In some writing systems you can place the characters vertically from the top to the bottom to form you sentences. Most users know this from Chinese, Japanese and Korean texts. What does it take to use the vertical writing mode in the PDF? And, what does it mean for a viewer to display the characters of vertical text correctly?
The creator of a PDF document has the choice of using a so called CJK (Chinese, Japanese, Korean) font either in horizontal or vertical mode. The mode is part of the encoding property in the font object. In the ISO standard you can find a list of predefined encodings, each of them intended either for horizontal or vertical writing. Alternatively, you can embed your own encoding in form of a CMAP stream in the PostScript language which contains, among other information, the vertical writing mode.
Furthermore, the same embedded font program can be used both in horizontal and vertical writing mode if there are two different font objects that have different a own encoding but share the same font file.
The vertical writing mode has an effect on how the characters of a text string appear on the page. The first and obvious effect is that the characters are positioned from the top to the bottom instead of from the left to the right. A closer look to the character positioning reveals that not only the writing direction has changed but also the origin of the character. In vertical writing mode it is the middle of the characters top bounding line whereas in horizontal mode it is on the left side on the base line of the character.
The second and not so obvious effect is that some characters appear rotated such as the parentheses characters '(', ')', '[', ']' and some punctuation characters '.', ',' etc.
To my big surprise, however, I found out that the rotation of the character is independent of the vertical writing mode. If you use a horizontal writing mode and you place the characters from the top to the bottom using text positioning operators then Acrobat rotates the characters as well.
After this somehow disturbing experience I searched the ISO standard for some explanation but found nothing about character rotation in vertical writing mode. Obviously this topic is left up to the viewer application.
Does your viewer correctly rotate the characters in vertical writing mode? Please, let me know about your experiences in this matter and post a comment.