How to create a lossless jpg using WIC in Delphi

Question

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

function SaveWICBitmapToJpgFile(WICFactory: IWICImagingFa         


        

Answer 1

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);

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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?