Re: lineHeight

Thanks for the very prompt solution proposal, Glenn.

Yes, personally I think this is the solution that is needed. It makes it clearer and as you said it meets the expectation of authors that are coming from CSS and XSL:FO.


Am 23.08.2013 um 19:50 schrieb Glenn Adams:
> How about we change the 8.2.12 (in TTML1) to read as follows?
> 
> If the value of this attribute is normal, then the computed value of the style property must be considered to be the same as 120% of the largest font size that applies to the element and its descendant elements in the intermediate synchronic document as determined by 9.3.2 Intermediate Synchronic Document Construction.
> 
> This is basically equivalent to the 1.2 multiplier used by most existing HTML browsers.
> 
> Should we be considered about potentially breaking interoperability with existing presentation processors? Since our existing TTML test suite does not include reference image output, this change wouldn't alter test results (as presently written). This change would also improve compatibility with TTML and author expectations based on their use of line-height in HTML and XSL-FO.
> 
> 
> 
> 
> 
> On Fri, Aug 23, 2013 at 10:48 AM, Glenn Adams <glenn@skynav.com> wrote:
> 
> On Fri, Aug 23, 2013 at 10:44 AM, Glenn Adams <glenn@skynav.com> wrote:
> 
> On Fri, Aug 23, 2013 at 8:01 AM, Andreas Tai <tai@irt.de> wrote:
> Thanks for the detailed walk through! It continues the discussion we had in a thread before [1].
> 
> To be clear that I got it right about the default value of line-height: If the computed line-height is the same as the largest font-size of a descendent and we assume the font-size of a p block is 14px for all descendant elements, than the computed line height is 14px. If we assume that the sum of the  computed values of the text-altitude and text-depth properties is not less than 14px than the leading is 0 or negative. Is this correct?
> 
> Since we don't expose the XSL-FO text-{altitude,depth} properties, then they will be determined according to font metrics, and it is a standard assumption that their sum is the same as the EM cell height, namely, the font size.
> 
> So in your example, if the font size for the block itself and its descendants are 14px and the line height on the block is 14px, then leading is zero, and the line heigh of individual lines will be 14px as well.
>  
> 
> If this is the case the default value for line-height would generate a representation that is not very accessible.
> 
> I guess you mean "not very readable".
>  
> 
> I am not sure how much we can take the font-size/line-height dependency in CSS as reference but at least here a two-line subtitle would not be very readable if font-size and line-height have the same value (see http://jsfiddle.net/tairt/YdsWd/).
> 
> There are two options: you as an author can specify something else for line height, say 120%. Or we can change the spec to say something else, such as resolve 'normal' to 120% of the maximum descendant font size.
> 
> That last part should better read as "resolve 'normal' to 120% of the maximum of the current element and its descendants' font size".
>  
>  
> 
> Best regards,
> 
> Andreas
> 
> 1] http://lists.w3.org/Archives/Public/public-tt/2013Jul/0090.html
> 
> 
> -------- Original-Nachricht --------
> Betreff:	Re: lineHeight
> Weitersenden-Datum:	Fri, 23 Aug 2013 02:55:03 +0000
> Weitersenden-Von:	public-tt@w3.org
> Datum:	Thu, 22 Aug 2013 20:54:09 -0600
> Von:	Glenn Adams <glenn@skynav.com>
> An:	Michael Dolan <mdolan@newtbt.com>
> Kopie (CC):	TTWG <public-tt@w3.org>
> 
> 
> Thanks for asking, as this is a complex area of functionality when having to wade through the more general models of both XSL-FO and CSS. See inline below for clarifications and answers, and at the end some additional useful information.
> 
> On Thu, Aug 22, 2013 at 4:55 PM, Michael Dolan <mdolan@newtbt.com> wrote:
> Glenn and all-
> 
>  
> A cursory reading of TTML suggests lineHeight=fontSize.  But digging deeper into XSL, one might reach different conclusions in red (also marked “CONCLUSION”).
> 
>  
> [TTML 8.2.12]:
> 
>  tts:lineHeight
> 
> If the value of this attribute is normal, then the initial value of the style property must be considered to be the same as the largest font size that applies to any descendant element.
> 
> ….
> 
> The semantics of the style property represented by this attribute are based upon that defined by [XSL 1.1], § 7.16.4. 
> 
> 
> First, we should quote the current TTML1 ED text, which was updated on July 12th [1] and 16th [2] to read as follows:
> 
> "If the value of this attribute is normal, then the computed value of the style property must be considered to be the same as the largest font size that applies to any descendant element in the intermediate synchronic document as determined by 9.3.2 Intermediate Synchronic Document Construction."
> 
> [1] https://dvcs.w3.org/hg/ttml/rev/00a9b799895d
> [2] https://dvcs.w3.org/hg/ttml/rev/0355e9473d7e
> 
> This text, about the "largest font size", is intended to correspond to the following language in:
> 
> XSL-FO [3]:
> 
> "When an element contains text that is rendered in more than one font, user agents should determine the "line-height" value according to the largest font size."
> 
> [3] http://www.w3.org/TR/2006/REC-xsl11-20061205/#line-height
> 
> and CSS2.1 10.8.1 [4]:
> 
> "When an element contains text that is rendered in more than one font, user agents may determine the 'normal' 'line-height' value according to the largest font size."
> 
> [4] http://www.w3.org/TR/CSS2/visudet.html#propdef-line-height
> 
> Note that these two citations are identical except that CSS2.1 added 'normal', while XSL-FO, which quotes CSS2, didn't make it explicit that this sentence was to be scoped by interpretation of what 'normal' means.
> 
> Note also the two changes [1][2] we made in TTML1 since PER:
> correct "initial value" to read "computed value";
> clarify that "descendant element" refers to descendants in the intermediate synchronic document, and not the source document;
> In other words, this changed language is about determining what the computed value of 'normal' means, and which descendants apply in making that determination. For example, one may have read the prior language as including the font size of descendant elements that are not temporally active or are selected into a different region, both of which should not apply when resolving the computed value for 'normal'.
> 
> [XSL 7.16.4]: 
> 
> Normal: Tells user agents to set the computed value to a "reasonable" value based on the font size of the element. The value has the same meaning as <number>. We recommend a computed value for "normal" between 1.0 to 1.2.
> 
> <length>: The box height is set to this length.
> 
>  
> CONCLUSION: Therefore as TTML states that a value of "normal" "must be considered to be the same as the largest font size", I assume that the XSL 7.16.4 definition that is in effect is "<length>" with the value of the largest font size.
> 
> Correct. For example, if one has:
> 
> <p id="p1" tts:fontSize="12pt" tts:lineHeight="normal">
> <span tts:fontSize="10pt">FOO</span>
> <span tts:fontSize="12pt">BAR</span>
> <span tts:fontSize="14pt">BAZ</span>
> </p>
> 
> Then the resolved computed value of tts:lineHeight on the paragraph p1 is "14pt". In other words, we could have specified the equivalent with:
> 
> <p id="p1" tts:fontSize="12pt" tts:lineHeight="14pt">...</p>
> 
> On the other hand, if this content mapped were selected into two regions, such as:
> 
> <p tts:fontSize="12pt" tts:lineHeight="normal">
> <span region="r1" tts:fontSize="10pt">FOO</span>
> <span region="r1" tts:fontSize="12pt">BAR</span>
> <span region="r2" tts:fontSize="14pt">BAZ</span>
> </p>
> 
> Then the computed line height that applies to p1 in region r1 would be 12pt, but in region r2 would be 14pt.
> 
>  
> [TTML 8.2.12] continued:
> 
> 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.
> 
> Here's where things get complex to interpret (particularly when trying to decode the language in XSL-FO). However, before we do that, let's take note of some simplifying factors that hold in TTML than apply to XSL-FO and CSS:
> 
> In particular, in TTML1 (though perhaps not TTML2 when we introduce images):
> tts:lineHeight only applies to the paragraph element p, and does not apply to span or br;
> all inline children of p are non-replaceable (text) elements; i.e., there are no replaceable elements like <img/> [if some profile introduces replaceable elements in a superset extension, then this simplifying assumption may not hold];
> In general, in both XSL-FO and CSS, these assumptions don't hold, so the descriptive text in XSL-FO is more complex to interpret.
> 
> Before I attempt to explain the XSL-FO text, I will summarize the three line stacking strategies defined there:
> 
> font-height
> 
> Produces constant distance between the baselines of adjacent line areas. The height of a line area's allocation rectangle is the height of the nominal-requested-line-rectangle, which is the sum of the altitude and the depth, i.e., font size, of the font that applies to the containing block element (ignoring altitude and depth of the fonts that apply to the line area's child areas).
> 
> Line areas of a block are stacked (in the block progression dimension) so that the distance between baselines are constant and equal to the computed value of the line height property of the containing block element. This is accomplished by (1) computing the half-leading trait of a block area as half the difference between the computed value of the line-height property and the sum of the computed values of the text-altitude and text-depth properties, then (2) assigning this half-leading to the space-before and space-after traits of each line area generated by the block.
> 
> Note that half-leading is negative if line-height is less than altitude plus depth (font size), in which case line area rectangles will overlap.
> 
> max-height
> 
> Produces constant distance between the adjacent edges of consecutive line areas, i.e., between the after edge of line N and the before edge of line N+1. The height of a line area's allocation rectangle is the height of the maximum-line-rectangle, which is the greater of (a) the height of the nominal-requested-line-rectangle described above and (b) the sum of the maximum altitude and maximum depth of the allocation rectangles of the line area's child inline areas.
> 
> If no child inline area has an altitude (distance above baseline) greater than the altitude of the containing block's font and no child inline area has a depth (distance below baseline) greater than the depth of the containing block's font, then the result will be identical to the font-height strategy.
> 
> Note that the allocation rectangle of an inline area generated by an element comes in two flavors [5]:
> normal-allocation-rectangle
> large-allocation-rectangle
> In general, non-replaceable elements, e.g., text, generate normal allocation rectangles, while replaceable elements, e.g., images, generate large-allocation-rectangles. The former "normal" variety does not include padding and border when computing its height, while the latter "large" variety does include padding and border.
> 
> [5] http://www.w3.org/TR/2006/REC-xsl11-20061205/#area-geo
> 
> Note further, and importantly (for what follows), that this strategy does not take into account (a) a different line height property of any non-replaceable child element and (b) the space (margin) before (top) and after (bottom) properties of any replaceable child element.
> 
> Half-leading computation and usage follows the same rules described above for the font-height strategy, meaning that half-leading may be negative, and thus cause line area rectangle overlap.
> 
> line-height
> 
> Effectively the same as the max-height strategy, except that (a) half-leading is computed on a per-line basis, (b) the half-leading of non-replaceable child inline elements is applied (rather than ignored), and (c) the margin (space) in the block progression dimension of replaceable child inline elements is applied (rather than ignored).
> 
> More specifically, the height of a line area's allocation rectangle is the height of the per-inline-height-rectangle, which is the greater of (a) the height of the expanded-nominal-requested-line-rectangle (see below) and (b) the sum of the maximum altitude and maximum depth of the expanded-rectangle (see below) of each of the line area's child inline areas.
> 
> The expanded-nominal-requested-line-rectangle is the same as the nominal-requested-line-rectangle described above except that it is expanded in the before and after directions by an amount equal to the half leading of the containing block area. [If this half leading is negative, it will be contracted rather than expanded.]
> 
> The expanded-rectangle of an inline area child of a line area is the same as its allocation-rectangle described (under max-height strategy) above except that it is expanded in the before and after directions by either (1) an amount equal to the half leading of the inline area, if generated by a non-replaceable element, or (2) amounts equal to the margin-{top,bottom} (or space-{before,after}), respectively, of the inline area, if generated by a replaceable element.
> 
> Because the half-leading of the containing block is already taken into account by the expanded-nominal-requested-line-rectangle, the space-before and space-after traits of each line area are set to zero.
> 
> This strategy corresponds to the line stacking strategy adopted by CSS, and is adopted by TTML, Section 9.3.3, step (5), when mapping intermediate synchronic documents to XSL-FO [6].
> 
> [6] https://dvcs.w3.org/hg/ttml/raw-file/default/ttml1/spec/ttml1.html#semantics-region-layout-step-2
> 
> As mentioned above, however, TTML1 does not presently apply line-height to inline elements (span or br), and thus the half-leading of each inline element is zero, and does not support a replaceable element type. Consequently, the line-height strategy, when applied to TTML1, produces equivalent results as produced by the max-height strategy. That is, each per-inline-height-rectangle is the same height as the equivalent maximum-line-rectangle to which half-leading (of the block) has been added (instead of applying this half-leading to the space-{before,after} of the line area).
> 
> So, with TTML1, this produces line areas that abut one another in the block progression dimension, but whose height includes half leading both before and after, as contrasted with max-height, where line areas don't abut but rather use space before and after on line areas to represent half leading.
> 
>  
> [XSL 4.5]
> 
> The per-inline-height-rectangle for a line-area is the rectangle whose start-edge and end-edge are parallel to and coincident with the start-edge and end-edge of the nominal-requested-line-rectangle, and whose extent in the block-progression-dimension is determined as follows.
> 
> ….
> 
> 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; this may vary depending on the descendants of the line-area.
> 
> ….
> 
> The expanded-rectangle of an inline-area is the rectangle with start-edge and end-edge coincident with those of its allocation-rectangle, and whose before-edge and after-edge are outside those of its allocation-rectangle by a distance equal to either (a.) the half-leading, when the area's allocation-rectangle is specified to be the normal-allocation-rectangle by the description of the generating formatting object , or (b.) the space-before and space after (respectively), when the area's allocation-rectangle is specified to be the large-allocation-rectangle. 
> 
> …..
> 
> The expanded-nominal-requested-line-rectangle is the rectangle with start-edge and end-edge coincident with those of the nominal-requested-line-rectangle, and whose before-edge and after-edge are outside those of the nominal-requested-line-rectangle by a distance equal to the half-leading.
> 
> ……
> 
> The nominal-requested-line-rectangle for a line-area is the rectangle whose start-edge is parallel to the start-edge of the content-rectangle of the nearest ancestor reference-area and offset from it by the sum of the start-indent and the start-intrusion-adjustment of the line area, whose end-edge is parallel to the end-edge of the content-rectangle of the nearest ancestor reference-area and offset from it by the sum of the end-indent and the end-intrusion-adjustment of the line area, whose before-edge is separated from the baseline-start-point by the text-altitude of the parent block-area, and whose after-edge is separated from the baseline-start-point by the text-depth of the parent block-area. It has the same block-progression-dimension for each line-area child of a block-area.
> 
> …
> 
> [XSL 6.5.2]
> 
> 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. The .precedence and ..conditionality components are copied from the line-height property.
> 
>  
> CONCLUSION: Therefore lineHeight for each line of text in a TTML document is given by: largest fontSize on the line + half leading == 1.5 * largest fontSize
> 
> First, we must distinguish between the height of a line area and the computed value of the lineHeight property on an element. Here, you use the term "lineHeight", so I'm not sure to which of these you are referring.
> 
> As I indicated above, the computed value of the 'normal' lineHeight property value is derived from the maximum font size over the entire set of descendants of a paragraph element (in a particular intermediate synchronic document). Or, if the lineHeight property is specified as a length, then that is resolved as usual (without reference to any descendant element's font size).
> 
> Once the computed lineHeight value is resolved, now you can compute half leading and then derive individual line area rectangle heights according to the line stacking strategy rules.
> 
> So, if I were to state the correct conclusion, it would read:
> 
> CONCLUSION: In TTML1, the height of a line area rectangle generated by a paragraph (<p/>) element is the greater of (a) the computed value of the lineHeight property that applies to the paragraph and (b) the sum of the maximum altitude and maximum depth of each font that applies to the line area's child inline areas.
> 
> For those that wish to delve even deeper, I'd suggest reading:
> 
> [7] https://lists.w3.org/Archives/Member/w3c-xsl-fo-sg/2000Nov/0027.html (Member Only)
> [8] http://meyerweb.com/eric/css/inline-format.html
> 
> Regarding [7], I'll check with the author to see if he minds me resending his message to this public list.
> 
> G.
> 
> -- 
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> Andreas Tai
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