bitmap生成

通过常规api生成超大位图时，可能会因为一次性分配大量内存空间而导致失败，因此，需要通过文件流的形式，将多个较小的位图，合并成一个大位图。

bitmap文件结构：

    struct BmpInfo
    {
        unsigned short bfType;
        unsigned long bfSize;
        unsigned short bfReserver1;
        unsigned short bfReserver2;
        unsigned long bfOffBits;

        BmpInfo()
        {
            bfType = 0x4D42;
            bfSize = 0;
            bfReserver1 = 0;
            bfReserver2 = 0;
            bfOffBits = 54;
        }
    };

bitmaphead文件结构：

    struct BmpHeadInfo
    {
        unsigned long biSize;
        long biWidth;
        long biHeight;  // pixel height + orientation
        unsigned short biPlanes;
        unsigned short biBitCount;
        unsigned long biCompression;
        unsigned long biSizeImage;
        long biXPelsPerMeter;
        long biYPelsPerMeter;
        unsigned long biClrUsed;
        unsigned long biClrImportant;

        BmpHeadInfo()
        {
            biSize = 40;  // 40字节
            biWidth = 0;
            biHeight = 0;
            biPlanes = 1;
            biBitCount = 32; // 32位真彩
            biCompression = 0;
            biSizeImage = 0;
            biXPelsPerMeter = 0;
            biYPelsPerMeter = 0;
            biClrUsed = 0;
            biClrImportant = 0;
        }
    };

位图合并：

    SkBitmap bmp1;
    
    if(!SkImageDecoder::DecodeFile("d:\bmp\red.bmp", &bmp1, SkBitmap::kARGB_8888_Config, SkImageDecoder::kDecodePixels_Mode))
    {
        return;
    }

    SkBitmap bmp2;
    if(!SkImageDecoder::DecodeFile("d:\bmp\blue.bmp", &bmp2, SkBitmap::kARGB_8888_Config, SkImageDecoder::kDecodePixels_Mode))
    {
        return;
    }

    int w1, h1, w2,h2;
    w1 = bmp1.width();
    h1 = bmp1.height();
    w2 = bmp2.width();
    h2 = bmp2.height();
    SkASSERT(w1 = w2);
    int w = w1;

    int h = h1 + h2;

    long sz = w * h * 4;  // 每像素4字节
    BmpInfo bmpInfo;
    bmpInfo.bfSize = sz + bmpInfo.bfOffBits;

    BmpHeadInfo headinfo;
    headinfo.biWidth = w;
    headinfo.biHeight = -h;  // 修改扫描线顺序为从上到下，方便直接追加数据
    headinfo.biSizeImage = sz;

    std::ofstream f("d:\bmp\test.bmp", std::ios_base::binary);

    // 文件信息
    f.write((char*)(&bmpInfo.bfType), sizeof(unsigned short));
    f.write((char*)(&bmpInfo.bfSize), sizeof(unsigned long));
    f.write((char*)(&bmpInfo.bfReserver1), sizeof(unsigned short));
    f.write((char*)(&bmpInfo.bfReserver2), sizeof(unsigned short));
    f.write((char*)(&bmpInfo.bfOffBits), sizeof(unsigned long));

    // 文件头信息

    f.write((char*)(&headinfo.biSize), sizeof(unsigned long));
    f.write((char*)(&headinfo.biWidth), sizeof(long));
    f.write((char*)(&headinfo.biHeight), sizeof(long));
    f.write((char*)(&headinfo.biPlanes), sizeof(unsigned short));
    f.write((char*)(&headinfo.biBitCount), sizeof(unsigned short));
    f.write((char*)(&headinfo.biCompression), sizeof(unsigned long));
    f.write((char*)(&headinfo.biSizeImage), sizeof(unsigned long));
    f.write((char*)(&headinfo.biXPelsPerMeter), sizeof(long));
    f.write((char*)(&headinfo.biYPelsPerMeter), sizeof(long));
    f.write((char*)(&headinfo.biClrUsed), sizeof(unsigned long));
    f.write((char*)(&headinfo.biClrImportant), sizeof(unsigned long));

    f.write((char *)(bmp1.getPixels()), bmp1.getSafeSize());

    f.write((char *)(bmp2.getPixels()), bmp2.getSafeSize());

示例中为纵向方式合并图片，且需要满足合并的图片宽度相同，且同为32位真彩色。