Converting between different image formats (using GDI+)

Given an input stream of some type of image, the following function converts that image into another type of image given the destination format. The destination format should be a proper mime-type such as "image/jpeg", "image/png".

Gdiplus::Status imageToImage(
  IStream *pStreamIn, IStream *pStreamOut, BSTR wszOutputMimeType)
{
    namespace G = Gdiplus;
    G::Status status = G::Ok;
    G::Image imageSrc(pStreamIn);
    status = imageSrc.GetLastStatus();
    if (G::Ok != status) {
      return status;
    }
    UINT numEncoders = 0;
    UINT sizeEncodersInBytes = 0;
    status = G::GetImageEncodersSize(&numEncoders, &sizeEncodersInBytes);
    if (status != G::Ok) {
      return status;
    }
    G::ImageCodecInfo *pImageCodecInfo = 
         (G::ImageCodecInfo *) malloc(sizeEncodersInBytes);
    status = G::GetImageEncoders(
          numEncoders, sizeEncodersInBytes, pImageCodecInfo);
    if (status != G::Ok) {
      return status;
    }
    CLSID clsidOut;
    status = G::UnknownImageFormat;
    for (UINT j = 0; j < numEncoders; j ++) {
      if (0 == wcscmp(pImageCodecInfo[j].MimeType, wszOutputMimeType)) {
        clsidOut = pImageCodecInfo[j].Clsid;
        status = G::Ok;
        break;
      }    
    }
    free(pImageCodecInfo);
    if (status != G::Ok) {
      return status;
    }
    return imageSrc.Save(pStreamOut, &clsidOut);
  }

It can be used like so:

extern IStream *pSomeImg; // source image format is not important
extern IStream *pMyJpeg;
if (Gdiplus::Ok == imageToImage(pSomeImg, pMyJpeg, L"image/jpeg")) {
// pMyJpeg holds the converted jpeg.
}

If there is a need to put/retrieve data into/from IStream in byte-array format (such as char*), it can by done by using CreateStreamOnHGlobal, GlobalAlloc, GlobalLock Win32 API functions. See this tip for more details.

Note: List of supported formats: BMP, ICON, GIF, JPEG, Exif, PNG, TIFF, WMF, and EMF.