[angvelem]

Смотрим MSDN, там всё описано:

struct DDS_PIXELFORMAT {
DWORD dwSize;
DWORD dwFlags;
DWORD dwFourCC;
DWORD dwRGBBitCount;
DWORD dwRBitMask;
DWORD dwGBitMask;
DWORD dwBBitMask;
DWORD dwABitMask;
};
...
dwRBitMask
Type: DWORD

Red (or lumiannce or Y) mask for reading color data. For instance, given the A8R8G8B8 format, the red mask would be 0x00ff0000.

dwGBitMask
Type: DWORD

Green (or U) mask for reading color data. For instance, given the A8R8G8B8 format, the green mask would be 0x0000ff00.

dwBBitMask
Type: DWORD

Blue (or V) mask for reading color data. For instance, given the A8R8G8B8 format, the blue mask would be 0x000000ff.