Bitmap:bmWidthBytes以及Bitmap的详细说明

/* Bitmap Header Definition */
typedef struct tagBITMAP
{
    LONG        bmType;
    LONG        bmWidth;
    LONG        bmHeight;
    LONG        bmWidthBytes;//本文说明此参数的计算方法
    WORD        bmPlanes;
    WORD        bmBitsPixel;
    LPVOID      bmBits;
} BITMAP, *PBITMAP, NEAR *NPBITMAP, FAR *LPBITMAP;

//typedef struct tagBITMAP 中的各个数据类型是什么意思

typedef    struct    tagBITMAP    {    
   
LONG bmType; //必需为0
           LONG    bmWidth;              //位图的宽度(以像素为单位)  
           LONG    bmHeight;            //位图的高度(以像素为单位)  
   
           /*指定每条光栅所占字节数。此值必须取偶数，  
              因为图形设备接口（GDI）默认为一个位图的位值组成一个2字节的整数数组。*/  
           LONG    bmWidthBytes;    
   
           WORD    bmPlanes;            //    位图调色板颜色数  
   
           WORD    bmBitsPixel;      //    一个点在每个调色板上接近的颜色位数  
   
           LPVOID    bmBits;            //   指向存储像素阵列的数组  
   
   }    BITMAP;

bmWidthBytes:

BitMap结构中的bmWidthBytes总算搞懂了，其实很简单：一行像素所占的字节数，一行像素的存储必须按word对齐，所以该值必须为2的倍数。所以计算方式中应体现出向字对齐来，即以16位为一划分。bmWidthBytes=((bitsPerLine+15)/16)*2，其中bitsPerLine=bmWidth*bmBitsPixel。一切搞定。

这是网上的一句话。Specifies the number of bytes in each scan line. This value must be divisible by 2, because the system assumes that the bit values of a bitmap form an array that is word aligned.

Programming Windows里总算找到了个错误，设置bmWidthBytes错误导致无法建立位图，导致程序整个几乎完全失败。以下是错误代码：

HBITMAP StretchBitmap (HBITMAP hBitmap1)
{
     BITMAP     bm1, bm2 ;
     HBITMAP    hBitmap2 ;
     HDC        hdc, hdcMem1, hdcMem2 ;
     int        cxChar, cyChar ;

          // Get the width and height of a system font character

     cxChar = LOWORD (GetDialogBaseUnits ()) ;
     cyChar = HIWORD (GetDialogBaseUnits ()) ;

          // Create 2 memory DCs compatible with the display
    
     hdc = CreateIC (TEXT ("DISPLAY"), NULL, NULL, NULL) ;
     hdcMem1 = CreateCompatibleDC (hdc) ;
     hdcMem2 = CreateCompatibleDC (hdc) ;
     DeleteDC (hdc) ;

          // Get the dimensions of the bitmap to be stretched
    
     GetObject (hBitmap1, sizeof (BITMAP), (PTSTR) &bm1) ;

          // Scale these dimensions based on the system font size
    
     bm2 = bm1 ;
     bm2.bmWidth      = (cxChar * bm2.bmWidth) / 4 ;
     bm2.bmHeight     = (cyChar * bm2.bmHeight) / 8 ;
     bm2.bmWidthBytes = ((bm2.bmWidth+15)/16)*2 ;
                       ~~~~~~~~~~~~~~~~少了*bm2.bmBitsPixel
          // Create a new bitmap of larger size
    
     hBitmap2 = CreateBitmapIndirect (&bm2) ;

          // Select the bitmaps in the memory DCs and do a StretchBlt
    
     SelectObject (hdcMem1, hBitmap1) ;
     SelectObject (hdcMem2, hBitmap2) ;
     StretchBlt (hdcMem2, 0, 0, bm2.bmWidth, bm2.bmHeight,
                 hdcMem1, 0, 0, bm1.bmWidth, bm1.bmHeight, SRCCOPY) ;

          // Clean up
    
     DeleteDC (hdcMem1) ;
     DeleteDC (hdcMem2) ;
     DeleteObject (hBitmap1) ;
    
     return hBitmap2 ;
}

来自：http://hi.baidu.com/ustc_/blog/item/3b49cca98ec32dfa1e17a29d.html