How to create a lossless jpg using WIC in Delphi

I have the following code working to create a jpg file out of a IWICBitmapSource with defaults options:

function SaveWICBitmapToJpgFile(WICFactory: IWICImagingFactory;
  WICBitmap: IWICBitmapSource; SrcRect: TRect; FileName: string): HRESULT;
var
  hr: HRESULT;
  Encoder: IWICBitmapEncoder;
  Frame: IWICBitmapFrameEncode;
  PropBag: IPropertyBag2;
  S: IWICStream;
  PixelFormatGUID: WICPixelFormatGUID;
  R: WICRect;
begin

  hr := WICFactory.CreateStream(S);

  if Succeeded(hr) then begin
    hr := S.InitializeFromFilename(PChar(FileName), GENERIC_WRITE);
  end;

  if Succeeded(hr) then begin
    hr := WICFactory.CreateEncoder(GUID_ContainerFormatJpeg, GUID_NULL,
      Encoder);
  end;

  if Succeeded(hr) then begin
    hr := Encoder.Initialize(S, WICBitmapEncoderNoCache);
  end;

  if Succeeded(hr) then begin
    hr := Encoder.CreateNewFrame(Frame, PropBag);
  end;

  if Succeeded(hr) then begin
    hr := Frame.Initialize(PropBag);
  end;

  if Succeeded(hr) then begin
    hr := Frame.SetSize(SrcRect.Width, SrcRect.Height);
  end;

  if Succeeded(hr) then begin
    PixelFormatGUID := GUID_WICPixelFormat24bppBGR;
    hr := Frame.SetPixelFormat(PixelFormatGUID);
  end;

  if Succeeded(hr) then begin
    hr := IfThen(PixelFormatGUID = GUID_WICPixelFormat24bppBGR, S_OK, E_FAIL);
  end;

  if Succeeded(hr) then begin
    R.X := SrcRect.Left;
    R.Y := SrcRect.Top;
    R.Width := SrcRect.Width;
    R.Height := SrcRect.Height;
    Frame.WriteSource(WICBitmap, @R);
  end;

  if Succeeded(hr) then begin
    hr := Frame.Commit;
  end;

  if Succeeded(hr) then begin
    hr := Encoder.Commit;
  end;

  Result := hr;

end;

I would like to change the codec options to produce a lossless jpg. As I understood what I read in MSDN, I have to use IPropertyBag2 to achieve this. Despite not having a clue on how to do it, I tried inserting this code between the Frame creation and the Frame initialization:

var
[...]
  PropBagOptions: TPropBag2;
  V: Variant;
[...]

if Succeeded(hr) then begin
  FillChar(PropBagOptions, SizeOf(PropBagOptions), 0);
  PropBagOptions.pstrName := 'ImageQuality';
  V := 1.0;
  hr := PropBag.Write(1, @PropBagOptions, @V);
end;

It did not work: hr = 0x88982F8E (WINCODEC_ERR_PROPERTYUNEXPECTEDTYPE). MSDN says here that the type of the "ImageQuality" property is VT_R4, but as I never used a Variant before, I am not sure how to write that. And I am not even sure that the compression will indeed be lossless.

How could I modify my code to make it produce a quality 1.0 jpg, and will it be lossless?

I've no experience of this, but here's my best suggestions based on reading the documentation that you have linked to.

First of all, I think you need to specify more of the members of PropBagOptions:

• dwType needs to be set to PROPBAG2_TYPE_DATA.
• vt needs to be set to VT_R4.

You also need to make sure that the variant really is VT_R4. That's a 4 byte real, in Delphi terms a variant of type varSingle.

V := VarAsType(1.0, varSingle);

I think that should get it done.

Putting it all together, it looks like this:

FillChar(PropBagOptions, SizeOf(PropBagOptions), 0);
PropBagOptions.pstrName := 'ImageQuality';
PropBagOptions.dwType := PROPBAG2_TYPE_DATA;
PropBagOptions.vt := VT_R4;
V := VarAsType(1.0, varSingle);
hr := PropBag.Write(1, @PropBagOptions, @V);

As regards whether or not JPEG quality 1.0 is lossless, it is not. That's a question already well covered here: Is JPEG lossless when quality is set to 100?