font-metrics-sizing

Problem

Some fonts may be displayed differently by different OS. Unix-like OS's use hhea metrics table but Windows use OS/2 table instead to display font. So if tables contain different vertical metrics values then Unix-like and Windows systems will render this font not the same way.

Solution

This tool was created to fix this issue for TrueType and OpenType fonts. The solution is to recalculate some vertical metrics values for hhea and OS/2 tables.

We have three sets of formula for three cases:

• when os2TypoLineGap = 0 and (os2TypoAsc + |os2TypoDesc|) > UPM:

    os2TypoAsc = os2TypoAsc + upperLowerAddUnits;
    os2TypoDesc = os2TypoDesc - upperLowerAddUnits;
    hheaAsc = hheaAsc + upperLowerAddUnits;
    hheaDesc = hheaDesc - upperLowerAddUnits;
    os2WinAsc = hheaAsc;
    os2WinDesc = -hheaDesc;

• when os2TypoLineGap = 0 and (os2TypoAsc + |os2TypoDesc|) = UPM:

    hheaAsc = hheaAsc + upperLowerAddUnits;
    hheaDesc = hheaDesc - upperLowerAddUnits;
    os2WinAsc = hheaAsc;
    os2WinDesc = -hheaDesc;

• otherwise:

    os2TypoLineGap = lineSpacingUnits;
    hheaAsc = Math.round(os2TypoAsc + (os2TypoLineGap / 2));
    hheaDesc = -1 * (totalHeight - hheaAsc);
    os2WinAsc = hheaAsc;
    os2WinDesc = -hheaDesc;

As you can see we used some parameters that you can't find in font tables such as: upperLowerAddUnits, lineSpacingUnits, totalHeight. How we get it:

    hheaAscDescDelta = hheaAsc + Math.abs(hheaDesc)
    lineSpacingUnits = Math.round(factor * UPM);
    totalHeight = lineSpacingUnits + UPM;

    deltaHeight = totalHeight - hheaAscDescDelta;
    upperLowerAddUnits = Math.round(deltaHeight / 2);

factor is the second parameter for module function.

Usage

• add module to your package.json dependencies:

{
    "dependencies": {
        ...
        
        "font-metrics-sizing": "git+https://github.com/crello/font-metrics-sizing.git",
       
        ...
    }
}

• install it:

    $ npm install

• use module in your code:

    const fontMetricsSizing = require('font-metrics-sizing');
    const fs = require('fs');
    
    const linespaceFactor = 10; // percent of scaling font height, default is 10
    
    const fileBuffer = fs.readFileSync('./path/to/font.ttf');
    const newFileBuffer = await fontMetricsSizing(fileBuffer, linespaceFactor);