Module kas_text::fonts[−][src]

Expand description

Font selection and loading

Fonts are managed by the FontLibrary, of which a static singleton exists and can be accessed via fonts.

FontId and the default font

The FontId type is a numeric identifier for a selected font. It may be default-constructed to access the default font, with number 0.

To make this work, the user of this library must load the default font before all other fonts and before any operation requiring font metrics:

if let Err(e) = kas_text::fonts::fonts().select_default() {
    panic!("Error loading font: {}", e);
}
// from now on, kas_text::fonts::FontId::default() identifies the default font

(It is not technically necessary to lead the first font with FontLibrary::select_default; whichever font is loaded first has number 0. If doing this, select_default must not be called at all. It is harmless to attempt to load any font multiple times, whether with select_default or another method.)

FaceId vs FontId

Why do both FaceId and FontId exist? Font fallbacks. A FontId identifies a list of font faces; when selecting glyphs the first face which includes that glyph is selected. Thus, when iterating over glyphs for rendering purposes, each has an associated FaceId.

Font sizes

Typically, font sizes are specified in “Points”. Several other units and measures come into play here. Lets start with those dating back to the early printing press:

1 Point = 1/72 inch (~0.35mm), by the usual DTP standard
1 Em is the width of a capital M (inclusive of margin) in a font
The point size of a font refers to the number of points per em in this font

Thus, with a “12 point font”, one ‘M’ occupies 12/72 of an inch on paper.

In digital typography, one must translate to/from pixel sizes. Here we have:

DPI (Dots Per Inch) is the number of pixels per inch
A scale factor is a measure of the number of pixels relative to a standard DPI, usually 96

We introduce two measures used by this library:

DPP (Dots Per Point): dpp = dpi / 72 = scale_factor × (96 / 72)
DPEM (Dots Per Em): dpem = point_size × dpp

Warning: on MacOS and Apple systems, a point sometimes refers to a (virtual) pixel, yielding dpp = 1 (or dpp = 2 on Retina screens, or something else entirely on iPhones). On any system, DPI/DPP/scale factor values may be set according to the user’s taste rather than physical measurements.

Finally, note that digital font files have an internally defined unit known as the font unit. We introduce one final unit: