Decompressing Compressed Textures

As with compressing a texture surface, decompressing a compressed texture is performed through DirectDraw blitting services. The HEL performs decompressing blits between system memory surfaces, so these always supported. Likewise, the HEL always performs blits for compressed managed textures (the DDSCAPS2_TEXTUREMANAGE capability). For other situations, the restrictions discussed in the following paragraphs apply.

If the driver supports the creation of compressed video-memory surfaces, then the driver can also perform decompressing blits from a compressed video-memory surface to an uncompressed video or system memory surface, so long as the destination surface has the DDSCAPS_OFFSCREENPLAIN capability.

Blits from compressed system-memory surfaces to uncompressed video-memory surfaces are largely unsupported and should not be attempted, even when the driver supports compressed textures. This does not mean that it is impossible to decompress a compressed system-memory surface and move its contents into a video memory surface; it merely requires an additional step:

To decompress a system memory surface into video memory:

1. Create an uncompressed, offscreen-plain, surface in system memory of the desired dimensions and pixel format.
2. Blit from the compressed system-memory surface to the uncompressed system-memory surface. (The DirectDraw HEL performs decompression in this case.)
3. Blit the uncompressed surface to the uncompressed video-memory surface.