How to create lossless jpg using WIC in Delphi

Question

I have the following code that works to create a jpg file from IWICBitmapSource with default parameters:

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 settings to create lossless jpg. Since I realized what I read on MSDN, I must use IPropertyBag2 for this. Despite the lack of a hint on how to do this, I tried to paste this code between frame creation and 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;

This did not work: hr = 0x88982F8E (WINCODEC_ERR_PROPERTYUNEXPECTEDTYPE). MSDN says here that the ImageQuality property type is VT_R4, but since I have never used an option before, I'm not sure how to write that. And I'm not even sure that compression will really be lossless.

, 1.0 jpg, ?

Answer 1

, , .

, , PropBagOptions:

• dwType PROPBAG2_TYPE_DATA.
• vt VT_R4.

, VT_R4. 4 , Delphi varSingle.

V := VarAsType(1.0, varSingle);

, .

, :

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);

---

JPEG 1.0 , . : JPEG , 100?