使用 StretchBlt 制作 缩略图标

//获取图片缩略图//hSrcBmp 源位图//cx cy 目标大小（像素）//bkColor 背景色//hDstBmp 目标位图BOOL GetImageThumb(HBITMAP hSrcBmp, int cx, int cy, COLORREF bkColor, HBITMAP *hDstBmp){  BOOL bRet = FALSE;  do   {    if(hSrcBmp == NULL || hDstBmp == NULL)      break;    //获取原始图片信息    BITMAP bitmap = {0};    if(GetObject(hSrcBmp, sizeof(bitmap), &bitmap) != sizeof(bitmap))      break;    //计算坐标 (锁定比例 & 居中显示)    int iSrcWidth = bitmap.bmWidth, iSrcHeight = bitmap.bmHeight;    double fZoom = min((double)cx/(double)iSrcWidth, (double)cy/(double)iSrcHeight);    int iDstWidth = (int)(iSrcWidth * fZoom);    int iDstHeight = (int)(iSrcHeight * fZoom);    int ixDst = (cx - iDstWidth)/2;    int iyDst = (cy - iDstHeight)/2;        //创建内存参考DC    HDC hRefDC = CreateDC(_T("DISPLAY"), NULL, NULL, NULL);        HBITMAP hRefBmp = CreateBitmap(64, 64, 1, 32, NULL);    HBITMAP hOldRefBmp = (HBITMAP)SelectObject(hRefDC, hRefBmp);    //源图内存DC & 选入源图    HDC hSrcDC = CreateCompatibleDC(hRefDC);    HBITMAP hOldSrcBmp = (HBITMAP)SelectObject(hSrcDC, hSrcBmp);    //目标内存DC & 选入目标位图    DeleteObject(*hDstBmp);    HDC hDstDC = CreateCompatibleDC(hRefDC);    *hDstBmp = CreateCompatibleBitmap(hRefDC, cx, cy);    HBITMAP hOldDstBmp = (HBITMAP)SelectObject(hDstDC, *hDstBmp);        SetStretchBltMode(hDstDC, HALFTONE); //缩放半色调    //填充背景色    HBRUSH hBursh = CreateSolidBrush(bkColor);    RECT rcFill = {0, 0, cx, cy};    FillRect(hDstDC, &rcFill, hBursh);    DeleteObject(hBursh); hBursh = NULL;    //贴图到内存DC    StretchBlt(hDstDC, //dst DC      ixDst, iyDst, iDstWidth, iDstHeight,  //目标位置      hSrcDC,      0, 0, iSrcWidth, iSrcHeight, //源位置      SRCCOPY);    //结束清理    SelectObject(hDstDC, hOldDstBmp);    DeleteDC(hDstDC);     SelectObject(hSrcDC, hSrcBmp);    DeleteDC(hSrcDC);    SelectObject(hRefDC, hOldRefBmp);    DeleteObject(hRefBmp);    DeleteDC(hRefDC);    bRet = TRUE;  } while (0);  return bRet;}

//测试用例， 使用CFileDialog 选择图像文件， 在C盘下生成 tThumb.bmp#include <atlimage.h>void CT2DDlg::OnBnClickedButton2(){  // TODO: 在此添加控件通知处理程序代码  CFileDialog dlg(TRUE);  if(dlg.DoModal() == IDOK)  {    CImage srcImage, dstImage;    if(srcImage.Load(dlg.GetPathName()) == S_OK)    {      HBITMAP hDstBmp = NULL;      if(GetImageThumb((HBITMAP)srcImage, 64, 64, RGB(255,255,255), &hDstBmp))      {        dstImage.Attach(hDstBmp);        dstImage.Save(_T("C:\\tThumb.bmp"));        dstImage.Detach();      }      if(hDstBmp)      {        DeleteObject(hDstBmp); hDstBmp = NULL;      }    }  }}