Re: Line Height stacking strategy algorithm - approval needed

Hi Glenn,

Thanks for reviewing.

For background, IMSC Section 7.2.3 is intended to reflect lessons
learned in CFF-TT (see [1] for latest revision). The objective is to
inform implementers of (a) expected line heights in general and
especially when multiple font sizes are present on the same line, (b)
the meaning of "normal" and (c) rounding of computed values of
tts:lineHeight.

I would rather point to existing prose than duplicate it in IMSC. In
fact, in my mind, Section 7.2.3 would ideally not exist, or merely
point to prose elsewhere.

The latest TTML 2 draft provides guidance on (b) and (c) -- it looks
like the guidance is slightly different than intended in CFF-TT, but
that is a different issue.

I think the more (informative) guidance and/or examples we can provide
on (a) the better.

For instance, how do we expect <p tts:fontSize="10px"
tts:fontFamily="arial" tts:lineHeight="125%"><span
tts:fontSize="48px">...</span>...</p> to behave, e.g. will the height
of each line be the same and, if so, what will the value be? If
tts:lineHeight="125%" and tts:fontSize="10px", what will be the height
of three lines? This is used to accurately determine the region
extent.

Maybe all this has been discussed before, in which case I will look
into referencing it.

Best,

-- Pierre

[1] http://www.uvvuwiki.com/images/c/cb/CFFMediaFormat-1.1r1.pdf

On Mon, Jan 20, 2014 at 5:06 PM, Glenn Adams <glenn@skynav.com> wrote:
>
> Copying public-tt due to general interest.
>
> On Mon, Jan 20, 2014 at 3:41 PM, Pierre-Anthony Lemieux <pal@sandflow.com>
> wrote:
>>
>> Hi Glenn,
>>
>> I am interested in understanding the impact of the proposed text on
>> IMSC Section 7.2.3. Specifically,
>>
>> > tts:lineHeight values expressed using the percentage metric shall
>> > be relative to the largest font size on the line.
>
>
> I'm afraid that was never the case, in none of TTML, XSL-FO, or CSS.
>
> TTML1 1e [1] specified that percentage on tts:lineHeight is "relative to
> this element's font size". This hasn't changed in TTML1 2e [2] or TTML2 [3].
> Further, this is consistent with both XSL-FO, which defines [4] percentage
> on this property by referring to CSS2 text [5]:
>
> "The computed value of the property is this percentage multiplied by the
> element's computed font size. Negative values are illegal."
>
> The "element" referred to by "element's computed font size" is the element
> on which tts:lineHeight is specified. So, if a percentage is specified on
> tt:p, e.g.,
>
> <tt:p tts:lineHeight="120%"/>
>
> then % is resolved with respect to the computed font size of tt:p.
>
> [1]
> http://www.w3.org/TR/2010/REC-ttaf1-dfxp-20101118/#style-attribute-lineHeight
> [2] http://www.w3.org/TR/2013/REC-ttml1-20130924/#style-attribute-lineHeight
> [3]
> https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml2/spec/ttml2.html#style-attribute-lineHeight
> [4] http://www.w3.org/TR/2006/REC-xsl11-20061205/#line-height
> [5] http://www.w3.org/TR/CSS2/visudet.html#propdef-line-height
>
>>
>>
>> Is this consistent with the following?
>>
>> "Furthermore, it is the intention of this specification that the
>> allocation rectangle of a line be consistent with the
>> per-inline-height-rectangle as defined by [XSL 1.1],  4.5, i.e., that
>> a CSS-style line box stacking strategy be used."
>
>
> Keep in mind that you now have transitioned from talking about (1) the
> property tts:lineHeight to (2) the algorithm for computing the height of
> each line area (box) which uses the property tts:lineHeight as a parameter.
>
>>
>>
>> > White space in excess of the largest font size on the line shall be
>> > allocated
>> > evenly top and bottom (half-leading). Note that inter-baseline
>> > separation
>> >  includes any leading.
>
>
> I gather that this text was written by you in IMSC, and synthesized from
> your working understanding, yes?
>
> First, we need to distinguish between the leading that applies to line areas
> and the leading that applies to an inline area child of a line area.
>
> To diagram this a bit, we have:
>
> (1) block areas generated by formatting an fo:block (tt:p)
>
> BA0 ... BAm-1
>
> (2) line area children of BA0 ... BAm-1
>
> LA0,0 ... LA0,i-1 - line area children of BA0
> LA1,0 ... LA1,j-1- line area children of BA1
> LA2,0 ... LA2,k-1 - line area children of BA2
> ...
>
> (3) inline area children of LAm,n
>
> IA0,0,0 ... IA0,0,p-1 - inline area children of LA0,0
> IA0,1,0 ... IA0,1,q-1 - inline area children of LA0,1
> ...
>
> Using XSL-FO terminology [6], we need to distinguish between the leading of
> an expanded-nominal-requested-line-rectangle (of a line area) and the
> leading of an expanded-rectangle (of each inline area child of a line area).
>
> The former leading is derived from the half-leading trait that applies to
> each block area BAm generated by tt:p, and this trait is defined as follows
> [7]:
>
> "The .minimum, .optimum, and .maximum components of the half-leading trait
> are set to 1/2 the difference of the computed value of the line-height
> property and the computed value of the sum of the text-altitude and
> text-depth properties."
>
> Here, the properties referenced (line-height, text-altitude, and text-depth)
> refer to the computed values of these properties on the fo:block element
> (our tt:p element), which, in turn are based on the computed value of the
> font-size and font-family properties [that apply to this block element].
>
> So, for example, if one has:
>
> <tt:p tts:fontSize="10px" tts:fontFamily="arial" tts:lineHeight="120%"/>
>
> Then we have:
>
> computed(fontSize) := 10px
> computed(lineHeight) := 12px
> computed(textAltitude) := scale(depthMetricOf(arial), 10px), let's say this
> is 8px
> computed(textDepth) := scale(depthMetricOf(arial), 10px), let's say this is
> 2px
>
> Assuming for a moment that the sum of computed(textAltitude) and
> computed(textDepth) is 10px (which need not necessarily hold since we don't
> control the font's metrics), then the half leading trait for each BAm
> generated by this tt:p would be 1px, i.e., 1/2 of (12px - 10px).
>
> Thus, the expanded-nominal-requested-line-rectangle of each line area
> generated from this tt:p will have a half leading of 1px, or in diagram:
>
> __ (BEFORE EDGE)
>
> 1px - before half leading
> __
>
> 8px - text altitude of scaled font that applies to tt:p
> __ (BASELINE)
>
> 2px - text depth of scaled font that applies to tt:p
> __
>
> 1px - after half leading
> __ (AFTER EDGE)
>
> [6] http://www.w3.org/TR/2006/REC-xsl11-20061205/#area-line
> [7] http://www.w3.org/TR/2006/REC-xsl11-20061205/#fo_block
>
> Now, we have to look at the expanded-rectangle of each inline area child of
> a line area. Since (at present) we have no replaced inline elements in TTML,
> we will focus on non-replaced elements, namely immediate text node or
> tt:span children of tt:p. These children will generate inline areas that are
> assigned to specific line area children of the sequence of block areas
> generated by tt:p. For this discussion, we will assume, without loss of
> generality, that each inline area contains only glyph areas (and does not
> contain a nested non-glyph, inline area).
>
> Each glyph area child of a generated inline area is associated with a font
> and a font size. It is also associated with a "normal" line height. We don't
> actually say this anywhere, but this follows from the CSS definitions of
> line height calculations [8], which follows from XSL-FO references to CSS
> semantics.
>
> In TTML, we could define (but have not yet defined) that style property
> tts:lineHeight additionally applies to tt:span; however, at present we
> define that it only applies to tt:p. Nevertheless, both XSL-FO and CSS
> assume (or make explicit) that each glyph area is associated with a line
> height parameter from which a leading for that glyph area might computed.
> For example, see the following [8] (emphasis added):
>
> "User agent must align the glyphs in a non-replaced inline box to each other
> by their relevant baselines. Then, for each glyph, determine the A and D.
> Note that glyphs in a single element may come from different fonts and thus
> need not all have the same A and D."
>
> "Still for each glyph, determine the leading L to add, where L =
> 'line-height' - AD. Half the leading is added above A and the other half
> below D, giving the glyph and its leading a total height above the baseline
> of A' = A + L/2 and a total depth of D' = D + L/2."
>
> "The height of the inline box encloses all glyphs and their half-leading on
> each side and is thus exactly 'line-height'."
>
> [8] http://www.w3.org/TR/CSS2/visudet.html#line-height
>
> Actual implementations of CSS (and XSL-FO) use a similar algorithm presently
> defined in TTML2 [9] to determine the line height parameter for each glyph
> (or inline) area, including the derived half-leading parameter. I say
> similar because the algorithm in [9] is currently expressed only with
> reference to the block (paragraph) and not each inline element, such as an
> explicit or anonymous span. Let's just say that the same algorithm is used,
> i.e., LH for a glyph area is determined by using the ascent (altitude),
> descent (depth), and line gap (leading) metrics, or in their absence, by
> using a fixed percentage of the scaled font size, say 125%.
>
> [9]
> https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml2/spec/ttml2.html#style-attribute-lineHeight
>
> Now, we can determine a half leading for each glyph area that appears in a
> line area, and thus, can determine the height of each expanded-rectangle by
> aligning the baseline of the glyphs in the line, then computing the maximum
> ascent, maximum descent, and the half leading that applies to each (locally
> contiguous) run of glyphs associated with the same font and font size.
>
> This finally gives enough information to complete the final step defined by
> XSL-FO in determining the height of the per-inline-height-rectangle of a
> line [6] as:
>
> "The extent of the per-inline-height-rectangle in the
> block-progression-direction is then defined to be the minimum required to
> enclose both the expanded-nominal-requested-line-rectangle and the
> expanded-rectangles of all the inline-areas stacked within the line-area."
>
>>
>>
>> So "per-inline-height-rectangle" calls for "half-leading" if the
>> "area's allocation-rectangle is specified to be the
>> normal-allocation-rectangle by the description of the generating
>> formatting object". Is this the case?
>
>
> Yes, provided you keep in mind that there is more than one potential
> "half-leading" that applies: (1) the half-leading that applies to the
> expanded-nominal-requested-line-rectangle, derived from the font, font size,
> and line height that applies to the block (tt;p) as a whole and (2) the
> half-leading that applies to each expanded-rectangle on a line, derived from
> the font, font size, and line height that applies to each glyph inline area
> in the line.
>
> Further, it isn't just a matter of maximizing these line heights, but must
> take into account baseline alignment and determining the maximum ascent and
> descent points in a line and the half leadings that apply to those points.
>
>>
>>
>> > If the computed value of tts:lineHeight is not supported, then a
>> > presentation processor
>> > shall use the next highest supported value.
>>
>> Would this be subject to the round half-up rule of Section 8.2.13, and
>> so not necessarily the "next highest supported value"?
>
>
> In some ways, talking about "support" for a specific lineHeight computed
> value is a bit of a red herring. What we probably should do instead is talk
> about support for a specific line area's height after performing the
> algorithms described above. For example, one might imagine that some devices
> will want to align lines to a device pixel value, while others, perhaps
> those using anti-aliasing, might choose do not enforce such a constraint.
>
> So what we have in the text at present is definitely not perfect, and very
> likely misleading.
>
>>
>>
>> Looking forward to your thoughts.
>>
>> Thanks,
>>
>> -- Pierre
>>
>> ---------- Forwarded message ----------
>> From: Nigel Megitt <nigel.megitt@bbc.co.uk>
>> Date: Fri, Jan 17, 2014 at 6:17 AM
>> Subject: Line Height stacking strategy algorithm - approval needed
>> To: Timed Text Working Group <public-tt@w3.org>
>>
>>
>> CONTEXT
>>
>> As discussed in yesterday's TTWG meeting, as part of the fulfilment of
>> actions 219 [1] and 221 [2] Glenn has edited the TTML2 editor's draft
>> to incorporate an algorithm for computing the value of line height,
>> based on the code used by Mozilla, blink and webkit. This can
>> additionally be considered part of the solution to the line height
>> problem which has also been raised in issues 275 [3] and 284 [4].
>>
>> WHAT HAS CHANGED
>>
>> The algorithm can be found at [5] in section 8.2.13 tts:lineHeight.
>>
>> I won't attempt to repeat Glenn's explanation of this algorithm, which
>> I scribed as best I could yesterday, except to note what may appear at
>> first to be a substantive change to some, that the "125% of fontSize"
>> rule does not always apply, but applies only when the font is not
>> associated with "font metrics that specify altitude A, descent D, and
>> line gap G".
>>
>> FOLLOW-UP
>>
>> This algorithm is absent from CSS but, if this group's members are
>> happy with it, we should communicate it to the CSS WG, to maximise the
>> time for them to raise any potential issues, or indeed choose to adopt
>> it into their own output documentation, which we could then
>> potentially reference. Formally they will report any problems anyway
>> at Last Call stage but I'd rather seek early agreement when possible.
>>
>> ACTION REQUIRED NOW
>>
>> Please notify the group if there are any queries or objections to this
>> algorithm as it stands.
>>
>> We noted in yesterday's call that it was not possible to verify that
>> IE adopts the same algorithm, so it would be particularly useful if
>> someone from Microsoft could confirm that they are happy with it 
>> Jerry, this probably means you!
>>
>> Rather than making this an open-ended action I'll close this off and
>> state that we have approval if there are no outstanding objections by
>> the end of our next call on 30th Jan 2014.
>>
>>
>>
>> This email satisfies Action-260 [6].
>>
>>
>> References
>>
>> [1] Action-219 https://www.w3.org/AudioVideo/TT/tracker/actions/219
>> [2] Action-221 https://www.w3.org/AudioVideo/TT/tracker/actions/221
>> [3] Issue-275 https://www.w3.org/AudioVideo/TT/tracker/issues/275
>> [4] Issue-284 https://www.w3.org/AudioVideo/TT/tracker/issues/284
>> [5] TTML2 Editor's draft, section on lineHeight
>>
>> https://dvcs.w3.org/hg/ttml/raw-file/default/ttml2/spec/ttml2.html#style-attribute-lineHeight
>> [6] Action-260 https://www.w3.org/AudioVideo/TT/tracker/actions/260
>>
>>
>>
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Received on Tuesday, 21 January 2014 16:53:45 UTC