Ensure font fallback is to "font" and not just to "typeface"

[MatteoH2O1999]

Right now, when a font is not found and a fallback is used, as in the default fontLoader

if (fontName == "0")
{
    return this.Typeface0;
}

return this.TypefaceA;

the scaling matrices are not applied, as it is just a fallback on the SKTypeface, not the font.

Thus we can change the logic of the fontloader (backwards compatible): if it returns null, the font is considered not found and a fallback can be used instead if the consumer chooses so.

An output parameter allows us to know whether we are using the fallback or not.
This way, we are truly falling back on font A, not just its typeface.

This changes nothing for fontLoaders that never return null.

[MatteoH2O1999]

cc @primo-ppcg

[primo-ppcg]

Do you have a explicit example of an incorrect label that this PR corrects?

[MatteoH2O1999]

^XA
^CFI,30,30^FO300,300^FDTestTest^FS
^XZ

of course you then need to edit the font loader:

if (fontName == "0")
{
    return this.Typeface0;
}

if (fontName == "I")
{
    return null;
}

return this.TypefaceA;

otherwise the old behavior is mainteined.

We are trying to replicate labelary, so we use the following fontLoader:

public static readonly Func<string, SKTypeface?> FontLoader = fontName =>
            fontName switch
            {
                "0" => font0,
                "A" => fontA,
                "B" => fontB,
                "C" or "D" => fontCD,
                "E" => fontE,
                "F" => fontF,
                "G" => fontG,
                "H" => fontH,
                "J" => fontJ,
                "L" => fontL,
                "N" => fontN,
                "P" => fontP,
                "Q" => fontQ,
                "R" => fontR,
                "S" => fontS,
                "T" => fontT,
                "U" => fontU,
                "V" => fontV,
                _ => null,
            };

[primo-ppcg]

We are trying to replicate labelary

We are not trying to replicate labelary. We are attempting to replicate the output of a ZPL enabled printer.

[MatteoH2O1999]

If a zpl printer does not find a font, it uses a fallback font. Not a fallback typeface with no scaling matrix.

Our specific use case was just one explicit example (like you requested) where the library has a wrong fallback behavior

[MatteoH2O1999]

Once again, this is a backwards compatible change: if the fontLoader could not return null, nothing changes, if it could, it would have thrown before so it was already broken.

In addition, this is the correct interpretation according to the ZPL standard as you can see here.
Citing from there:

If you do not change the alphanumeric default font and do not use any alphanumeric field command (^AF) or enter an invalid font value; any data you specify prints in font A.

As you can see, it says it uses font A. Not just its typeface. Font A. Together with its scaling matrix. Something that right now this library does not allow.