[css-writing-modes] Support vertical CJK + ttb Arabic (rtl scripts generally)

[Zhang-Junzhi]

In Issue 2754 I proposed a new value horizontal-tb-rl to writing-mode.

Now I would like to bring up an improved proposal to replace the older one:

Instead of adding a new value horizontal-tb-rl to writing-mode, it would be better to add a new property cjk-dir.

Let's think about vertical CJK text together with LTR text. It is reasonably rendered in a consecutiveness way, because vertical CJK characters are "by default" mapped as Top-Left/Bottom-Right in vertical mode:

But when it comes to displaying vertical CJK together with RTL text, such as Persian, it is not as readably rendered as with LTR Latin text, because CJK and Persian text are rendered bi-directionally:

However, in vertical mode, it shouldn't have had to be in that way, bacause CJK characters can be mapped Top-Right/Bottom-Left as well as Top-Left/Bottom-Right. So here comes the new property cjk-dir, it changes the CJK characters' default directionality(remap them as Top-Right/Bottom-Left), without any necessary rendering change for CJK characters themselves:

The image above shows the vertical text with cjk-dir: ltr on the left, and shows the other one with cjk-dir: rtl on the right(being obviously more readable than the left bidi one).

cjk-dir can also as perfectly slove Issue 2754, which was about RTL CJK in horizontal mode.

[Zhang-Junzhi]

I am looking forward to your thoughts. @fantasai @frivoal @kojiishi @r12a @upsuper

[frivoal]

Leaving aside the question of which property to use to trigger this behavior, I'd like to focus on the use case first.

When we were working on the current version of writing-modes, we used to have text-orientation: sideways-left, which would support this combination of arabic/persian/hebrew/... and chinese/japanese/... where everything goes downwards.

Implementing it was a little more complicated than the rest so browsers were reluctant, but I thought that this made a lot of sense, so I tried to look for evidence that this was needed.

However, after asking around, I could only find people who agreed in theory that this could be nice, but no actual person who actually wanted to do that, or examples of (non-web) documents where this style was used (other than a limited body of Chinese Islamic calligraphic art). To be fair, my research was not particularly deep, but I asked a number of people who should know better than me, and nobody came back with anything useful.

Given the lack of evidence of an actual use case, and the difficulty of implementing it, we dropped text-orientation: sideways-left.

Now, I still think it makes sense in theory to support, so I'd be very interested to hear if you have evidence of an actual need (either people asking for it, or documents using it).

[Zhang-Junzhi]

Thank you for your opinion. @frivoal

First, I, for one, am looking forward to applying this in my case. :) That's why I brought up this idea: I am currently learning Persian/Farsi. I also have a specific plan to develop a rich web editor which is adjustable to writing mode. So I want to design a Chinese/Japanese/Korean/Persian/Arabic language-learning website where people can learn from each other, and I want to make the whole website writing-mode-responsive.

Second, one of the main reason of the lack of the reality of combining non-CJK characters in vertical-writing mode is simply the lack of the reality of the vertical-writing mode itself, which is, in my opinion, due to some other remaining issues needed to resolve, for example, you usually need to make greater effort to scroll a horizontal bar than the vertical one in PC. So this makes designers hesitant to design a whole lengthy vertical-writing page. So I don't think the lack of combining non-CJK characters in vertical-writing mode can be independently interpreted as lack of interest, multiple factors interplay.

Third, this property is not only for vertical mode, it's also for RTL CJK characters in horizontal mode(See #2754).

Fourth, I agree with you that this feature makes sense, especially because CSS is an international standard, since this feature is all about better internationalization, isn't it? ;)

Fifth, for vertical mode, this is only about readablity improvement, not an actual functionality, even if UA implementers aren't reluctant in implementing this feature, nothing is going to be hurt at worst.

These are my two cents.

[frivoal]

even if UA implementers aren't reluctant in implementing this feature, nothing is going to be hurt at worst.

Writing specifications take time. Writing tests take more time. Discussing these with with lots of potential implementers...

So we generally don't write specifications "just in case".

As far as I can tell, people who actually use vertical CJK mixed with arabic/persian generally use the bidirectional version where the rtl language goes bottom to top. I find it a little surprising, but that's what it is. So CSS as it is today does support international usage.

When I say I looked for evidence that the writing mode where both CJK and rtl languages go down, I didn't just look on the web. I asked a number of professor of arabic studies in Japanese universities, and the like, looking for any document that would be written like that. Maybe I just asked the wrong people, but nobody had seen anything like that before.

Even if it makes sense to me, I'd be reluctant to draft the feature without seeing a single real world document written like that.

I suspect they exist, or at least used to. The current system mostly make sense of you think of non-CJK text as primarily being english, and when you put some arabic into it, you do the usual bidi thing. But China has had many centuries of contact with Persia and the rest of the Arabic speaking world way before english was relevant, and I wouldn't be surprised if documents from back then where written so that everything flows down. But I have yet to see one.

[lianghai]

Unclear how this is a CJK-specific issue.

The figures in the original post seem to be about whether RTL scripts take a TTB inline progression in vertical lines. A rather typical use case is how RTL/TTB scripts (such as historical scripts Sogdian and Old Uyghur) can be rendered properly.

This is about how to override the default bidi behavior (as today’s common implementations of vertical writing simply rotate a whole horizontal line without changing the bidi behavior) with a unified direction for inline progression.

[Zhang-Junzhi]

Unclear how this is a CJK-specific issue.

The figures in the original post seem to be about whether RTL scripts take a TTB inline progression in vertical lines. A rather typical use case is how RTL/TTB scripts (such as historical scripts Sogdian and Old Uyghur) can be rendered properly.

This is about how to override the default bidi behavior (as today’s common implementations of vertical writing simply rotate a whole horizontal line without changing the bidi behavior) with a unified direction for inline progression.

Thank you for the reply.

It's really about RTL/LTR, in CSS.

"writing-mode: vertical-*" combined with "direction: ltr" creates TTB inline progression,
while "writing-mode: vertical-*" combined with "direction: rtl" creates BTT inline progression.

It means in vertical mode in CSS, left is "mapped" to top, and right "mapped" to bottom.

"cjk-dir" changes the left to the right mapping in vertical mode.

[Zhang-Junzhi]

I wanted to generalize the property to be also useful in horizontal mode.

"cjk-dir: rtl" simply indicates CJK characters are treated like any other RTL characters, such as Arabic, Persian, so they goes the same direction all the way down, regardless in vertical or horizontal mode.

[frivoal]

Sorry to insist, but before we discuss the detail of how the property should work or should be called, I think sharing examples of documents (screenshots, pictures, scans...) would be very useful.

As for Sogdian and Old Uyghur, aren't they already supported, as Mongolian and Manchu are, using writing-mode: vertical-lr? (and if not, why not?).