EncoderParameters encoderParameters; 

ULONG nValue = 8; 

encoderParameters.Count = 1; 

encoderParameters.Parameter[0].Guid = EncoderColorDepth; 

encoderParameters.Parameter[0].Type = EncoderParameterValueTypeLong; 

encoderParameters.Parameter[0].NumberOfValues = 1; 

encoderParameters.Parameter[0].Value = &nValue; 

//原位图位深度是4

Image  *pImage = Image::FromFile(L"test_4.bmp");

CLSID clsid; 

GetEncoderClsid(L"image/bmp",&clsid);

//encoderParameters将位深度改为8

pImage->Save(L"test_8.bmp", &clsid, &encoderParameters);