W3C home > Mailing lists > Public > public-tt@w3.org > December 2013

Re: Issue-286: application of padding to p etc

From: Glenn Adams <glenn@skynav.com>
Date: Tue, 10 Dec 2013 05:28:04 +0800
Message-ID: <CACQ=j+eJGhdw_GOPTV=SqqbJSD0Wy7H9jdjzKDYVPRyyTTFg8A@mail.gmail.com>
To: Nigel Megitt <nigel.megitt@bbc.co.uk>
Cc: Timed Text Working Group <public-tt@w3.org>
On Tue, Dec 10, 2013 at 12:05 AM, Nigel Megitt <nigel.megitt@bbc.co.uk>wrote:

>  On 06/12/2013 18:34, "Glenn Adams" <glenn@skynav.com> wrote:
>
>
>
> On Fri, Dec 6, 2013 at 10:52 PM, Nigel Megitt <nigel.megitt@bbc.co.uk>wrote:
>
>>   On 05/12/2013 23:25, "Glenn Adams" <glenn@skynav.com> wrote:
>>
>>      body: Since tts:padding is not inheritable and body can not contain
>>> content (i.e. what's in the spec as Inline.class)  I'm not sure how it
>>> would ever be applied.
>>>
>>
>>  yes, body contains content, in fact, all content; body maps to fo:block
>> in XSL-FO and will map to an outer div in HTML
>>
>>
>>  Which rendered text would the padding apply to, given that the only
>> content that body can contain is zero or more divs, and divs have a padding
>> attribute of their own, which is not inheritable?
>>
>
>  not related to inheritance, each generated block area (box) has its own
> padding; think of it as a set of nested blocks, each with separate padding:
>
>  body padding
>
>
>  Is this the first instance of a box being explicitly required for the
> body, rather than merely an implicit concept?
>

In TTML1, we allowed padding on region only. In TTML2 it has been proposed
to support padding on content elements, which means body, div, p, span
(doesn't make sense on br though). It would be strange to define only for
some subset of body, div, p, span.


>
>       div1 padding
>     div1.1 padding
>       p1 padding
>
>
>>
>>      div: Could apply to the set of rendered lines within the div, taken
>>> as a single rectangle? This is subtly different from padding on region,
>>> which doesn't take into account the width of the rendered lines at all, so
>>> I can see it being useful.
>>>
>>
>>  again, the XSL-FO (=CSS) model applies; padding applies to the block
>> areas (boxes) generated by div
>>
>>
>>  Is that different in effect from the box tightly surrounding all the
>> (i.e. not "each of the…") rendered lines generated from content contained
>> in the div?
>>
>
>  not sure what you mean by *the* box; each of body, div.outer ...
> div.inner, p generate block areas (boxes), and each has independent
> padding; see diagram at [1]; the body has its own padding, then each of its
> div children generate areas placed into the body's content rectangle, where
> each div has its own padding
>
>  [1] http://www.w3.org/TR/2006/REC-xsl11-20061205/#area-intro
>
>  e.g. in the following example, there would be 175px of padding to the
> left of FIRST LINE with respect to the left edge of the region
>
>  <region xml:id="r1" tts:padding="100px"/>
> ...
> <body tts:padding="5px" region="r1">
>   <div tts:padding="10px">
>     <div tts:padding="20px">
>        <p tts:padding="40px">FIRST LINE</p>
>     </div>
>   </div>
> </div>
>
>
>>  Just want to double check that we have the same understanding on this
>> one. Also that it's not in effect identical to padding on region.
>>
>
>  Definitely not identical.
>
>
>>
>>      p: as per proposal, applies separately to each rendered line within
>>> the p.
>>>
>>
>>  not exactly, padding applies to the (non-line) block areas (boxes)
>> generated by p
>>
>>
>>  This appears to be a duplicate of the functionality provided by padding
>> on div.
>>
>
>  Don't know what you mean by duplicate. It is separate and independent.
>
>
>  Forgive my phrasing – I mean if you want to create padding on the block
> that contains all the child elements' (non-line) block areas you can do
> this with div. There doesn't seem to be an option that permits padding on
> each of the child elements' line block areas separately.
>

Line block areas (boxes) are generated only by p. The padding on p applies
to p as a whole, which means it consumes area around its content area into
which it may place line block areas.

Line block areas are not individually addressable (or styleable) in
general, so one can't specify a padding for, say, line 2 of a paragraph.


> I think it's possible from your proposal to put each line in a separate
> span and apply padding that way – is that how you expect the requirement to
> be met?
>

The requirement is focused on being able to paint a background color behind
text such that it extends outside of the text in the inline progression
axis. This appears to be satisfiable with either whitespace characters or
padding (on span) or a combination of both.


>
>
>     In the absence of an explicitly line-based entity we haven't yet met
>> the use case: padding on p seems to be the best place to put it given the
>> duplication.
>>
>
>  Not sure what you mean by duplication.
>
>  As above.
>
>
>
>>  It can't go on span because line breaks are not predictable, unless we
>> add the specification that in mapping to CSS box-decoration-break shall be
>> "clone". Padding on span also has the problem described below.
>>
>>      span: applies to the contained text within the span.
>>>
>>
>>  more accurately, applies to the inline areas (boxes) generated by span
>>
>>
>>  Right, yes. I meant that.
>>
>>      Adds to or overrides p-based tts:padding values?
>>>
>>
>>  padding on p and spans are treated separately and independentlty
>>
>>
>>  Does this affect the interpretation of lineHeight?
>>
>
>  It doesn't affect the computed value of tts:lineHeight. As for the
> actual height assigned to specific line areas (boxes), and since spans are
> non-replaced content, the following (from CSS2.1) applies [2]:
>
>  "Although margins, borders, and padding of non-replaced elements do not
> enter into the line box calculation, they are still rendered around inline
> boxes."
>
>  [2] http://www.w3.org/TR/CSS2/visudet.html#leading
>
>
>>  I.e. is new lineHeight effectively increased by the padding on span to
>> be lineHeight + padding_before + padding_after?
>>
>
>  I think by "lineHeight" here you mean "line area (box) height". So the
> answer is no. Please keep in mind that the "line area(box) height" is not
> necessarily the same as the computed value of the tts:lineHeight property;
> that is, the following holds:
>
>  line area box height >= computed value(tts:lineHeight)
>
>
>  That's true but not what I was trying to get at.
>
>  Is the baseline spacing = line area box height + padding_before +
> padding_after, where line area box height takes into account leading
> already?
>

No. First padding (on either p or span) doesn't affect height of line
areas. Second, baseline spacing depends on which fonts are used on a line.
If we include leading in the height of a line box, then the height of a
given line area is computed as:

height(line) :=
  max(
    leading(line) + max(ascent(inline area)) + max(descent(inline area)),
    computedValue(tts:lineHeight)
  )

This value may vary between lines, so baseline spacing is (in general)
non-constant over a paragraph. It would be constant if the same font
applies to all content in the paragraph or if computed value of
tts:lineHeight is greater than the maximum of all fonts' ascent plus
descent plus line gap.



>
>           Either way this is the most problematic one: the padding
>>> creates spacing that is normally created using a spacing text character,
>>> which gives the author a no-win problem: either create spans with padding
>>> to make spacing correct, and remove space characters from text, or have
>>> unwanted extra spacing, dependent on line wrapping.
>>>
>>
>>  i don't see any overlap between padding and spacing characters; they
>> are quite distinct
>>
>>
>>  Start and end padding creates a similar visual effect to spacing
>> characters, and the two act additively in how much space is created, so
>> they're not distinct when rendered. The "colour the background behind a
>> word and extend it to half way between that word and the adjacent word's
>> background" use case that John described shows how they are not distinct,
>> at least if padding affects layout. To demonstrate:
>>
>>  <p tts:backgroundColor="black">
>> <span tts:padding="0c 0.5c 0c 1c">A word on a</span>
>> <span tts:backgroundColor="green" tts:padding="0c 0.5c 0c
>> 0.5c">green</span>
>> <span tts:padding="0c 1c 0c 0.5c">background</span>
>> </p>
>>
>>  would render (in laid out foreground text) the same as:
>>
>>  <p tts:backgroundColor="black">
>> <span>A word on a green background</span>
>> </p>
>>
>>  assuming that 1c has been arranged to be equal to the width of one
>> space character. The former example has two space characters missing. The
>> background colours could have been omitted altogether to give exactly the
>> same rendering.
>>
>
>  Are you trying to achieve something like the attached?
>
>
>  Yes. Actually it's a secondary use case. The primary use case is the
> spacing at the beginning and end of a line, where that's known.
>

Unfortunately CSS doesn't provide any way to address individual line areas,
so one can achieve this using only indirect means.


>
>           The cost of removing space characters from the text is that
>>> meaning is removed – some processors might for example pre-process by
>>> removing all formatting (e.g. for indexing), leading to weird compound
>>> words where the space has been removed.
>>>
>>      Agree this could be a problem, as shown in the third example HTML
> in the attached, where it was necessary to remove spaces between spans.
>
>  However, as shown in the fourth example in the attached, this can be
> fixed by inserting ZERO-WIDTH SPACE (&#x200B;) to ensure the presence of a
> space boundary.
>
>
>  This works because you were able to discover that the width of a space
> character is 24px for a 40px monospace font, but wouldn't work in general
> if the width of the space character is not predictable.
>

Correct.


>
>
>>>  Kind regards,
>>>
>>>  Nigel
>>>
>>>
>
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Received on Monday, 9 December 2013 21:28:53 UTC

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