MSDN's truetype font article (https://learn.microsoft.com/en-us/typography/opentype/otspec160/ttch01) gives the following for converting FUnits to pixels:
Values in the em square are converted to values in the pixel coordinate system by multiplying them by a scale. This scale is:
pointSize * resolution / ( 72 points per inch * units_per_em )
where pointSize is the size at which the glyph is to be displayed, and resolution is the resolution of the output device. The 72 in the denominator reflects the number of points per inch.
For example, assume that a glyph feature is 550 FUnits in length on a 72 dpi screen at 18 point. There are 2048 units per em. The following calculation reveals that the feature is 4.83 pixels long.
550 * 18 * 72 / ( 72 * 2048 ) = 4.83
Questions:
Unfortunately, Apple's documentation is more or less the same, and other than that there are barely any resources other than just reading the source code of stb_truetype.
It says "pointSize is the size at which the glyph is to be displayed." How does one compute this, and what units is it in?
You don’t compute the point size, you set it. It’s the nominal size you want the font to be displayed in (think the font menu in a text editor). The ‘point size’ is a traditional typographical measurement system, with ‘point’ being roughly 1/72 of an inch. This brings the other question:
It says "72 in the denominator reflects the number of points per inch." Is this related to DPI or no?
No. Again, these are typographical points — the same unit you set the point size with. That’s why it’s part of the denominator in the first place: the point size is expressed in a measurement system of 72 points to an inch, and that has to be somehow taken into account in the equation.
Now, the typographical points are different from the output device’s dots or pixels. While in the early days of desktop publishing it was common to have a screen resolution of 72 pixels per inch that indeed corresponded to typographical system of 72 points per inch (no coincidence in that), these days the output resolution can, of course, vary quite dramatically, so it’s important to keep the point vs pixel distinction in mind.
In the example, it says '18 point'. Is this 18 used in computing the resolution or the pointSize?
Neither. It is the point size; see above. The entire example could be translated as follows. With a font based on 2048 units per em, if a particular glyph feature is 550 em units long and the glyph gets displayed at the size of 18 points (that is, 18/72 of an inch) on a device with screen resolution of 72 pixels per inch, the pixel size of that feature will be 4.84.
It says "resolution is the resolution of the output device". Is this in DPI? Where would I get this information?
It’s DPI/PPI, yes. You have to query some system API for that information or just hardcode the value if you’re targeting a specific device.