DDS文件格式解析

DDS是DirectDrawSurface的缩写，它是DirectX纹理压缩（DirectX TextureCompression，简称DXTC）的产物。

DXTC减少了纹理内存消耗的50%甚至更多，有3种DXTC的格式可供使用，分别是DXT1，DXT3和DXT5。

DXT1 压缩比例：1:8

压缩比最高，它只有1BitAlpha，Alpha通道信息几乎完全丧失。一般将不带Alpha通道的图片压缩成这种格式。如WorldWind用的卫星图片

DXT3 压缩比例：1:4

使用了4BitAlpha，可以有16个Alpha值，可很好地用于alpha通道锐利、对比强烈的半透和镂空材质。

DXT5 压缩比例：1:4

使用了线形插值的4BitAlpha，特别适合Alpha通道柔和的材质，比如高光掩码材质。

 

dds格式分为2个部分：
1、dds头
2、dxt压缩数据

 

dds的头文件信息如下：

typedef struct _DDCOLORKEY
{
    uint32      dwColorSpaceLowvalue;   // lowboundary of color space that is to
                                        //be treated as Color Key, inclusive
    uint32      dwColorSpaceHighvalue;  // highboundary of color space that is
                                        //to be treated as Color Key, inclusive
} DDCOLORKEY;

typedef struct _DDSCAPS2
{
    uint32      dwCaps;        // capabilities of surface wanted
    uint32      dwCaps2;
    uint32      dwCaps3;
    union
    {
        uint32      dwCaps4;
        uint32      dwVolumeDepth;
    }DUMMYUNIONNAMEN(1);
} DDSCAPS2;

typedef struct _DDPIXELFORMAT
{
    uint32      dwSize;                // size of structure
    uint32      dwFlags;                //pixel format flags
    uint32      dwFourCC;              // (FOURCC code)
    union
    {
        uint32  dwRGBBitCount;          //how many bits per pixel
        uint32  dwYUVBitCount;          //how many bits per pixel
        uint32  dwZBufferBitDepth;      //how many total bits/pixel in z buffer (including any stencilbits)
        uint32  dwAlphaBitDepth;        //how many bits for alpha channels
        uint32  dwLuminanceBitCount;    //how many bits per pixel
        uint32  dwBumpBitCount;        // how many bits per "buxel", total
        uint32  dwPrivateFormatBitCount;// Bits per pixel of private driverformats. Only valid in texture
                                        //format list and if DDPF_D3DFORMAT is set
    }DUMMYUNIONNAMEN(1);
    union
    {
        uint32  dwRBitMask;            // mask for red bit
        uint32  dwYBitMask;            // mask for Y bits
        uint32  dwStencilBitDepth;      //how many stencil bits (note: dwZBufferBitDepth-dwStencilBitDepth istotal Z-only bits)
        uint32  dwLuminanceBitMask;    // mask for luminance bits
        uint32  dwBumpDuBitMask;        //mask for bump map U delta bits
        uint32  dwOperations;          // DDPF_D3DFORMAT Operations
    }DUMMYUNIONNAMEN(2);
    union
    {
        uint32  dwGBitMask;            // mask for green bits
        uint32  dwUBitMask;            // mask for U bits
        uint32  dwZBitMask;            // mask for Z bits
        uint32  dwBumpDvBitMask;        //mask for bump map V delta bits
        struct
        {
            uint16    wFlipMSTypes;      // Multisample methods supported via flip for this D3DFORMAT
            uint16    wBltMSTypes;        //Multisample methods supported via blt for this D3DFORMAT
        }MultiSampleCaps;

    }DUMMYUNIONNAMEN(3);
    union
    {
        uint32  dwBBitMask;            // mask for blue bits
        uint32  dwVBitMask;            // mask for V bits
        uint32  dwStencilBitMask;      // mask for stencil bits
        uint32  dwBumpLuminanceBitMask; // mask for luminance in bump map
    }DUMMYUNIONNAMEN(4);
    union
    {
        uint32  dwRGBAlphaBitMask;      //mask for alpha channel
        uint32  dwYUVAlphaBitMask;      //mask for alpha channel
        uint32  dwLuminanceAlphaBitMask;// mask for alpha channel
        uint32  dwRGBZBitMask;          //mask for Z channel
        uint32  dwYUVZBitMask;          //mask for Z channel
    }DUMMYUNIONNAMEN(5);
} DDPIXELFORMAT;

typedef struct _DDSURFACEDESC2
{
    uint32              dwSize;                // size of the DDSURFACEDESC structure
    uint32              dwFlags;                //determines what fields are valid
    uint32              dwHeight;              // height of surface to be created
    uint32              dwWidth;                //width of input surface
    union
    {
        int32            lPitch;                // distance to start of next line (return value only)
        uint32          dwLinearSize;          // Formless late-allocated optimized surface size
    }DUMMYUNIONNAMEN(1);
    union
    {
        uint32          dwBackBufferCount;      //number of back buffers requested
        uint32          dwDepth;                //the depth if this is a volume texture
    }DUMMYUNIONNAMEN(5);
    union
    {
        uint32          dwMipMapCount;          //number of mip-map levels requestde
                                                //dwZBufferBitDepth removed, use ddpfPixelFormat one instead
        uint32          dwRefreshRate;          //refresh rate (used when display mode is described)
        uint32          dwSrcVBHandle;          //The source used in VB::Optimize
    }DUMMYUNIONNAMEN(2);
    uint32              dwAlphaBitDepth;        //depth of alpha buffer requested
    uint32              dwReserved;            // reserved
    void*              lpSurface;              //pointer to the associated surface memory
    union
    {
        DDCOLORKEY      ddckCKDestOverlay;      //color key for destination overlay use
        uint32          dwEmptyFaceColor;      // Physical color for empty cubemap faces
    }DUMMYUNIONNAMEN(3);
    DDCOLORKEY          ddckCKDestBlt;          //color key for destination blt use
    DDCOLORKEY          ddckCKSrcOverlay;      // color key for source overlay use
    DDCOLORKEY          ddckCKSrcBlt;          // color key for source blt use
    union
    {
        DDPIXELFORMAT  ddpfPixelFormat;        //pixel format description of the surface
        uint32          dwFVF;                  //vertex format description of vertex buffers
    }DUMMYUNIONNAMEN(4);
    DDSCAPS2            ddsCaps;                //direct draw surface capabilities
    uint32              dwTextureStage;        // stage in multitexture cascade
} DDSURFACEDESC2;

struct DDS_IMAGE_DATA
{
    int  width;
    int  height;
    int    components;
    unsignedint   format;
    int      numMipMaps;
    unsignedchar *pixels;//这个是压缩的图像信息文件,glCompressedTexImage2DARB就是加载这个。
};