Confusion over font size and DPI on Apple Mac

[carl_caulkett]

Hello, I'm currently writing a font selector control, and for the purposes of setting a default size value, I am using the following code:

Code: Pascal  [Select][+][-]

1. procedure TcaFontSelecter.InitSelectedFontProperties;
2. var
3.   FontData: TFontData;
4.   PPI: Integer;
5. begin
6.   FontData := GetFontData(FLabel.Font.Handle);
7.   PPI := Forms.Screen.PixelsPerInch;
8.   FontData.Height := -FontData.Height;  // GetFontData returns Height as a +ve value
9.   FSelectedFont.Name := FontData.Name;
10.   FSelectedFont.Size := -MulDiv(FontData.Height, 72, PPI) // This should give Size as a +ve value
11. end;
12.  

The weird thing is that FLabel, the source of the GetFontData call, is made visible in this component, and visually, at least, appears to have the same size font as a standard TLabel, dropped straight on the form. I would expect, therefore, for the Height as returned by GetFontData to be -13 (as noted, GetFontData is returning a +ve value), and the resultant value of FSelectedFont.Size to be 10.

For some reason, which I suspect is to do with of DPI related factors, the Height as returned by GetFontData is actually 16, giving a FSelectedFont.Size to be 12. I'm sure I read somewhere recently, that recent changes to Lazarus on the Apple Mac, mean that adjustments have to be made in order for font sizes to work properly but I didn't make a note of it.

Can anyone shed some light on this question?

Thanks,
Carl