AFM metrics

An AFM file (Adobe Font Metrics) gives the metrics of a PostScript font without its glyphs: the widths and the boxes of its characters, its kerning pairs and its ligatures. A PDF writer uses them to measure text in the 14 standard fonts of PDF, which PDF readers provide.

Measure text with a standard font

The metrics of the 14 standard fonts are embedded: AfmFontMetrics.GetStandardFont gives them by name, and StandardFontNames lists the names.

// The metrics of the 14 standard fonts of PDF are embedded
var helvetica = AfmFontMetrics.GetStandardFont("Helvetica-Bold")!;

// The width of a text at 12 points: the widths are in 1/1000 of the point size
var units = 0.0;
string? previous = null;
foreach (var character in "Total")
{
    var metrics = helvetica.GetCharacterByCode(character)!; // the codes of the standard encoding of the font
    units += helvetica.GetWidth(metrics)!.Value.X;
    if (previous is not null)
        units += helvetica.GetKerning(previous, metrics.Name!).X;

    previous = metrics.Name;
}

Assert.Equal(27.708, units * 12 / 1000);
Assert.HasCount(14, AfmFontMetrics.StandardFontNames);

• The metrics are in 1/1000 of the point size.
• GetStandardFont returns null for another name; the names are case-sensitive (Helvetica, not helvetica).
• A character is found by its name (GetCharacter, GetWidth) or by its code in the encoding of the font (GetCharacterByCode).

Read an AFM file

AfmFontMetrics.Parse reads the text of a file, and Load and LoadAsync read a stream or bytes:

// An AFM file gives the metrics of a PostScript font without its glyphs, here two characters of Helvetica.
var metrics = AfmFontMetrics.Parse("""
    StartFontMetrics 4.1
    FontName Helvetica
    StartCharMetrics 2
    C 65 ; WX 667 ; N A ; B 14 0 654 718 ;
    C 86 ; WX 667 ; N V ; B 20 0 647 718 ;
    EndCharMetrics
    StartKernData
    StartKernPairs 1
    KPX A V -70
    EndKernPairs
    EndKernData
    EndFontMetrics
    """);

// The metrics are in 1/1000 of the point size: the width of "AV" at 12 points, with its kerning.
var units = metrics.GetWidth("A")!.Value.X + metrics.GetKerning("A", "V").X + metrics.GetWidth("V")!.Value.X;
var width = units * 12 / 1000;
Assert.Equal(15.168, width);

The Load methods take FontResourceLimits: a file from an untrusted source is bounded like a font. The multiple master (AMFM) and composite (ACFM) metrics files are not supported.