To start off, latin scripts are generally not written vertically. For example, let’s say you’re writing on the spine of a book. Usually, you wouldn’t stack the letters on top of each other; instead you would lay the book down horizontally and then write on the spine. In this situation, you’re simply writing horizontally on a canvas which has been rotated.
However, many East Asian scripts are written vertically. In this case, the characters are stacked on top of each other. Laying out text like this is a little different than laying it out horizontally.
You’re probably familiar with the concept of each character having an advance, which represents the displacement from one character to the next. Well, this displacement vector is different if you’re stacking letters on top of each other. In particular, there are two different kinds of advances: horizontal advances and vertical advances. With CoreText, CTFontGetAdvancesForGlyphs() takes a CTFontOrientation parameter, where you can specify if you want horizontal advances or vertical advances.
In general, the pen starts out horizontally centered above the letter, and the vertical advance takes you to a place that is horizontally centered below the letter.
This is actually the same concept as a “baseline”, where pen keeps returning to the same line after each character. The difference is that this baseline is in the middle of the letters, rather than underneath (some of) them. This is known as an “ideographic baseline,” which is distinct from the “alphabetic baseline” we are used to. A font file can define where these (and more) baselines should occur in the ‘bsln’ or ‘BASE’ table.
However, when you tell a glyph to draw at a particular point, that local origin isn’t at these pen locations.
Therefore, if you are going to draw a glyph, you need some way of going from this pen location to the glyph’s local origin. With CoreText, you can do this with the API function CTFontGetVerticalTranslationsForGlyphs().
So that should be enough to lay out a line of pure text, but in Web content, you may have stacked letters in the same line as non stacked letters (for example: an English word in a Chinese sentence). Laying out these two situations correctly is possible because of the relationship between the alphabetic baseline and the ideographic baseline.
Keep in mind baselines progress in the direction of the line, not the characters. The ideographic baseline and the alphabetic baseline are parallel to each other. The font's ascent and descent tell you how much space the letters take up orthogonally to the baseline. Therefore, in vertical text, the ascent measures to the right of the baseline and the descent measures to the left of the baseline, even though the letters are stacked.