Why is compressionQuality parameter ignored in Img32.Fmt.PNG?

[idw-git]

Hi Angus, I recently installed your latest version of Image32 and am wondering why Img32.Fmt.PNG says on line 28 that "SaveToStream: the compressionQuality parameter is ignored here"? Does this have any specific reason or was it actually something postponed for later implementation? Of course, it's easy enough to apply compressionQuality in SaveToStream for the non-FPC section by myself, but I'm still wondering.

[AngusJohnson]

The generic (ancestor) TImageFormat class from which TImageFormat_PNG class derives is also the ancestor of TImageFormat_JPG. PNG compression is loseless, unlike JPG compression, so the amount that PNG images can be compressed is generally fixed *.

I had a choice of overloading or overriding the TImageFormat.SaveToStream method, and I chose the latter option.

*Edit: Nevertheless, I concede that PNG images are still compressed using ZLIB compression, though most usually using the default level (6 or 7 depending where you look 😉).

[idw-git]

Ok, so you're rather disscussing the naming of this parameter instead of an option for using it as a compressionLevel parameter for PNGs. But since it's just a plain integer type anyway it could easily be used as an open "Option" parameter for different overrides.

Here's what I did: First of all add png.CompressionLevel := compressionQuality; in Img32.Fmt.PNG's "SaveToStream" method, then create a derived "TImageFormat_PNGX" like so:

  private
    class var FCompressionLevel: TCompressionLevel;
  public
    procedure SaveToStream(stream: TStream; img32: TImage32; CompressionLevel: Integer); override;
    class property CompressionLevel: TCompressionLevel read FCompressionLevel write FCompressionLevel;
  end;

implementation

procedure TImageFormat_PNGX.SaveToStream(stream: TStream; img32: TImage32; CompressionLevel: Integer);
begin
  inherited SaveToStream(stream, img32, FCompressionLevel);
end;

initialization
  TImage32.RegisterImageFormatClass('PNG', TImageFormat_PNGX, cpHigh);

end.```

[AngusJohnson]

I don't believe your code above would work because the CompressionLevel parameter is still ignored.
Nevertheless I've modified the code so the compression level is no longer ignored 😁.

[idw-git]

My code does work: It would not work if using TImageFormat_PNGX.SaveToStream() directly (according to my tests), but if using TImage32.SaveToStream() it does, provided the CompressionLevel property has been set using an initialization routine like this: TImageFormat_PNGX(GetImageFormatClass(PNG)).CompressionLevel := PNGCompressionLevel in a component derived from TImage32.

It is still required after your latest fix and I think I now understand why: While TImage.SaveToFile() has a quality parameter TImage.SaveToStream() does not: So there's currenly no way to pass this parameter to the TImageFormat.SaveToStream() proc while you can certainly pass it to TImageFormat.SaveToFile().

[AngusJohnson]

My code does work

As far as I can see in your code above, the CompressionLevel param is ignored because you're using the CompressionLevel property inside your SaveToStream method.

TImage.SaveToFile() has a quality parameter TImage.SaveToStream() does not

OK, I will investigate 😁.