. DirectDraw does the compression for you.
• Load the compressed data from a file and copy it directly into the surface memory. (See Accessing Surface Memory Directly.) You can create and convert compressed texture (DDS) files using the DirectX Texture Tool (Dxtex.exe) supplied with the Programmer's Reference. You can also create your own DDS files and either copy the data from compressed surfaces or else use your own routines to convert regular bitmap data to one of the compressed formats.

Note  When you call IDirectDrawSurface4::Lock or IDirectDrawSurface4::GetSurfaceDesc on a compressed surface, the DDSD_LINEARSIZE flag is set in the dwFlags member of the DDSURFACEDESC structure, and the dwLinearSize member contains the number of bytes allocated to contain the compressed surface data. The dwLinearSize parameter resides in a union with the lPitch parameter, so these parameters are mutually exclusive, as are the flags DDSD_LINEARSIZE and DDSD_PITCH.

The advantage of this behavior is that an application can copy the contents of a compressed surface to a file without having to calculate for itself how much storage is required for a surface of a particular width and height in the specific format.

The following table shows the five types of compressed textures. For more information on how the data is stored (you need to know this only if you are writing your own compression routines) see Compressed Texture Formats.

Note  When you blit from a non-premultiplied format to a premultiplied format, DirectDraw scales the colors based on the alpha values. Blitting from a premultiplied format to a non-premultiplied format is not supported. If you try to blit from a premultiplied-alpha source to a non-premultiplied-alpha destination, the method will return DDERR_INVALIDPARAMS. If you blit from a premultiplied-alpha source to a destination that has no alpha, the source color components, which have been scaled by alpha, will be copied as is.