使用GDI+保存带Alpha通道的图像
带Alpha通道的图像(ARBG)在通过GDIPlus::Bitmap::FromHBITMAP等转为GDI+位图,再存储时,透明区域会变成纯黑(也有可能是纯白?)。
网上找了两段保持透明的实现代码,列在下边,经测试,第一段无效,第二段有效,这两段代码正好可以对比说明:FromHBITMAP在拷贝图像数据时,原图中的Alpha数据确实没有Copy过来,而并非是未设置图像属性的问题。
第一段的思路是:直接用FromHBITMAP创建一个GDI+位图,新建另一个带PixelFormat32bppARGB标识的位图,再从前者拷贝数据到后者;
第二段的思路是:获取BITMAP数据,新建一个带PixelFormat32bppARGB标识的位图,再从前者拷贝数据到后者;
第二段代码也不够好,应该先判断一下位图是不是32位的,带Alpha通道的只有ARGB格式,ARGB是32位的,所以不是32位的不需要用这种方式来存储。如果要拷贝数据的话,不是32位的格式必须处理行边界对齐的问题。
bool ImageUtil :: CreateGdiplusBmpFromHBITMAP_Alpha ( HBITMAP hBmp , Gdiplus :: Bitmap ** bmp ) { BITMAP bitmap ; GetObject ( hBmp , sizeof ( BITMAP ), & bitmap ); if ( bitmap . bmBitsPixel != 32) { return false ; } Gdiplus :: Bitmap * pWrapBitmap = Gdiplus :: Bitmap :: FromHBITMAP ( hBmp , NULL ); if ( pWrapBitmap ) { Gdiplus :: BitmapData bitmapData ; Gdiplus :: Rect rcImage (0, 0, pWrapBitmap -> GetWidth (), pWrapBitmap -> GetHeight ()); pWrapBitmap -> LockBits (& rcImage , Gdiplus :: ImageLockModeRead , pWrapBitmap -> GetPixelFormat (), & bitmapData ); * bmp = new Gdiplus :: Bitmap ( bitmapData . Width , bitmapData . Height , bitmapData . Stride , PixelFormat32bppARGB , ( BYTE *) bitmapData . Scan0 ); pWrapBitmap -> UnlockBits (& bitmapData ); delete pWrapBitmap ; return true ; } return false ; }
Gdiplus :: Bitmap * ImageUtil :: CreateBitmapFromHBITMAP ( IN HBITMAP hBitmap ) { BITMAP bmp = { 0 }; if ( 0 == GetObject ( hBitmap , sizeof ( BITMAP ), ( LPVOID )& bmp ) ) { return FALSE ; } // Although we can get bitmap data address by bmp.bmBits member of BITMAP // which is got by GetObject function sometime, // we can determine the bitmap data in the HBITMAP is arranged bottom-up // or top-down, so we should always use GetDIBits to get bitmap data. BYTE * piexlsSrc = NULL ; LONG cbSize = bmp . bmWidthBytes * bmp . bmHeight ; piexlsSrc = new BYTE [ cbSize ]; BITMAPINFO bmpInfo = { 0 }; // We should initialize the first six members of BITMAPINFOHEADER structure. // A bottom-up DIB is specified by setting the height to a positive number, // while a top-down DIB is specified by setting the height to a negative number. bmpInfo . bmiHeader . biSize = sizeof ( BITMAPINFOHEADER ); bmpInfo . bmiHeader . biWidth = bmp . bmWidth ; bmpInfo . bmiHeader . biHeight = bmp . bmHeight ; // 正数,说明数据从下到上,如未负数,则从上到下 bmpInfo . bmiHeader . biPlanes = bmp . bmPlanes ; bmpInfo . bmiHeader . biBitCount = bmp . bmBitsPixel ; bmpInfo . bmiHeader . biCompression = BI_RGB ; HDC hdcScreen = CreateDC ( L "DISPLAY" , NULL , NULL , NULL ); LONG cbCopied = GetDIBits ( hdcScreen , hBitmap , 0, bmp . bmHeight , piexlsSrc , & bmpInfo , DIB_RGB_COLORS ); DeleteDC ( hdcScreen ); if ( 0 == cbCopied ) { delete [] piexlsSrc ; return FALSE ; } // Create an GDI+ Bitmap has the same dimensions with hbitmap Bitmap * pBitmap = new Bitmap ( bmp . bmWidth , bmp . bmHeight , PixelFormat32bppPARGB ); // Access to the Gdiplus::Bitmap's pixel data BitmapData bitmapData ; Rect rect (0, 0, bmp . bmWidth , bmp . bmHeight ); if ( Ok != pBitmap -> LockBits (& rect , ImageLockModeRead , PixelFormat32bppPARGB , & bitmapData ) ) { delete ( pBitmap ); return NULL ; } BYTE * pixelsDest = ( BYTE *) bitmapData . Scan0 ; int nLinesize = bmp . bmWidth * sizeof ( UINT ); int nHeight = bmp . bmHeight ; // Copy pixel data from HBITMAP by bottom-up. for ( int y = 0; y < nHeight ; y ++ ) { // 从下到上复制数据,因为前面设置高度时是正数。 memcpy_s ( ( pixelsDest + y * nLinesize ), nLinesize , ( piexlsSrc + ( nHeight - y - 1) * nLinesize ), nLinesize ); } // Copy the data in temporary buffer to pBitmap if ( Ok != pBitmap -> UnlockBits (& bitmapData ) ) { delete pBitmap ; } delete [] piexlsSrc ; return pBitmap ; }
使用ATL::CImage来保存图像也会存在黑底的问题,但这并非上述丢失Alpha的问题,事实上,ATL::CImage::Attach不会创建位图的副本。下面是ATL::CImage在Attach到一个位图名柄后执行的操作,从ATL::Image::UpdateBitmapInfo的实现中可见,它的问题在于默认认为图像是不带Alpha通道的(置m_bHasAlphaChannel为false),继而在保存操作中,转为Gdiplus::Bitmap时没有使用PixelFormat32bppARGB标志。
最后,我从ATL::CImage里边的代码提取了一个函数,应该比上边的靠谱并且效率高点。
bool ImageUtil::SavePng( HBITMAP hBmp, LPCTSTR lpszFilePath ) { DIBSECTION dibsection; int nBytes = ::GetObject( hBmp, sizeof( DIBSECTION ), &dibsection ); Gdiplus::Bitmap* bitmap = 0; if(nBytes != sizeof(DIBSECTION) || dibsection.dsBm.bmBitsPixel != 32) { // Bitmap with plate or non-ARGB(32bpp) bitmap = Gdiplus::Bitmap::FromHBITMAP(hBmp); } else { int width, height, bits_per_pixel, pitch; LPVOID bits; width = dibsection.dsBmih.biWidth; height = abs( dibsection.dsBmih.biHeight ); bits_per_pixel = dibsection.dsBmih.biBitCount; pitch = (((width*bits_per_pixel)+31)/32)*4; //计算行宽,四字节对齐 ATL::CImage::ComputePitch bits = dibsection.dsBm.bmBits; if( dibsection.dsBmih.biHeight > 0 ) { bits = LPBYTE( bits )+((height-1)*pitch); pitch = -pitch; } bitmap = new Gdiplus::Bitmap(width, height, pitch, PixelFormat32bppARGB, static_cast< BYTE* >(bits )); } bool ret = false; CLSID clsid = GetGdiplusEncoderClsid(NULL, &Gdiplus::ImageFormatPNG); if(clsid != CLSID_NULL) { ret = (Gdiplus::Ok == bitmap->Save(lpszFilePath, &clsid, NULL)); } delete bitmap; return ret; }
补充关于Gdi+里图像编码器的一点:每个解码器都有CLSID和FORMAT两个数据,实际上都是GUID,不能搞混了,Gdiplus::Bitmap::Save中接收的是CLSID,Gdiplus中提供的预定义ImageFormatXXX是指解码器的Format,很奇葩,细看了一下代码才搞清楚。见ATL::CImage::FindCodecForFileType。
经测试,此代码亦存在问题,见下一篇日记。