Question

关于HBITMAP

在VC中一般显示bmp要先转换为HBITMAP，为什么不能够直接显示呢？他们又是哪一个与设备有关，哪一个与设备无关呢？
谢谢各位，可是我想问的不是怎么实现，而是为什么这样实现(VC++)：
程序所用的函数大致是：CreatDIBitmap,Seletjob,Bitblt; CreatDIBitmap返回值是HBITMAP吧,我就是这里不明白，为什么要这样转换？
书上说这是位图句柄，我知道什么是句柄，那位图句柄怎么理解？

Answer 1

VC中显示bmp要用到CBitmap类，Windows 里显示/画东西还要与device context建立关系。建bitmap GDI object 要调用CBitmap类的函数Attach
（）例如：
bitmap.Attach( hBmp );
hBmp 是HBITMAP

bmp本身与设备无关。但显示时它要与设备支持的颜色种类做color mapping.

下面是我的一个子程序，用于我的程序VPIC,你可直接使用。这个子程序打开bmp文件，建bitmap GDI object，也就是得到CBitmap object。

/* =========================================================================*
* LoadBMPImage - Loads a BMP file and creates a bitmap GDI object
* also creates logical palette for it.
* Returns - TRUE for success (check success or not)
* sBMPFile - Full path of the BMP file, (input file name)
* bitmap - The bitmap object to initialize, (bring back img in it)
* pPal - Will hold the logical palette. Can be NULL (bring back)
*==========================================================================*/

BOOL CVPICDoc::LoadBMPImage(LPCTSTR sBMPFile, CBitmap &bitmap, CPalette *pPal)
{

BITMAPFILEHEADER bmfHeader;

// Read file header

CFile file;
if( !file.Open( sBMPFile, CFile::modeRead) ) return FALSE;

if (file.Read((LPSTR)&bmfHeader, sizeof(bmfHeader)) != sizeof(bmfHeader))
return FALSE;

// Check File type should be 'BM'
if (bmfHeader.bfType != ((WORD) ('M' << 8) | 'B'))
return FALSE;

read_bmp_o = bmfHeader.bfOffBits / 8;

// Get length of the remainder of the file and allocate memory
DWORD nPackedDIBLen = file.GetLength() - sizeof(BITMAPFILEHEADER);
HGLOBAL hDIB = ::GlobalAlloc(GMEM_FIXED, nPackedDIBLen);
if (hDIB == 0)
return FALSE;

// Read the remainder of the bitmap file.
// BITMAPINFOHEADER + possible-Color-Table + DIB-bits
if (file.ReadHuge((LPSTR)hDIB, nPackedDIBLen) != nPackedDIBLen )
{
::GlobalFree(hDIB);
return FALSE;
}

BITMAPINFOHEADER &bmiHeader = *(LPBITMAPINFOHEADER)hDIB ;
BITMAPINFO &bmInfo = *(LPBITMAPINFO)hDIB ;

read_bmp_w = bmiHeader.biWidth;
read_bmp_h = bmiHeader.biHeight;
read_bmp_c = bmiHeader.biBitCount;
read_bmp_size = bmiHeader.biSizeImage;
if (read_bmp_size == 0) {
read_bmp_size = ((((read_bmp_w * read_bmp_c) + 31) & ~31) >> 3) * read_bmp_h;
};
read_bmp_l = read_bmp_size / read_bmp_h; // scan line width

// If bmiHeader.biClrUsed is zero we have to infer the number
// of colors from the number of bits used to specify it.
int nColors = bmiHeader.biClrUsed ? bmiHeader.biClrUsed :
1 << bmiHeader.biBitCount;

LPVOID lpDIBBits;
if( bmInfo.bmiHeader.biBitCount > 8 )
lpDIBBits = (LPVOID)((LPDWORD)(bmInfo.bmiColors + bmInfo.bmiHeader.biClrUsed) +
((bmInfo.bmiHeader.biCompression == BI_BITFIELDS) ? 3 : 0));
else
lpDIBBits = (LPVOID)(bmInfo.bmiColors + nColors);

// Create the logical palette
if( pPal != NULL )
{
// Create the palette
if( nColors <= 256 )
{
UINT nSize = sizeof(LOGPALETTE) + (sizeof(PALETTEENTRY) * nColors);
LOGPALETTE *pLP = (LOGPALETTE *) new BYTE[nSize];

pLP->palVersion = 0x300;
pLP->palNumEntries = nColors;

for( int i=0; i < nColors; i++)
{
pLP->palPalEntry[i].peRed = bmInfo.bmiColors[i].rgbRed;
pLP->palPalEntry[i].peGreen = bmInfo.bmiColors[i].rgbGreen;
pLP->palPalEntry[i].peBlue = bmInfo.bmiColors[i].rgbBlue;
pLP->palPalEntry[i].peFlags = 0;
}

pPal->CreatePalette( pLP );

delete[] pLP;
}
}

CClientDC dc(NULL);
CPalette* pOldPalette = NULL;
if( pPal )
{
pOldPalette = dc.SelectPalette( pPal, FALSE );
dc.RealizePalette();
}

HBITMAP hBmp = CreateDIBitmap( dc.m_hDC,// handle to device context
&bmiHeader, // pointer to BITMAPINFOHEADER
CBM_INIT, // initialization flag
lpDIBBits, // pointer to initialization data
&bmInfo, // pointer to bitmap color-format data
DIB_RGB_COLORS); // color-data usage
bitmap.Attach( hBmp );

if( pOldPalette )
dc.SelectPalette( pOldPalette, FALSE );

::GlobalFree(hDIB);
return TRUE;

}

Answer 2

不是转换成HBITMAP呀，是要取得BMP图象的句柄的！

Answer 3

楼上的，佩服。