[webkit-dev] Ruby Text Enhancements

Tue Sep 21 00:52:44 PDT 2010

Great to see someone else interested in doing ruby implementation! :)

I did the original ruby implementation, so I'm very happy to help with any
questions/problems/issues (bugs? there are not bugs!). BTW, please note that
there is another ruby patch in the review pipeline:
https://bugs.webkit.org/show_bug.cgi?id=41040 (had to modify and re-submit
this one after in-flight clashes with another patch) that might affect the
implementation.

On Tue, Sep 21, 2010 at 3:12 AM, David Hyatt <hyatt at apple.com> wrote:

> On Sep 17, 2010, at 8:07 PM, Eric Mader wrote:
>
> > Hi,
> >
> > I'm working on making the following enhancements to Ruby Text:
> >
> > 1) Implement the behavior of ruby-overhang:auto
>

Oh vey, that's ambituous! :)  There's so many corner cases I foresee on this
one that I was just too happy to postpone it when we originally discussed to
leave out CSS3 ruby stuff from the initial implementation, which is purely
based off HTML5 - including supporting multiple base/text pairs within a
single ruby, and line-breaking within the ruby.

> > 2) implement the behavior of ruby-line-stacking:exclude-ruby
>

Which way do you intend to implement this? AFAICT the current consensus
seems to go towards having ruby included by default rather than excluded.

> > 3) Add some Mac OS specific character properties to the ruby text
> >
> > 4) Turn off the underline when the ruby text is in a link
> >
> > I've looked at the code enough to know that the layout or ruby text is
> done by the normal block stacking in BlockLayout. I'm guessing that I can do
> at lest the first two tasks by changing the RenderRuby code to report a
> different width and / or height for the ruby block. Does this seem like the
> right way to do what I want?
>
>
> > Assuming for the moment that it is, I have some questions:
> >
> > 1) What methods should I subclass to report the adjusted width and
> height?
> >
>
> I'm very hazy on the Ruby implementation.  I believe it makes an
> inline-block with two block children vertically stacked, and then it uses
> text-align:center to center the ruby base.  If so, this behavior has to be
> preserved when the ruby text is wider than the base.
>

Yes, that's the basic layout for a single ruby text/base pair. Note that
multiple such pairs may be contained within a single <ruby> element (which
is normally an inline element, unless it's floated or positioned). A <ruby>
may also include renderers for :before and :after content, which are outside
of the inline-blocks for base/text pairs. This is implemented in the
aforementioned patch for 41040, which also fixes some issues with
RenderRubyAsBlock and supersedes the patch for 43722).

> I think a reasonable way to implement overhang therefore would be with
> negative margins applied to the ruby run.  This way the correct layout of
> the Ruby object is preserved, and the surrounding text will just naturally
> get pushed inside the Ruby object to overlap it.
>
> Basically you can compare the delta in width between the base and the text
> and then apply margins to either side of the ruby run based off how you want
> to overhang.
>

That's also what I'd suggest. However, there are the following additional
things to consider:

.) The margin on the left side may need to be reduced because of line start
(setting it to 0), or neighboring elements that reduce how much it can
overhand (larger text or other element, a neighboring <ruby> element,
etc.).

.) The same goes for the right margin - however, this one is vastly more
tricky, since the following elements are probably not yet layouted.
Depending on what follows (text you can overhang, stuff that turns out you
can't overhang, the line end) you might need to increase the projected width
of the whole ruby run, which in turn might push out following elements.
There might even be extreme cases where ruby text overhangs multiple
following objects if those are very small.

You also need to be careful caching any values inside used for this inside
the ruby, because whether a ruby text may/may not overhang (or by how much)
may change dependent on changes inside _neighboring_ elements.

Also, please note that currently the size of <rt> is set at 60% of the base,
which is different from the "standard" 50%. This was chosen because it
improves readability on (low dpi) screens. If you implement ruby overhang
properly, esp. for the purposes of CJK rendering (see
http://www.w3.org/TR/2009/NOTE-jlreq-20090604/#en-subheading2_3_3), you may
want/need to reconsider this, or adjust your implementation to take the
difference into consideration.

> 2) If the ruby text is wider than the ruby base and I report the width of
> the base as the width of the whole block will some of the ruby text get
> clipped, or will it all still draw?
> >
>
> It would all still draw as long as you set up overflow correctly.   You can
> look for addLayoutOverflow methods.  I think you may be able to use negative
> margins for overhang though without altering your reported width.
>
> > 3) Ruby text is only allowed to overhang the base in some cases. To know
> when it's OK, I'll need to examine the neighboring text. Can I always find
> the neighboring text by walking the render tree?
> >
>
> This is going to be tricky. You basically want to walk the line box tree
> rather than the renderobject tree and then look at surrounding text.
>
> > About turning off the underline if the ruby is in a link: I've looked at
> the styles and tried adding code to change the parts that I think relate to
> this, but haven't found anything that makes a difference. It's also occurred
> to me that I might be able to do this by writing a rule in
> <WebKit>WebCore/css/html.css, but I can't figure out exactly what the rule
> would look like. I tried adding "text-decoration: none" to the "ruby > rt"
> section, but that doesn't do it.
> >
> > (at first, I thought that is was probably overkill, but now I think that
> turning text-decoration off for all ruby text is probably right.)
>
> There is no way to do this.  The closest I see is:
>
> http://dev.w3.org/csswg/css3-text/#text-decoration-skip

> This is not implemented in WebKit yet.
>
> We'd probably need to add a new value to that property if Ruby is supposed
> to be skipped.
>

Ergh.... Looking at it, I'm not sure that's a good proposal at all - at
least it has still lots to address (it doesn't address list bullets, or
:before/:after generated content, for one). I think that the best approach
for ruby would be to view the whole ruby run (i.e., base and text combined)
as the main object for text-decoration, and not the base and text
individually. That is:

.) underline: line painted below the base only, over the width of base and
text (but excluding any overhangs!)
.) overline: overline painted above the text, same as above - note that the
line width doesn't (!) change
.) line-through: either just the base is decorated, or both base and text. I
can see arguments for either way, although I think painting a line-through
through the text may overly obscure it, since it's quite small - note that
the line width for the ruby text would need to be different in this case as
well, which in turn probably means amending the spec.
.) blink: all blinks ;)

This however means that a rule for <rt> would need to affect the ruby text
separately, independent of the decoration of the whole thing (which IMHO
would be a good thing anyway).

Cheers,

- Roland
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