GDI+ 屏幕截图转JPG, 为何总从0,0左边开始？如果改开始坐标??

void CBbbDlg::OnButton1()
{
// TODO: Add your control notification handler code here
HWND hwnd = ::GetDesktopWindow();
HDC hdc = ::GetWindowDC(NULL);
int x = GetDeviceCaps(hdc, HORZRES);
int y = GetDeviceCaps(hdc, VERTRES);
HBITMAP hbmp = ::CreateCompatibleBitmap(hdc, x, y), hold;
HDC hmemdc = ::CreateCompatibleDC(hdc);
hold = (HBITMAP)::SelectObject(hmemdc, hbmp);
BitBlt(hmemdc, 0, 0, x, y, hdc, 0, 0, SRCCOPY);
SelectObject(hmemdc, hold);

const unsigned short *pszFileName=L"C:\\new.jpg"; //保存路径
int xs= GetSystemMetrics(SM_CXSCREEN);
int ys= GetSystemMetrics(SM_CYSCREEN);

Bitmap bit2(hbmp, NULL), bit(400, 300); //截屏生成的jpg 文件，长度，宽度，，，为何总从0,0起，，改其他参数总是不性？？
Graphics g(&bit);
g.ScaleTransform((float)xs/x, (float)ys/y);
g.DrawImage(&bit2, 0, 0);

CLSID encoderClsid;
ULONG quality;
quality = 50; //转换压缩率

EncoderParameters encoderParameters;

encoderParameters.Count = 1;
encoderParameters.Parameter[0].Guid = EncoderQuality;
encoderParameters.Parameter[0].Type = EncoderParameterValueTypeLong;
encoderParameters.Parameter[0].NumberOfValues = 1;
encoderParameters.Parameter[0].Value = &quality;

GetEncoderClsid(L"image/jpeg",&encoderClsid);
bit.Save(pszFileName, &encoderClsid, &encoderParameters);

::DeleteObject(hbmp);
::DeleteObject(hmemdc);

AfxMessageBox("ok");
return;

}

int CBbbDlg::GetEncoderClsid(const WCHAR* format, CLSID* pClsid)
{
UINT num = 0; // number of image encoders
UINT size = 0; // size of the image encoder array in bytes

ImageCodecInfo* pImageCodecInfo = NULL;
GetImageEncodersSize(&num,&size);
if(size == 0) return -1; // Failure

pImageCodecInfo = (ImageCodecInfo*)(malloc(size));
if(pImageCodecInfo == NULL) return -1; // Failure

GetImageEncoders(num, size, pImageCodecInfo);

for(UINT j =0; j<num; ++j)
{
if(wcscmp(pImageCodecInfo[j].MimeType, format) ==0 )
{
*pClsid = pImageCodecInfo[j].Clsid;
free(pImageCodecInfo);
return j; // Success
}
}
free(pImageCodecInfo);
return -1; // Failure
}