Wrong font size for dymo labels

[MarcelVincourt]

Hello,

I have been trying to convert my dymo .label files to glabels however I found that the scaling of the font size is incorrect. For example Arial with size 24 was to large compared to what Dymo label software made. (I also tested printing it in libreoffice writer and that had the same size as Dymo label software).

Eventually I found that changing the virtual_dpi in ModelTextObject.ccp fixed the issue. Below you see the function I changed. Originally the value of virtual_dpi is 96 but changing it to 72 fixed the issue.

        ///
        /// Calculate pixel size
        ///
        /// Assume a virtual DPI of 96 pixels/inch for all QPainter contexts.
        /// Ideally, we should use pointSizes for device independence, but as
        /// Qt-6.4 on X11, Wayland, and MacOS this approach has better results.
        ///
        int pixelSize( double pointSize )
        {
                const double virtual_dpi = 72;
                return qMax( 1, qRound( pointSize * virtual_dpi/72.0 ) );
        }

[j-evins]

I think you are right. When chasing the font size differences between the wayland and xcb Qt backends, I ended up normalizing on the wayland sizing when bypassing the backend's font metrics calculation (#272).

I am going to fix this, but it's going to be very disruptive to folks using recent versions, in the same way this was disruptive to your task. So I want to document my decision process here.

1. I brought the LiberationSansRegular font into FontForge to find a reference measurement that can be validated on the
   printed page.
   
   The em-height for this font is 2048 units.
   The height of a capital "L" is 1409 units, or approximately 0.688 em. If rendered at 1 em = 96 pt, this is 66 pt or about 0.92 in.

2. Next I printed a test page in glabels with 96 pt LiberationSans text. The height of the capital "L" is approximately 1.2 inches.
   

3. I printed a similar test page in LibreOffice and made the same measurement. The height of the capital "L" is approximately 0.9 in, which is much more consistent with size identified in FontForge.
   

4. Conclusion: the wayland Qt backend scaled font sizes by 125%, and it was a mistake to normalize on its results.